update devcontainer extensions

* wip

* reorg

* update sv int

* apb4 working

* apb3 working

* version bump + ignore runner warning

* remove redundant check

* adding log on failure

* cleaning up verilator version issue

* devcontainer

* Fix missing libpython in GitHub Actions CI environment (#21)

* Initial plan

* Install libpython in GitHub Actions for cocotb tests

Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

.devcontainer/Dockerfile

@@ -0,0 +1,22 @@
+FROM verilator/verilator:latest
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        python3 \
+        python3-venv \
+        python3-pip \
+        python3-dev \
+        build-essential \
+        pkg-config \
+        git \
+        curl \
+        ca-certificates && \
+    rm -rf /var/lib/apt/lists/*
+
+ENV UV_INSTALL_DIR=/usr/local/bin
+ENV UV_LINK_MODE=copy
+# Install uv globally so both VS Code and terminals can use it
+RUN curl -LsSf https://astral.sh/uv/install.sh | sh
+RUN uv --version

.devcontainer/devcontainer.json

@@ -0,0 +1,36 @@
+{
+    "name": "PeakRDL BusDecoder",
+    "build": {
+        "dockerfile": "Dockerfile"
+    },
+    "runArgs": [
+        "--init"
+    ],
+    "features": {
+        "ghcr.io/devcontainers/features/common-utils:2": {
+            "username": "vscode",
+            "uid": "1000",
+            "gid": "1000",
+            "installZsh": "false",
+            "installOhMyZsh": "false"
+        }
+    },
+    "remoteUser": "vscode",
+    "postCreateCommand": "uv sync --frozen --all-extras --group tools --group test",
+    "customizations": {
+        "vscode": {
+            "settings": {
+                "python.defaultInterpreterPath": ".venv/bin/python",
+                "terminal.integrated.shell.linux": "/bin/bash"
+            },
+            "extensions": [
+                "ms-python.python",
+                "ms-python.vscode-pylance",
+                "ms-vscode.cpptools",
+                "charliermarsh.ruff",
+                "astral-sh.ty",
+                "meta.pyrefly"
+            ]
+        }
+    }
+}

.github/workflows/release.yml

@@ -40,7 +40,6 @@ jobs:
       
       - name: Publish to PyPI
         if: startsWith(github.ref, 'refs/tags/')
-        env:
-          TWINE_USERNAME: __token__
-          TWINE_PASSWORD: ${{ secrets.PYPI_API_TOKEN }}
-        run: uvx twine upload dist/*
+        uses: pypa/gh-action-pypi-publish@release/v1
+        with:
+          skip-existing: true

.github/workflows/test.yml

@@ -14,6 +14,8 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    container:
+      image: verilator/verilator:latest
     permissions:
       contents: read
     strategy:
@@ -27,19 +29,21 @@ jobs:
         uses: astral-sh/setup-uv@v3
         with:
           python-version: ${{ matrix.python-version }}
+          enable-cache: true
 
-      - name: Install Verilator
+      - name: Check Verilator version
+        run: verilator --version
+      
+      - name: Install Python development packages
         run: |
-          sudo apt-get update
-          sudo apt-get install -y verilator
-          verilator --version
+          apt-get update && apt-get install -y python3-dev libpython3-dev
       
       - name: Install dependencies
         run: |
           uv sync --all-extras --group test
       
       - name: Run tests
-        run: uv run pytest tests/ -v --cov=peakrdl_busdecoder --cov-report=xml --cov-report=term
+        run: uv run pytest tests/ --cov=peakrdl_busdecoder --cov-report=xml --cov-report=term
       
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v4

pyproject.toml

@@ -4,11 +4,11 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "peakrdl-busdecoder"
-version = "0.2.0"
+version = "0.5.0"
 requires-python = ">=3.10"
 dependencies = ["jinja2>=3.1.6", "systemrdl-compiler~=1.30.1"]
 
-authors = [{ name = "Alex Mykyta" }]
+authors = [{ name = "Arnav Sacheti" }]
 description = "Generate a SystemVerilog bus decoder from SystemRDL for splitting CPU interfaces to multiple sub-address spaces"
 readme = "README.md"
 license = { text = "LGPLv3" }
@@ -114,3 +114,4 @@ markers = [
     "simulation: marks tests as requiring cocotb simulation (deselect with '-m \"not simulation\"')",
     "verilator: marks tests as requiring verilator simulator (deselect with '-m \"not verilator\"')",
 ]
+filterwarnings = ["error", "ignore::UserWarning"]

src/peakrdl_busdecoder/__peakrdl__.py

@@ -116,7 +116,9 @@ class Exporter(ExporterSubcommandPlugin):
             type=int,
             default=1,
             help="""Maximum depth for address decoder to descend into nested
-            addressable components. Default is 1.
+            addressable components. Value of 0 decodes all levels (infinite depth).
+            Value of 1 decodes only top-level children. Value of 2 decodes top-level
+            and one level deeper, etc. Default is 1.
             """,
         )
 

src/peakrdl_busdecoder/cpuif/apb3/apb3_cpuif.py

@@ -52,8 +52,16 @@ class APB3Cpuif(BaseCpuif):
             fanin["cpuif_rd_ack"] = "'0"
             fanin["cpuif_rd_err"] = "'0"
         else:
-            fanin["cpuif_rd_ack"] = self.signal("PREADY", node, "i")
-            fanin["cpuif_rd_err"] = self.signal("PSLVERR", node, "i")
+            # Use intermediate signals for interface arrays to avoid
+            # non-constant indexing of interface arrays in procedural blocks
+            if self.is_interface and node.is_array and node.array_dimensions:
+                # Generate array index string [i0][i1]... for the intermediate signal
+                array_idx = "".join(f"[i{i}]" for i in range(len(node.array_dimensions)))
+                fanin["cpuif_rd_ack"] = f"{node.inst_name}_fanin_ready{array_idx}"
+                fanin["cpuif_rd_err"] = f"{node.inst_name}_fanin_err{array_idx}"
+            else:
+                fanin["cpuif_rd_ack"] = self.signal("PREADY", node, "i")
+                fanin["cpuif_rd_err"] = self.signal("PSLVERR", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))
 
@@ -62,6 +70,23 @@ class APB3Cpuif(BaseCpuif):
         if node is None:
             fanin["cpuif_rd_data"] = "'0"
         else:
-            fanin["cpuif_rd_data"] = self.signal("PRDATA", node, "i")
+            # Use intermediate signals for interface arrays to avoid
+            # non-constant indexing of interface arrays in procedural blocks
+            if self.is_interface and node.is_array and node.array_dimensions:
+                # Generate array index string [i0][i1]... for the intermediate signal
+                array_idx = "".join(f"[i{i}]" for i in range(len(node.array_dimensions)))
+                fanin["cpuif_rd_data"] = f"{node.inst_name}_fanin_data{array_idx}"
+            else:
+                fanin["cpuif_rd_data"] = self.signal("PRDATA", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))
+
+    def fanin_intermediate_assignments(
+        self, node: AddressableNode, inst_name: str, array_idx: str, master_prefix: str, indexed_path: str
+    ) -> list[str]:
+        """Generate intermediate signal assignments for APB3 interface arrays."""
+        return [
+            f"assign {inst_name}_fanin_ready{array_idx} = {master_prefix}{indexed_path}.PREADY;",
+            f"assign {inst_name}_fanin_err{array_idx} = {master_prefix}{indexed_path}.PSLVERR;",
+            f"assign {inst_name}_fanin_data{array_idx} = {master_prefix}{indexed_path}.PRDATA;",
+        ]

src/peakrdl_busdecoder/cpuif/apb3/apb3_cpuif_flat.py

@@ -35,15 +35,15 @@ class APB3CpuifFlat(BaseCpuif):
 
     def fanout(self, node: AddressableNode) -> str:
         fanout: dict[str, str] = {}
-        fanout[self.signal("PSEL", node)] = (
-            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'i')}|cpuif_rd_sel.{get_indexed_path(self.exp.ds.top_node, node, 'i')}"
+        fanout[self.signal("PSEL", node, "gi")] = (
+            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'gi')}|cpuif_rd_sel.{get_indexed_path(self.exp.ds.top_node, node, 'gi')}"
         )
-        fanout[self.signal("PENABLE", node)] = self.signal("PENABLE")
-        fanout[self.signal("PWRITE", node)] = (
-            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'i')}"
+        fanout[self.signal("PENABLE", node, "gi")] = self.signal("PENABLE")
+        fanout[self.signal("PWRITE", node, "gi")] = (
+            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'gi')}"
         )
-        fanout[self.signal("PADDR", node)] = self.signal("PADDR")
-        fanout[self.signal("PWDATA", node)] = "cpuif_wr_data"
+        fanout[self.signal("PADDR", node, "gi")] = self.signal("PADDR")
+        fanout[self.signal("PWDATA", node, "gi")] = "cpuif_wr_data"
 
         return "\n".join(map(lambda kv: f"assign {kv[0]} = {kv[1]};", fanout.items()))
 
@@ -53,8 +53,8 @@ class APB3CpuifFlat(BaseCpuif):
             fanin["cpuif_rd_ack"] = "'0"
             fanin["cpuif_rd_err"] = "'0"
         else:
-            fanin["cpuif_rd_ack"] = self.signal("PREADY", node)
-            fanin["cpuif_rd_err"] = self.signal("PSLVERR", node)
+            fanin["cpuif_rd_ack"] = self.signal("PREADY", node, "i")
+            fanin["cpuif_rd_err"] = self.signal("PSLVERR", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))
 
@@ -63,6 +63,6 @@ class APB3CpuifFlat(BaseCpuif):
         if node is None:
             fanin["cpuif_rd_data"] = "'0"
         else:
-            fanin["cpuif_rd_data"] = self.signal("PRDATA", node)
+            fanin["cpuif_rd_data"] = self.signal("PRDATA", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))

src/peakrdl_busdecoder/cpuif/apb3/apb3_tmpl.sv

@@ -26,6 +26,13 @@ assign {{cpuif.signal("PSLVERR")}} = cpuif_rd_err | cpuif_rd_sel.cpuif_err | cpu
 // Fanout CPU Bus interface signals
 //--------------------------------------------------------------------------
 {{fanout|walk(cpuif=cpuif)}}
+{%- if cpuif.is_interface %}
+
+//--------------------------------------------------------------------------
+// Intermediate signals for interface array fanin
+//--------------------------------------------------------------------------
+{{fanin_intermediate|walk(cpuif=cpuif)}}
+{%- endif %}
 
 //--------------------------------------------------------------------------
 // Fanin CPU Bus interface signals

src/peakrdl_busdecoder/cpuif/apb4/apb4_cpuif.py

@@ -55,8 +55,16 @@ class APB4Cpuif(BaseCpuif):
             fanin["cpuif_rd_ack"] = "'0"
             fanin["cpuif_rd_err"] = "'0"
         else:
-            fanin["cpuif_rd_ack"] = self.signal("PREADY", node, "i")
-            fanin["cpuif_rd_err"] = self.signal("PSLVERR", node, "i")
+            # Use intermediate signals for interface arrays to avoid
+            # non-constant indexing of interface arrays in procedural blocks
+            if self.is_interface and node.is_array and node.array_dimensions:
+                # Generate array index string [i0][i1]... for the intermediate signal
+                array_idx = "".join(f"[i{i}]" for i in range(len(node.array_dimensions)))
+                fanin["cpuif_rd_ack"] = f"{node.inst_name}_fanin_ready{array_idx}"
+                fanin["cpuif_rd_err"] = f"{node.inst_name}_fanin_err{array_idx}"
+            else:
+                fanin["cpuif_rd_ack"] = self.signal("PREADY", node, "i")
+                fanin["cpuif_rd_err"] = self.signal("PSLVERR", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))
 
@@ -65,6 +73,23 @@ class APB4Cpuif(BaseCpuif):
         if node is None:
             fanin["cpuif_rd_data"] = "'0"
         else:
-            fanin["cpuif_rd_data"] = self.signal("PRDATA", node, "i")
+            # Use intermediate signals for interface arrays to avoid
+            # non-constant indexing of interface arrays in procedural blocks
+            if self.is_interface and node.is_array and node.array_dimensions:
+                # Generate array index string [i0][i1]... for the intermediate signal
+                array_idx = "".join(f"[i{i}]" for i in range(len(node.array_dimensions)))
+                fanin["cpuif_rd_data"] = f"{node.inst_name}_fanin_data{array_idx}"
+            else:
+                fanin["cpuif_rd_data"] = self.signal("PRDATA", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))
+
+    def fanin_intermediate_assignments(
+        self, node: AddressableNode, inst_name: str, array_idx: str, master_prefix: str, indexed_path: str
+    ) -> list[str]:
+        """Generate intermediate signal assignments for APB4 interface arrays."""
+        return [
+            f"assign {inst_name}_fanin_ready{array_idx} = {master_prefix}{indexed_path}.PREADY;",
+            f"assign {inst_name}_fanin_err{array_idx} = {master_prefix}{indexed_path}.PSLVERR;",
+            f"assign {inst_name}_fanin_data{array_idx} = {master_prefix}{indexed_path}.PRDATA;",
+        ]

src/peakrdl_busdecoder/cpuif/apb4/apb4_cpuif_flat.py

@@ -35,17 +35,17 @@ class APB4CpuifFlat(BaseCpuif):
 
     def fanout(self, node: AddressableNode) -> str:
         fanout: dict[str, str] = {}
-        fanout[self.signal("PSEL", node)] = (
-            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'i')}|cpuif_rd_sel.{get_indexed_path(self.exp.ds.top_node, node, 'i')}"
+        fanout[self.signal("PSEL", node, "gi")] = (
+            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'gi')}|cpuif_rd_sel.{get_indexed_path(self.exp.ds.top_node, node, 'gi')}"
         )
-        fanout[self.signal("PENABLE", node)] = self.signal("PENABLE")
-        fanout[self.signal("PWRITE", node)] = (
-            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'i')}"
+        fanout[self.signal("PENABLE", node, "gi")] = self.signal("PENABLE")
+        fanout[self.signal("PWRITE", node, "gi")] = (
+            f"cpuif_wr_sel.{get_indexed_path(self.exp.ds.top_node, node, 'gi')}"
         )
-        fanout[self.signal("PADDR", node)] = self.signal("PADDR")
-        fanout[self.signal("PPROT", node)] = self.signal("PPROT")
-        fanout[self.signal("PWDATA", node)] = "cpuif_wr_data"
-        fanout[self.signal("PSTRB", node)] = "cpuif_wr_byte_en"
+        fanout[self.signal("PADDR", node, "gi")] = self.signal("PADDR")
+        fanout[self.signal("PPROT", node, "gi")] = self.signal("PPROT")
+        fanout[self.signal("PWDATA", node, "gi")] = "cpuif_wr_data"
+        fanout[self.signal("PSTRB", node, "gi")] = "cpuif_wr_byte_en"
 
         return "\n".join(map(lambda kv: f"assign {kv[0]} = {kv[1]};", fanout.items()))
 
@@ -55,8 +55,8 @@ class APB4CpuifFlat(BaseCpuif):
             fanin["cpuif_rd_ack"] = "'0"
             fanin["cpuif_rd_err"] = "'0"
         else:
-            fanin["cpuif_rd_ack"] = self.signal("PREADY", node)
-            fanin["cpuif_rd_err"] = self.signal("PSLVERR", node)
+            fanin["cpuif_rd_ack"] = self.signal("PREADY", node, "i")
+            fanin["cpuif_rd_err"] = self.signal("PSLVERR", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))
 
@@ -65,6 +65,6 @@ class APB4CpuifFlat(BaseCpuif):
         if node is None:
             fanin["cpuif_rd_data"] = "'0"
         else:
-            fanin["cpuif_rd_data"] = self.signal("PRDATA", node)
+            fanin["cpuif_rd_data"] = self.signal("PRDATA", node, "i")
 
         return "\n".join(map(lambda kv: f"{kv[0]} = {kv[1]};", fanin.items()))

src/peakrdl_busdecoder/cpuif/apb4/apb4_tmpl.sv

@@ -29,6 +29,13 @@ assign {{cpuif.signal("PSLVERR")}} = cpuif_rd_err | cpuif_rd_sel.cpuif_err | cpu
 // Fanout CPU Bus interface signals
 //--------------------------------------------------------------------------
 {{fanout|walk(cpuif=cpuif)}}
+{%- if cpuif.is_interface %}
+
+//--------------------------------------------------------------------------
+// Intermediate signals for interface array fanin
+//--------------------------------------------------------------------------
+{{fanin_intermediate|walk(cpuif=cpuif)}}
+{%- endif %}
 
 //--------------------------------------------------------------------------
 // Fanin CPU Bus interface signals

src/peakrdl_busdecoder/cpuif/axi4lite/axi4_lite_cpuif.py

@@ -4,14 +4,14 @@ from systemrdl.node import AddressableNode
 
 from ...utils import get_indexed_path
 from ..base_cpuif import BaseCpuif
-from .axi4lite_interface import AXI4LiteSVInterface
+from .axi4_lite_interface import AXI4LiteSVInterface
 
 if TYPE_CHECKING:
     from ...exporter import BusDecoderExporter
 
 
 class AXI4LiteCpuif(BaseCpuif):
-    template_path = "axi4lite_tmpl.sv"
+    template_path = "axi4_lite_tmpl.sv"
 
     def __init__(self, exp: "BusDecoderExporter") -> None:
         super().__init__(exp)
@@ -68,9 +68,17 @@ class AXI4LiteCpuif(BaseCpuif):
             fanin["cpuif_rd_ack"] = "'0"
             fanin["cpuif_rd_err"] = "'0"
         else:
-            # Read side: ack comes from RVALID; err if RRESP[1] is set (SLVERR/DECERR)
-            fanin["cpuif_rd_ack"] = self.signal("RVALID", node, "i")
-            fanin["cpuif_rd_err"] = f"{self.signal('RRESP', node, 'i')}[1]"
+            # Use intermediate signals for interface arrays to avoid
+            # non-constant indexing of interface arrays in procedural blocks
+            if self.is_interface and node.is_array and node.array_dimensions:
+                # Generate array index string [i0][i1]... for the intermediate signal
+                array_idx = "".join(f"[i{i}]" for i in range(len(node.array_dimensions)))
+                fanin["cpuif_rd_ack"] = f"{node.inst_name}_fanin_ready{array_idx}"
+                fanin["cpuif_rd_err"] = f"{node.inst_name}_fanin_err{array_idx}"
+            else:
+                # Read side: ack comes from RVALID; err if RRESP[1] is set (SLVERR/DECERR)
+                fanin["cpuif_rd_ack"] = self.signal("RVALID", node, "i")
+                fanin["cpuif_rd_err"] = f"{self.signal('RRESP', node, 'i')}[1]"
 
         return "\n".join(f"{lhs} = {rhs};" for lhs, rhs in fanin.items())
 
@@ -79,6 +87,23 @@ class AXI4LiteCpuif(BaseCpuif):
         if node is None:
             fanin["cpuif_rd_data"] = "'0"
         else:
-            fanin["cpuif_rd_data"] = self.signal("RDATA", node, "i")
+            # Use intermediate signals for interface arrays to avoid
+            # non-constant indexing of interface arrays in procedural blocks
+            if self.is_interface and node.is_array and node.array_dimensions:
+                # Generate array index string [i0][i1]... for the intermediate signal
+                array_idx = "".join(f"[i{i}]" for i in range(len(node.array_dimensions)))
+                fanin["cpuif_rd_data"] = f"{node.inst_name}_fanin_data{array_idx}"
+            else:
+                fanin["cpuif_rd_data"] = self.signal("RDATA", node, "i")
 
         return "\n".join(f"{lhs} = {rhs};" for lhs, rhs in fanin.items())
+
+    def fanin_intermediate_assignments(
+        self, node: AddressableNode, inst_name: str, array_idx: str, master_prefix: str, indexed_path: str
+    ) -> list[str]:
+        """Generate intermediate signal assignments for AXI4-Lite interface arrays."""
+        return [
+            f"assign {inst_name}_fanin_ready{array_idx} = {master_prefix}{indexed_path}.RVALID;",
+            f"assign {inst_name}_fanin_err{array_idx} = {master_prefix}{indexed_path}.RRESP[1];",
+            f"assign {inst_name}_fanin_data{array_idx} = {master_prefix}{indexed_path}.RDATA;",
+        ]

src/peakrdl_busdecoder/cpuif/axi4lite/axi4_lite_cpuif_flat.py

@@ -4,7 +4,7 @@ from systemrdl.node import AddressableNode
 
 from ...utils import get_indexed_path
 from ..base_cpuif import BaseCpuif
-from .axi4lite_interface import AXI4LiteFlatInterface
+from .axi4_lite_interface import AXI4LiteFlatInterface
 
 if TYPE_CHECKING:
     from ...exporter import BusDecoderExporter
@@ -13,7 +13,7 @@ if TYPE_CHECKING:
 class AXI4LiteCpuifFlat(BaseCpuif):
     """Verilator-friendly variant that flattens the AXI4-Lite interface ports."""
 
-    template_path = "axi4lite_tmpl.sv"
+    template_path = "axi4_lite_tmpl.sv"
 
     def __init__(self, exp: "BusDecoderExporter") -> None:
         super().__init__(exp)

src/peakrdl_busdecoder/cpuif/axi4lite/axi4_lite_tmpl.sv

@@ -53,6 +53,13 @@ assign {{cpuif.signal("BRESP")}}  = (cpuif_wr_err | cpuif_wr_sel.cpuif_err | cpu
 // Fanout CPU Bus interface signals
 //--------------------------------------------------------------------------
 {{fanout|walk(cpuif=cpuif)}}
+{%- if cpuif.is_interface %}
+
+//--------------------------------------------------------------------------
+// Intermediate signals for interface array fanin
+//--------------------------------------------------------------------------
+{{fanin_intermediate|walk(cpuif=cpuif)}}
+{%- endif %}
 
 //--------------------------------------------------------------------------
 // Fanin CPU Bus interface signals

src/peakrdl_busdecoder/cpuif/base_cpuif.py

@@ -7,6 +7,7 @@ from systemrdl.node import AddressableNode
 
 from ..utils import clog2, get_indexed_path, is_pow2, roundup_pow2
 from .fanin_gen import FaninGenerator
+from .fanin_intermediate_gen import FaninIntermediateGenerator
 from .fanout_gen import FanoutGenerator
 
 if TYPE_CHECKING:
@@ -24,11 +25,7 @@ class BaseCpuif:
 
     @property
     def addressable_children(self) -> list[AddressableNode]:
-        return [
-            child
-            for child in self.exp.ds.top_node.children(unroll=self.unroll)
-            if isinstance(child, AddressableNode)
-        ]
+        return self.exp.ds.get_addressable_children_at_depth(unroll=self.unroll)
 
     @property
     def addr_width(self) -> int:
@@ -97,6 +94,7 @@ class BaseCpuif:
             "ds": self.exp.ds,
             "fanout": FanoutGenerator,
             "fanin": FaninGenerator,
+            "fanin_intermediate": FaninIntermediateGenerator,
         }
 
         template = jj_env.get_template(self.template_path)
@@ -116,3 +114,24 @@ class BaseCpuif:
 
     def readback(self, node: AddressableNode | None = None) -> str:
         raise NotImplementedError
+
+    def fanin_intermediate_assignments(
+        self, node: AddressableNode, inst_name: str, array_idx: str, master_prefix: str, indexed_path: str
+    ) -> list[str]:
+        """Generate intermediate signal assignments for interface array fanin.
+
+        This method should be implemented by cpuif classes that use interfaces.
+        It returns a list of assignment strings that copy signals from interface
+        arrays to intermediate unpacked arrays using constant (genvar) indexing.
+
+        Args:
+            node: The addressable node
+            inst_name: Instance name for the intermediate signals
+            array_idx: Array index string (e.g., "[gi0][gi1]")
+            master_prefix: Master interface prefix
+            indexed_path: Indexed path to the interface element
+
+        Returns:
+            List of assignment strings
+        """
+        return []  # Default: no intermediate assignments needed

src/peakrdl_busdecoder/cpuif/fanin_gen.py

@@ -27,6 +27,17 @@ class FaninGenerator(BusDecoderListener):
     def enter_AddressableComponent(self, node: AddressableNode) -> WalkerAction | None:
         action = super().enter_AddressableComponent(node)
 
+        should_generate = action == WalkerAction.SkipDescendants
+        if not should_generate and self._ds.max_decode_depth == 0:
+            for child in node.children():
+                if isinstance(child, AddressableNode):
+                    break
+            else:
+                should_generate = True
+
+        if not should_generate:
+            return action
+
         if node.array_dimensions:
             for i, dim in enumerate(node.array_dimensions):
                 fb = ForLoopBody(

src/peakrdl_busdecoder/cpuif/fanin_intermediate_gen.py

@@ -0,0 +1,142 @@
+"""Generator for intermediate signals needed for interface array fanin.
+
+When using SystemVerilog interface arrays, we cannot use variable indices
+in procedural blocks (like always_comb). This generator creates intermediate
+signals that copy from interface arrays using generate loops, which can then
+be safely accessed with variable indices in the fanin logic.
+"""
+
+from collections import deque
+from typing import TYPE_CHECKING
+
+from systemrdl.node import AddressableNode
+from systemrdl.walker import WalkerAction
+
+from ..body import Body, ForLoopBody
+from ..design_state import DesignState
+from ..listener import BusDecoderListener
+from ..utils import get_indexed_path
+
+if TYPE_CHECKING:
+    from .base_cpuif import BaseCpuif
+
+
+class FaninIntermediateGenerator(BusDecoderListener):
+    """Generates intermediate signals for interface array fanin."""
+
+    def __init__(self, ds: DesignState, cpuif: "BaseCpuif") -> None:
+        super().__init__(ds)
+        self._cpuif = cpuif
+        self._declarations: list[str] = []
+        self._stack: deque[Body] = deque()
+        self._stack.append(Body())
+
+    def enter_AddressableComponent(self, node: AddressableNode) -> WalkerAction | None:
+        action = super().enter_AddressableComponent(node)
+
+        # Only generate intermediates for interface arrays
+        # Check if cpuif has is_interface attribute (some implementations don't)
+        is_interface = getattr(self._cpuif, "is_interface", False)
+        if not is_interface or not node.array_dimensions:
+            return action
+
+        # Generate intermediate signal declarations
+        self._generate_intermediate_declarations(node)
+
+        # Generate assignment logic using generate loops
+        if node.array_dimensions:
+            for i, dim in enumerate(node.array_dimensions):
+                fb = ForLoopBody(
+                    "genvar",
+                    f"gi{i}",
+                    dim,
+                )
+                self._stack.append(fb)
+
+        # Generate assignments from interface array to intermediates
+        self._stack[-1] += self._generate_intermediate_assignments(node)
+
+        return action
+
+    def exit_AddressableComponent(self, node: AddressableNode) -> None:
+        is_interface = getattr(self._cpuif, "is_interface", False)
+        if is_interface and node.array_dimensions:
+            for _ in node.array_dimensions:
+                b = self._stack.pop()
+                if not b:
+                    continue
+                self._stack[-1] += b
+
+        super().exit_AddressableComponent(node)
+
+    def _generate_intermediate_declarations(self, node: AddressableNode) -> None:
+        """Generate intermediate signal declarations for a node."""
+        inst_name = node.inst_name
+
+        # Array dimensions should be checked before calling this function
+        if not node.array_dimensions:
+            return
+
+        # Calculate total array size
+        array_size = 1
+        for dim in node.array_dimensions:
+            array_size *= dim
+
+        # Create array dimension string
+        array_str = "".join(f"[{dim}]" for dim in node.array_dimensions)
+
+        # Generate declarations for each fanin signal
+        # For APB3/4: PREADY, PSLVERR, PRDATA
+        # These are the signals read in fanin
+        self._declarations.append(f"logic {inst_name}_fanin_ready{array_str};")
+        self._declarations.append(f"logic {inst_name}_fanin_err{array_str};")
+        self._declarations.append(
+            f"logic [{self._cpuif.data_width - 1}:0] {inst_name}_fanin_data{array_str};"
+        )
+
+    def _generate_intermediate_assignments(self, node: AddressableNode) -> str:
+        """Generate assignments from interface array to intermediate signals."""
+        inst_name = node.inst_name
+        indexed_path = get_indexed_path(node.parent, node, "gi", skip_kw_filter=True)
+
+        # Get master prefix - use getattr to avoid type errors
+        interface = getattr(self._cpuif, "_interface", None)
+        if interface is None:
+            return ""
+        master_prefix = interface.get_master_prefix()
+
+        # Array dimensions should be checked before calling this function
+        if not node.array_dimensions:
+            return ""
+
+        # Create indexed signal names for left-hand side
+        array_idx = "".join(f"[gi{i}]" for i in range(len(node.array_dimensions)))
+
+        # Delegate to cpuif to get the appropriate assignments for this interface type
+        assignments = self._cpuif.fanin_intermediate_assignments(
+            node, inst_name, array_idx, master_prefix, indexed_path
+        )
+
+        return "\n".join(assignments)
+
+    def get_declarations(self) -> str:
+        """Get all intermediate signal declarations."""
+        if not self._declarations:
+            return ""
+        return "\n".join(self._declarations)
+
+    def __str__(self) -> str:
+        """Get all intermediate signal declarations and assignments."""
+        if not self._declarations:
+            return ""
+
+        # Output declarations first
+        output = "\n".join(self._declarations)
+        output += "\n\n"
+
+        # Then output assignments
+        body_str = "\n".join(map(str, self._stack))
+        if body_str and body_str.strip():
+            output += body_str
+
+        return output

src/peakrdl_busdecoder/cpuif/fanout_gen.py

@@ -23,6 +23,17 @@ class FanoutGenerator(BusDecoderListener):
     def enter_AddressableComponent(self, node: AddressableNode) -> WalkerAction | None:
         action = super().enter_AddressableComponent(node)
 
+        should_generate = action == WalkerAction.SkipDescendants
+        if not should_generate and self._ds.max_decode_depth == 0:
+            for child in node.children():
+                if isinstance(child, AddressableNode):
+                    break
+            else:
+                should_generate = True
+
+        if not should_generate:
+            return action
+
         if node.array_dimensions:
             for i, dim in enumerate(node.array_dimensions):
                 fb = ForLoopBody(

src/peakrdl_busdecoder/cpuif/interface.py

@@ -1,10 +1,13 @@
 """Interface abstraction for handling flat and non-flat signal declarations."""
 
+import re
 from abc import ABC, abstractmethod
 from typing import TYPE_CHECKING
 
 from systemrdl.node import AddressableNode
 
+from ..utils import get_indexed_path
+
 if TYPE_CHECKING:
     from .base_cpuif import BaseCpuif
 
@@ -93,7 +96,6 @@ class SVInterface(Interface):
         indexer: str | int | None = None,
     ) -> str:
         """Generate SystemVerilog interface signal reference."""
-        from ..utils import get_indexed_path
 
         # SVInterface only supports string indexers (loop variable names like "i", "gi")
         if indexer is not None and not isinstance(indexer, str):
@@ -166,6 +168,13 @@ class FlatInterface(Interface):
 
         # Is an array
         if indexer is not None:
+            if isinstance(indexer, str):
+                indexed_path = get_indexed_path(node.parent, node, indexer, skip_kw_filter=True)
+                pattern = r"\[.*?\]"
+                indexes = re.findall(pattern, indexed_path)
+
+                return f"{base}_{signal}{''.join(indexes)}"
+
             return f"{base}_{signal}[{indexer}]"
         return f"{base}_{signal}[N_{node.inst_name.upper()}S]"
 

src/peakrdl_busdecoder/decode_logic_gen.py

@@ -70,7 +70,9 @@ class DecodeLogicGenerator(BusDecoderListener):
         # Avoid generating a redundant >= 0 comparison, which triggers Verilator warnings.
         if not (l_bound.value == 0 and len(l_bound_comp) == 1):
             predicates.append(lower_expr)
-        predicates.append(upper_expr)
+        # Avoid generating a redundant full-width < max comparison, which triggers Verilator warnings.
+        if not (u_bound.value == (1 << addr_width) and len(u_bound_comp) == 1):
+            predicates.append(upper_expr)
 
         return predicates
 
@@ -85,6 +87,20 @@ class DecodeLogicGenerator(BusDecoderListener):
     def enter_AddressableComponent(self, node: AddressableNode) -> WalkerAction | None:
         action = super().enter_AddressableComponent(node)
 
+        should_decode = action == WalkerAction.SkipDescendants
+
+        if not should_decode and self._ds.max_decode_depth == 0:
+            # When decoding all levels, treat leaf registers as decode boundary
+            for child in node.children():
+                if isinstance(child, AddressableNode):
+                    break
+            else:
+                should_decode = True
+
+        # Only generate select logic if we're at the decode boundary
+        if not should_decode:
+            return action
+
         conditions: list[str] = []
         conditions.extend(self.cpuif_addr_predicate(node))
         conditions.extend(self.cpuif_prot_predicate(node))
@@ -146,6 +162,8 @@ class DecodeLogicGenerator(BusDecoderListener):
     def __str__(self) -> str:
         body = self._decode_stack[-1]
         if isinstance(body, IfBody):
+            if len(body) == 0:
+                return f"{self._flavor.cpuif_select}.cpuif_err = 1'b1;"
             with body.cm(...) as b:
                 b += f"{self._flavor.cpuif_select}.cpuif_err = 1'b1;"
 

src/peakrdl_busdecoder/design_state.py

@@ -1,6 +1,6 @@
 from typing import TypedDict
 
-from systemrdl.node import AddrmapNode
+from systemrdl.node import AddressableNode, AddrmapNode
 from systemrdl.rdltypes.user_enum import UserEnum
 
 from .design_scanner import DesignScanner
@@ -72,3 +72,56 @@ class DesignState:
         if user_addr_width < self.addr_width:
             msg.fatal(f"User-specified address width shall be greater than or equal to {self.addr_width}.")
         self.addr_width = user_addr_width
+
+    def get_addressable_children_at_depth(self, unroll: bool = False) -> list[AddressableNode]:
+        """
+        Get addressable children at the decode boundary based on max_decode_depth.
+
+        max_decode_depth semantics:
+        - 0: decode all levels (return leaf registers)
+        - 1: decode only top level (return children at depth 1)
+        - 2: decode top + 1 level (return children at depth 2)
+        - N: decode down to depth N (return children at depth N)
+
+        Args:
+            unroll: Whether to unroll arrayed nodes
+
+        Returns:
+            List of addressable nodes at the decode boundary
+        """
+        from systemrdl.node import RegNode
+
+        def collect_nodes(node: AddressableNode, current_depth: int) -> list[AddressableNode]:
+            """Recursively collect nodes at the decode boundary."""
+            result: list[AddressableNode] = []
+
+            # For depth 0, collect all leaf registers
+            if self.max_decode_depth == 0:
+                # If this is a register, it's a leaf
+                if isinstance(node, RegNode):
+                    result.append(node)
+                else:
+                    # Recurse into children
+                    for child in node.children(unroll=unroll):
+                        if isinstance(child, AddressableNode):
+                            result.extend(collect_nodes(child, current_depth + 1))
+            else:
+                # For depth N, collect children at depth N
+                if current_depth == self.max_decode_depth:
+                    # We're at the decode boundary - return this node
+                    result.append(node)
+                elif current_depth < self.max_decode_depth:
+                    # We haven't reached the boundary yet - recurse
+                    for child in node.children(unroll=unroll):
+                        if isinstance(child, AddressableNode):
+                            result.extend(collect_nodes(child, current_depth + 1))
+
+            return result
+
+        # Start collecting from top node's children
+        nodes: list[AddressableNode] = []
+        for child in self.top_node.children(unroll=unroll):
+            if isinstance(child, AddressableNode):
+                nodes.extend(collect_nodes(child, 1))
+
+        return nodes

src/peakrdl_busdecoder/exporter.py

@@ -89,7 +89,9 @@ class BusDecoderExporter:
             interface. By default, arrayed nodes are kept as arrays.
         max_decode_depth: int
             Maximum depth for address decoder to descend into nested addressable
-            components. By default, the decoder descends 1 level deep.
+            components. A value of 0 decodes all levels (infinite depth). A value
+            of 1 decodes only top-level children. A value of 2 decodes top-level
+            and one level deeper, etc. By default, the decoder descends 1 level deep.
         """
         # If it is the root node, skip to top addrmap
         if isinstance(node, RootNode):

src/peakrdl_busdecoder/listener.py

@@ -15,7 +15,12 @@ class BusDecoderListener(RDLListener):
     def should_skip_node(self, node: AddressableNode) -> bool:
         """Check if this node should be skipped (not decoded)."""
         # Check if current depth exceeds max depth
-        if self._depth > self._ds.max_decode_depth:
+        # max_decode_depth semantics:
+        # - 0 means decode all levels (infinite)
+        # - 1 means decode only top level (depth 0)
+        # - 2 means decode top + 1 level (depth 0 and 1)
+        # - N means decode down to depth N-1
+        if self._ds.max_decode_depth > 0 and self._depth >= self._ds.max_decode_depth:
             return True
 
         # Check if this node only contains external addressable children

src/peakrdl_busdecoder/module_tmpl.sv

@@ -51,7 +51,7 @@ module {{ds.module_name}}
     //--------------------------------------------------------------------------
     always_comb begin
         // Default all write select signals to 0
-        cpuif_wr_sel = '0;
+        cpuif_wr_sel = '{default: '0};
 
         if (cpuif_req && cpuif_wr_en) begin
             // A write request is pending
@@ -66,7 +66,7 @@ module {{ds.module_name}}
     //--------------------------------------------------------------------------
     always_comb begin
         // Default all read select signals to 0
-        cpuif_rd_sel = '0;
+        cpuif_rd_sel = '{default: '0};
 
         if (cpuif_req && cpuif_rd_en) begin
             // A read request is pending

src/peakrdl_busdecoder/struct_gen.py

@@ -3,7 +3,7 @@ from collections import deque
 from systemrdl.node import AddressableNode
 from systemrdl.walker import WalkerAction
 
-from .body import Body, StructBody
+from .body import StructBody
 from .design_state import DesignState
 from .identifier_filter import kw_filter as kwf
 from .listener import BusDecoderListener
@@ -16,42 +16,53 @@ class StructGenerator(BusDecoderListener):
     ) -> None:
         super().__init__(ds)
 
-        self._stack: deque[Body] = deque()
-        self._stack.append(StructBody("cpuif_sel_t", True, True))
+        self._stack: list[StructBody] = [StructBody("cpuif_sel_t", True, False)]
+        self._struct_defs: list[StructBody] = []
+        self._created_struct_stack: deque[bool] = deque()  # Track if we created a struct for each node
 
     def enter_AddressableComponent(self, node: AddressableNode) -> WalkerAction | None:
         action = super().enter_AddressableComponent(node)
 
-        self._skip = False
-        if action == WalkerAction.SkipDescendants:
-            self._skip = True
+        skip = action == WalkerAction.SkipDescendants
 
-        if node.children():
+        # Only create nested struct if we're not skipping and node has addressable children
+        has_addressable_children = any(isinstance(child, AddressableNode) for child in node.children())
+        if has_addressable_children and not skip:
             # Push new body onto stack
-            body = StructBody(f"cpuif_sel_{node.inst_name}_t", True, True)
+            body = StructBody(f"cpuif_sel_{node.inst_name}_t", True, False)
             self._stack.append(body)
+            self._created_struct_stack.append(True)
+        else:
+            self._created_struct_stack.append(False)
 
         return action
 
     def exit_AddressableComponent(self, node: AddressableNode) -> None:
         type = "logic"
 
-        if node.children():
+        # Pop the created_struct flag
+        created_struct = self._created_struct_stack.pop()
+
+        # Only pop struct body if we created one
+        if created_struct:
             body = self._stack.pop()
-            if body and isinstance(body, StructBody) and not self._skip:
-                self._stack.appendleft(body)
+            if body:
+                self._struct_defs.append(body)
                 type = body.name
 
         name = kwf(node.inst_name)
 
         if node.array_dimensions:
             for dim in node.array_dimensions:
-                name = f"[{dim - 1}:0]{name}"
+                name = f"{name}[{dim}]"
 
         self._stack[-1] += f"{type} {name};"
 
         super().exit_AddressableComponent(node)
 
     def __str__(self) -> str:
-        self._stack[-1] += "logic cpuif_err;"
-        return "\n".join(map(str, self._stack))
+        if "logic cpuif_err;" not in self._stack[-1].lines:
+            self._stack[-1] += "logic cpuif_err;"
+        bodies = [str(body) for body in self._struct_defs]
+        bodies.append(str(self._stack[-1]))
+        return "\n".join(bodies)

src/peakrdl_busdecoder/sv_int.py

@@ -1,3 +1,6 @@
+from typing import Literal
+
+
 class SVInt:
     def __init__(self, value: int, width: int | None = None) -> None:
         self.value = value
@@ -19,3 +22,27 @@ class SVInt:
             return SVInt(self.value + other.value, max(self.width, other.width))
         else:
             return SVInt(self.value + other.value, None)
+
+    def __sub__(self, other: "SVInt") -> "SVInt":
+        if self.width is not None and other.width is not None:
+            return SVInt(self.value - other.value, max(self.width, other.width))
+        else:
+            return SVInt(self.value - other.value, None)
+
+    def __len__(self) -> int:
+        if self.width is not None:
+            return self.width
+        else:
+            return self.value.bit_length()
+
+    def to_bytes(self, byteorder: Literal["little", "big"] = "little") -> bytes:
+        byte_length = (self.value.bit_length() + 7) // 8
+        return self.value.to_bytes(byte_length, byteorder)
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, SVInt):
+            return NotImplemented
+        return self.value == other.value and self.width == other.width
+
+    def __hash__(self) -> int:
+        return hash((self.value, self.width))

tests/cocotb/apb3/smoke/test_register_access.py

@@ -1,15 +1,19 @@
-"""APB3 smoke tests for generated multi-register design."""
+"""APB3 smoke tests generated from SystemRDL sources."""
+
+from __future__ import annotations
+
+import json
+import os
+from typing import Any, Iterable
 
 import cocotb
 from cocotb.triggers import Timer
 
-WRITE_ADDR = 0x0
-READ_ADDR = 0x8
-WRITE_DATA = 0xCAFEBABE
-READ_DATA = 0x0BAD_F00D
-
+from tests.cocotb_lib.handle_utils import SignalHandle, resolve_handle
 
 class _Apb3SlaveShim:
+    """Accessor for the APB3 slave signals on the DUT."""
+
     def __init__(self, dut):
         prefix = "s_apb"
         self.PSEL = getattr(dut, f"{prefix}_PSEL")
@@ -22,102 +26,161 @@ class _Apb3SlaveShim:
         self.PSLVERR = getattr(dut, f"{prefix}_PSLVERR")
 
 
-class _Apb3MasterShim:
-    def __init__(self, dut, base: str):
-        self.PSEL = getattr(dut, f"{base}_PSEL")
-        self.PENABLE = getattr(dut, f"{base}_PENABLE")
-        self.PWRITE = getattr(dut, f"{base}_PWRITE")
-        self.PADDR = getattr(dut, f"{base}_PADDR")
-        self.PWDATA = getattr(dut, f"{base}_PWDATA")
-        self.PRDATA = getattr(dut, f"{base}_PRDATA")
-        self.PREADY = getattr(dut, f"{base}_PREADY")
-        self.PSLVERR = getattr(dut, f"{base}_PSLVERR")
+def _load_config() -> dict[str, Any]:
+    payload = os.environ.get("RDL_TEST_CONFIG")
+    if payload is None:
+        raise RuntimeError("RDL_TEST_CONFIG environment variable was not provided")
+    return json.loads(payload)
 
 
-def _apb3_slave(dut):
-    return getattr(dut, "s_apb", None) or _Apb3SlaveShim(dut)
+def _resolve(handle, indices: Iterable[int]):
+    return resolve_handle(handle, indices)
 
 
-def _apb3_master(dut, base: str):
-    return getattr(dut, base, None) or _Apb3MasterShim(dut, base)
+def _set_value(handle, indices: Iterable[int], value: int) -> None:
+    _resolve(handle, indices).value = value
+
+
+def _get_int(handle, indices: Iterable[int]) -> int:
+    return int(_resolve(handle, indices).value)
+
+
+def _build_master_table(dut, masters_cfg: list[dict[str, Any]]) -> dict[str, dict[str, Any]]:
+    table: dict[str, dict[str, Any]] = {}
+    for master in masters_cfg:
+        prefix = master["port_prefix"]
+        entry = {
+            "indices": [tuple(idx) for idx in master["indices"]] or [tuple()],
+            "outputs": {
+                "PSEL": SignalHandle(dut, f"{prefix}_PSEL"),
+                "PENABLE": SignalHandle(dut, f"{prefix}_PENABLE"),
+                "PWRITE": SignalHandle(dut, f"{prefix}_PWRITE"),
+                "PADDR": SignalHandle(dut, f"{prefix}_PADDR"),
+                "PWDATA": SignalHandle(dut, f"{prefix}_PWDATA"),
+            },
+            "inputs": {
+                "PRDATA": SignalHandle(dut, f"{prefix}_PRDATA"),
+                "PREADY": SignalHandle(dut, f"{prefix}_PREADY"),
+                "PSLVERR": SignalHandle(dut, f"{prefix}_PSLVERR"),
+            },
+        }
+        table[master["inst_name"]] = entry
+    return table
+
+
+def _all_index_pairs(table: dict[str, dict[str, Any]]):
+    for name, entry in table.items():
+        for idx in entry["indices"]:
+            yield name, idx
+
+
+def _write_pattern(address: int, width: int) -> int:
+    mask = (1 << width) - 1
+    return ((address * 0x2041) ^ 0xCAFEBABE) & mask
+
+
+def _read_pattern(address: int, width: int) -> int:
+    mask = (1 << width) - 1
+    return ((address ^ 0x0BAD_F00D) + width) & mask
 
 
 @cocotb.test()
-async def test_apb3_read_write_paths(dut):
-    """Exercise APB3 slave interface and observe master fanout."""
-    s_apb = _apb3_slave(dut)
-    masters = {
-        "reg1": _apb3_master(dut, "m_apb_reg1"),
-        "reg2": _apb3_master(dut, "m_apb_reg2"),
-        "reg3": _apb3_master(dut, "m_apb_reg3"),
-    }
+async def test_apb3_address_decoding(dut) -> None:
+    """Exercise the APB3 slave interface against sampled register addresses."""
+    config = _load_config()
+    slave = _Apb3SlaveShim(dut)
+    masters = _build_master_table(dut, config["masters"])
 
-    s_apb.PSEL.value = 0
-    s_apb.PENABLE.value = 0
-    s_apb.PWRITE.value = 0
-    s_apb.PADDR.value = 0
-    s_apb.PWDATA.value = 0
+    slave.PSEL.value = 0
+    slave.PENABLE.value = 0
+    slave.PWRITE.value = 0
+    slave.PADDR.value = 0
+    slave.PWDATA.value = 0
 
-    for master in masters.values():
-        master.PRDATA.value = 0
-        master.PREADY.value = 0
-        master.PSLVERR.value = 0
+    for master_name, idx in _all_index_pairs(masters):
+        entry = masters[master_name]
+        _set_value(entry["inputs"]["PRDATA"], idx, 0)
+        _set_value(entry["inputs"]["PREADY"], idx, 0)
+        _set_value(entry["inputs"]["PSLVERR"], idx, 0)
 
     await Timer(1, units="ns")
 
-    # Write to reg1
-    masters["reg1"].PREADY.value = 1
-    s_apb.PADDR.value = WRITE_ADDR
-    s_apb.PWDATA.value = WRITE_DATA
-    s_apb.PWRITE.value = 1
-    s_apb.PSEL.value = 1
-    s_apb.PENABLE.value = 1
+    addr_mask = (1 << config["address_width"]) - 1
 
-    await Timer(1, units="ns")
+    for txn in config["transactions"]:
+        master_name = txn["master"]
+        index = tuple(txn["index"])
+        entry = masters[master_name]
 
-    assert int(masters["reg1"].PSEL.value) == 1, "reg1 should be selected for write"
-    assert int(masters["reg1"].PWRITE.value) == 1, "Write should propagate to master"
-    assert int(masters["reg1"].PADDR.value) == WRITE_ADDR, "Address should reach selected master"
-    assert int(masters["reg1"].PWDATA.value) == WRITE_DATA, "Write data should fan out"
+        address = txn["address"] & addr_mask
+        write_data = _write_pattern(address, config["data_width"])
 
-    for name, master in masters.items():
-        if name != "reg1":
-            assert int(master.PSEL.value) == 0, f"{name} must idle during reg1 write"
+        _set_value(entry["inputs"]["PREADY"], index, 1)
+        _set_value(entry["inputs"]["PSLVERR"], index, 0)
 
-    assert int(s_apb.PREADY.value) == 1, "Ready must reflect selected master"
-    assert int(s_apb.PSLVERR.value) == 0, "Write should not signal error"
+        slave.PADDR.value = address
+        slave.PWDATA.value = write_data
+        slave.PWRITE.value = 1
+        slave.PSEL.value = 1
+        slave.PENABLE.value = 1
 
-    s_apb.PSEL.value = 0
-    s_apb.PENABLE.value = 0
-    s_apb.PWRITE.value = 0
-    masters["reg1"].PREADY.value = 0
-    await Timer(1, units="ns")
+        await Timer(1, units="ns")
 
-    # Read from reg3
-    masters["reg3"].PRDATA.value = READ_DATA
-    masters["reg3"].PREADY.value = 1
-    masters["reg3"].PSLVERR.value = 0
+        assert _get_int(entry["outputs"]["PSEL"], index) == 1, f"{master_name} should assert PSEL for write"
+        assert _get_int(entry["outputs"]["PWRITE"], index) == 1, f"{master_name} should see write direction"
+        assert _get_int(entry["outputs"]["PADDR"], index) == address, f"{master_name} must receive write address"
+        assert _get_int(entry["outputs"]["PWDATA"], index) == write_data, f"{master_name} must receive write data"
 
-    s_apb.PADDR.value = READ_ADDR
-    s_apb.PSEL.value = 1
-    s_apb.PENABLE.value = 1
-    s_apb.PWRITE.value = 0
+        for other_name, other_idx in _all_index_pairs(masters):
+            if other_name == master_name and other_idx == index:
+                continue
+            other_entry = masters[other_name]
+            assert (
+                _get_int(other_entry["outputs"]["PSEL"], other_idx) == 0
+            ), f"{other_name}{other_idx} should remain idle during {txn['label']}"
 
-    await Timer(1, units="ns")
+        assert int(slave.PREADY.value) == 1, "Slave ready should mirror selected master"
+        assert int(slave.PSLVERR.value) == 0, "Write should complete without error"
 
-    assert int(masters["reg3"].PSEL.value) == 1, "reg3 should be selected for read"
-    assert int(masters["reg3"].PWRITE.value) == 0, "Read should clear write"
-    assert int(masters["reg3"].PADDR.value) == READ_ADDR, "Address should reach read target"
+        slave.PSEL.value = 0
+        slave.PENABLE.value = 0
+        slave.PWRITE.value = 0
+        _set_value(entry["inputs"]["PREADY"], index, 0)
+        await Timer(1, units="ns")
 
-    for name, master in masters.items():
-        if name != "reg3":
-            assert int(master.PSEL.value) == 0, f"{name} must idle during reg3 read"
+        # ------------------------------------------------------------------
+        # Read phase
+        # ------------------------------------------------------------------
+        read_data = _read_pattern(address, config["data_width"])
+        _set_value(entry["inputs"]["PRDATA"], index, read_data)
+        _set_value(entry["inputs"]["PREADY"], index, 1)
+        _set_value(entry["inputs"]["PSLVERR"], index, 0)
 
-    assert int(s_apb.PRDATA.value) == READ_DATA, "Read data should return to slave"
-    assert int(s_apb.PREADY.value) == 1, "Read should acknowledge"
-    assert int(s_apb.PSLVERR.value) == 0, "Read should not signal error"
+        slave.PADDR.value = address
+        slave.PWRITE.value = 0
+        slave.PSEL.value = 1
+        slave.PENABLE.value = 1
 
-    s_apb.PSEL.value = 0
-    s_apb.PENABLE.value = 0
-    masters["reg3"].PREADY.value = 0
-    await Timer(1, units="ns")
+        await Timer(1, units="ns")
+
+        assert _get_int(entry["outputs"]["PSEL"], index) == 1, f"{master_name} must assert PSEL for read"
+        assert _get_int(entry["outputs"]["PWRITE"], index) == 0, f"{master_name} should clear write during read"
+        assert _get_int(entry["outputs"]["PADDR"], index) == address, f"{master_name} must receive read address"
+
+        for other_name, other_idx in _all_index_pairs(masters):
+            if other_name == master_name and other_idx == index:
+                continue
+            other_entry = masters[other_name]
+            assert (
+                _get_int(other_entry["outputs"]["PSEL"], other_idx) == 0
+            ), f"{other_name}{other_idx} must stay idle during read of {txn['label']}"
+
+        assert int(slave.PRDATA.value) == read_data, "Read data should propagate back to the slave"
+        assert int(slave.PREADY.value) == 1, "Slave ready should acknowledge the read"
+        assert int(slave.PSLVERR.value) == 0, "Read should not signal an error"
+
+        slave.PSEL.value = 0
+        slave.PENABLE.value = 0
+        _set_value(entry["inputs"]["PREADY"], index, 0)
+        _set_value(entry["inputs"]["PRDATA"], index, 0)
+        await Timer(1, units="ns")

tests/cocotb/apb3/smoke/test_runner.py

@@ -1,5 +1,8 @@
-"""Pytest wrapper launching the APB3 cocotb smoke test."""
+"""Pytest wrapper launching the APB3 cocotb smoke tests."""
 
+from __future__ import annotations
+
+import json
 from pathlib import Path
 
 import pytest
@@ -11,20 +14,25 @@ try:  # pragma: no cover - optional dependency shim
 except ImportError:  # pragma: no cover
     from cocotb_tools.runner import get_runner
 
-from tests.cocotb_lib.utils import compile_rdl_and_export, get_verilog_sources
+from tests.cocotb_lib import RDL_CASES
+from tests.cocotb_lib.utils import get_verilog_sources, prepare_cpuif_case
 
 
 @pytest.mark.simulation
 @pytest.mark.verilator
-def test_apb3_smoke(tmp_path: Path) -> None:
-    """Compile the APB3 design and execute the cocotb smoke test."""
+@pytest.mark.parametrize(("rdl_file", "top_name"), RDL_CASES, ids=[case[1] for case in RDL_CASES])
+def test_apb3_smoke(tmp_path: Path, rdl_file: str, top_name: str) -> None:
+    """Compile each APB3 design variant and execute the cocotb smoke test."""
     repo_root = Path(__file__).resolve().parents[4]
+    rdl_path = repo_root / "tests" / "cocotb_lib" / "rdl" / rdl_file
+    build_root = tmp_path / top_name
 
-    module_path, package_path = compile_rdl_and_export(
-        str(repo_root / "tests" / "cocotb_lib" / "multiple_reg.rdl"),
-        "multi_reg",
-        tmp_path,
+    module_path, package_path, config = prepare_cpuif_case(
+        str(rdl_path),
+        top_name,
+        build_root,
         cpuif_cls=APB3CpuifFlat,
+        control_signal="PSEL",
     )
 
     sources = get_verilog_sources(
@@ -34,17 +42,18 @@ def test_apb3_smoke(tmp_path: Path) -> None:
     )
 
     runner = get_runner("verilator")
-    build_dir = tmp_path / "sim_build"
+    sim_build = build_root / "sim_build"
 
     runner.build(
         sources=sources,
         hdl_toplevel=module_path.stem,
-        build_dir=build_dir,
+        build_dir=sim_build,
     )
 
     runner.test(
         hdl_toplevel=module_path.stem,
         test_module="tests.cocotb.apb3.smoke.test_register_access",
-        build_dir=build_dir,
-        log_file=str(tmp_path / "sim.log"),
+        build_dir=sim_build,
+        log_file=str(build_root / "simulation.log"),
+        extra_env={"RDL_TEST_CONFIG": json.dumps(config)},
     )

tests/cocotb/apb3/smoke/test_variable_depth.py

@@ -0,0 +1,211 @@
+"""APB3 smoke tests for variable depth design testing max_decode_depth parameter."""
+
+import cocotb
+from cocotb.triggers import Timer
+
+
+class _Apb3SlaveShim:
+    def __init__(self, dut):
+        prefix = "s_apb"
+        self.PSEL = getattr(dut, f"{prefix}_PSEL")
+        self.PENABLE = getattr(dut, f"{prefix}_PENABLE")
+        self.PWRITE = getattr(dut, f"{prefix}_PWRITE")
+        self.PADDR = getattr(dut, f"{prefix}_PADDR")
+        self.PWDATA = getattr(dut, f"{prefix}_PWDATA")
+        self.PRDATA = getattr(dut, f"{prefix}_PRDATA")
+        self.PREADY = getattr(dut, f"{prefix}_PREADY")
+        self.PSLVERR = getattr(dut, f"{prefix}_PSLVERR")
+
+
+class _Apb3MasterShim:
+    def __init__(self, dut, base: str):
+        self.PSEL = getattr(dut, f"{base}_PSEL")
+        self.PENABLE = getattr(dut, f"{base}_PENABLE")
+        self.PWRITE = getattr(dut, f"{base}_PWRITE")
+        self.PADDR = getattr(dut, f"{base}_PADDR")
+        self.PWDATA = getattr(dut, f"{base}_PWDATA")
+        self.PRDATA = getattr(dut, f"{base}_PRDATA")
+        self.PREADY = getattr(dut, f"{base}_PREADY")
+        self.PSLVERR = getattr(dut, f"{base}_PSLVERR")
+
+
+def _apb3_slave(dut):
+    return getattr(dut, "s_apb", None) or _Apb3SlaveShim(dut)
+
+
+def _apb3_master(dut, base: str):
+    return getattr(dut, base, None) or _Apb3MasterShim(dut, base)
+
+
+@cocotb.test()
+async def test_depth_1(dut):
+    """Test max_decode_depth=1 - should have interface for inner1 only."""
+    s_apb = _apb3_slave(dut)
+    
+    # At depth 1, we should have m_apb_inner1 but not deeper interfaces
+    inner1 = _apb3_master(dut, "m_apb_inner1")
+    
+    # Default slave side inputs
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    s_apb.PWRITE.value = 0
+    s_apb.PADDR.value = 0
+    s_apb.PWDATA.value = 0
+    
+    inner1.PRDATA.value = 0
+    inner1.PREADY.value = 0
+    inner1.PSLVERR.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select inner1)
+    inner1.PREADY.value = 1
+    s_apb.PADDR.value = 0x0
+    s_apb.PWDATA.value = 0x12345678
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(inner1.PSEL.value) == 1, "inner1 must be selected"
+    assert int(inner1.PWRITE.value) == 1, "Write should propagate"
+    assert int(s_apb.PREADY.value) == 1, "Ready should mirror master"
+
+
+@cocotb.test()
+async def test_depth_2(dut):
+    """Test max_decode_depth=2 - should have interfaces for reg1 and inner2."""
+    s_apb = _apb3_slave(dut)
+    
+    # At depth 2, we should have m_apb_reg1 and m_apb_inner2
+    reg1 = _apb3_master(dut, "m_apb_reg1")
+    inner2 = _apb3_master(dut, "m_apb_inner2")
+    
+    # Default slave side inputs
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    s_apb.PWRITE.value = 0
+    s_apb.PADDR.value = 0
+    s_apb.PWDATA.value = 0
+    
+    reg1.PRDATA.value = 0
+    reg1.PREADY.value = 0
+    reg1.PSLVERR.value = 0
+    
+    inner2.PRDATA.value = 0
+    inner2.PREADY.value = 0
+    inner2.PSLVERR.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select reg1)
+    reg1.PREADY.value = 1
+    s_apb.PADDR.value = 0x0
+    s_apb.PWDATA.value = 0xABCDEF01
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg1.PSEL.value) == 1, "reg1 must be selected for address 0x0"
+    assert int(inner2.PSEL.value) == 0, "inner2 should not be selected"
+    
+    # Reset
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    reg1.PREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x10 (should select inner2)
+    inner2.PREADY.value = 1
+    s_apb.PADDR.value = 0x10
+    s_apb.PWDATA.value = 0x23456789
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(inner2.PSEL.value) == 1, "inner2 must be selected for address 0x10"
+    assert int(reg1.PSEL.value) == 0, "reg1 should not be selected"
+
+
+@cocotb.test()
+async def test_depth_0(dut):
+    """Test max_decode_depth=0 - should have interfaces for all leaf registers."""
+    s_apb = _apb3_slave(dut)
+    
+    # At depth 0, we should have all leaf registers: reg1, reg2, reg2b
+    reg1 = _apb3_master(dut, "m_apb_reg1")
+    reg2 = _apb3_master(dut, "m_apb_reg2")
+    reg2b = _apb3_master(dut, "m_apb_reg2b")
+    
+    # Default slave side inputs
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    s_apb.PWRITE.value = 0
+    s_apb.PADDR.value = 0
+    s_apb.PWDATA.value = 0
+    
+    for master in [reg1, reg2, reg2b]:
+        master.PRDATA.value = 0
+        master.PREADY.value = 0
+        master.PSLVERR.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select reg1)
+    reg1.PREADY.value = 1
+    s_apb.PADDR.value = 0x0
+    s_apb.PWDATA.value = 0x11111111
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg1.PSEL.value) == 1, "reg1 must be selected for address 0x0"
+    assert int(reg2.PSEL.value) == 0, "reg2 should not be selected"
+    assert int(reg2b.PSEL.value) == 0, "reg2b should not be selected"
+    
+    # Reset
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    reg1.PREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x10 (should select reg2)
+    reg2.PREADY.value = 1
+    s_apb.PADDR.value = 0x10
+    s_apb.PWDATA.value = 0x22222222
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg2.PSEL.value) == 1, "reg2 must be selected for address 0x10"
+    assert int(reg1.PSEL.value) == 0, "reg1 should not be selected"
+    assert int(reg2b.PSEL.value) == 0, "reg2b should not be selected"
+    
+    # Reset
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    reg2.PREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x14 (should select reg2b)
+    reg2b.PREADY.value = 1
+    s_apb.PADDR.value = 0x14
+    s_apb.PWDATA.value = 0x33333333
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg2b.PSEL.value) == 1, "reg2b must be selected for address 0x14"
+    assert int(reg1.PSEL.value) == 0, "reg1 should not be selected"
+    assert int(reg2.PSEL.value) == 0, "reg2 should not be selected"

tests/cocotb/apb3/smoke/test_variable_depth_runner.py

@@ -0,0 +1,128 @@
+"""Pytest wrapper launching the APB3 cocotb smoke test for variable depth."""
+
+from pathlib import Path
+
+import pytest
+
+from peakrdl_busdecoder.cpuif.apb3.apb3_cpuif_flat import APB3CpuifFlat
+
+try:  # pragma: no cover - optional dependency shim
+    from cocotb.runner import get_runner
+except ImportError:  # pragma: no cover
+    from cocotb_tools.runner import get_runner
+
+from tests.cocotb_lib.utils import compile_rdl_and_export, get_verilog_sources
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_apb3_variable_depth_1(tmp_path: Path) -> None:
+    """Test APB3 design with max_decode_depth=1."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=APB3CpuifFlat,
+        max_decode_depth=1,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "apb3_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.apb3.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth1.log"),
+        testcase="test_depth_1",
+    )
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_apb3_variable_depth_2(tmp_path: Path) -> None:
+    """Test APB3 design with max_decode_depth=2."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=APB3CpuifFlat,
+        max_decode_depth=2,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "apb3_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.apb3.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth2.log"),
+        testcase="test_depth_2",
+    )
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_apb3_variable_depth_0(tmp_path: Path) -> None:
+    """Test APB3 design with max_decode_depth=0 (all levels)."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=APB3CpuifFlat,
+        max_decode_depth=0,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "apb3_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.apb3.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth0.log"),
+        testcase="test_depth_0",
+    )

tests/cocotb/apb4/smoke/test_register_access.py

@@ -1,15 +1,19 @@
-"""APB4 smoke tests using generated multi-register design."""
+"""APB4 smoke tests generated from SystemRDL sources."""
+
+from __future__ import annotations
+
+import json
+import os
+from typing import Any, Iterable
 
 import cocotb
 from cocotb.triggers import Timer
 
-WRITE_ADDR = 0x4
-READ_ADDR = 0x8
-WRITE_DATA = 0x1234_5678
-READ_DATA = 0x89AB_CDEF
-
+from tests.cocotb_lib.handle_utils import SignalHandle, resolve_handle
 
 class _Apb4SlaveShim:
+    """Lightweight accessor for the APB4 slave side of the DUT."""
+
     def __init__(self, dut):
         prefix = "s_apb"
         self.PSEL = getattr(dut, f"{prefix}_PSEL")
@@ -24,115 +28,175 @@ class _Apb4SlaveShim:
         self.PSLVERR = getattr(dut, f"{prefix}_PSLVERR")
 
 
-class _Apb4MasterShim:
-    def __init__(self, dut, base: str):
-        self.PSEL = getattr(dut, f"{base}_PSEL")
-        self.PENABLE = getattr(dut, f"{base}_PENABLE")
-        self.PWRITE = getattr(dut, f"{base}_PWRITE")
-        self.PADDR = getattr(dut, f"{base}_PADDR")
-        self.PPROT = getattr(dut, f"{base}_PPROT")
-        self.PWDATA = getattr(dut, f"{base}_PWDATA")
-        self.PSTRB = getattr(dut, f"{base}_PSTRB")
-        self.PRDATA = getattr(dut, f"{base}_PRDATA")
-        self.PREADY = getattr(dut, f"{base}_PREADY")
-        self.PSLVERR = getattr(dut, f"{base}_PSLVERR")
+def _load_config() -> dict[str, Any]:
+    """Read the JSON payload describing the generated register topology."""
+    payload = os.environ.get("RDL_TEST_CONFIG")
+    if payload is None:
+        raise RuntimeError("RDL_TEST_CONFIG environment variable was not provided")
+    return json.loads(payload)
 
 
-def _apb4_slave(dut):
-    return getattr(dut, "s_apb", None) or _Apb4SlaveShim(dut)
+def _resolve(handle, indices: Iterable[int]):
+    """Index into hierarchical cocotb handles."""
+    return resolve_handle(handle, indices)
 
 
-def _apb4_master(dut, base: str):
-    return getattr(dut, base, None) or _Apb4MasterShim(dut, base)
+def _set_value(handle, indices: Iterable[int], value: int) -> None:
+    _resolve(handle, indices).value = value
+
+
+def _get_int(handle, indices: Iterable[int]) -> int:
+    return int(_resolve(handle, indices).value)
+
+
+def _build_master_table(dut, masters_cfg: list[dict[str, Any]]) -> dict[str, dict[str, Any]]:
+    table: dict[str, dict[str, Any]] = {}
+    for master in masters_cfg:
+        port_prefix = master["port_prefix"]
+        entry = {
+            "port_prefix": port_prefix,
+            "indices": [tuple(idx) for idx in master["indices"]] or [tuple()],
+            "outputs": {
+                "PSEL": SignalHandle(dut, f"{port_prefix}_PSEL"),
+                "PENABLE": SignalHandle(dut, f"{port_prefix}_PENABLE"),
+                "PWRITE": SignalHandle(dut, f"{port_prefix}_PWRITE"),
+                "PADDR": SignalHandle(dut, f"{port_prefix}_PADDR"),
+                "PPROT": SignalHandle(dut, f"{port_prefix}_PPROT"),
+                "PWDATA": SignalHandle(dut, f"{port_prefix}_PWDATA"),
+                "PSTRB": SignalHandle(dut, f"{port_prefix}_PSTRB"),
+            },
+            "inputs": {
+                "PRDATA": SignalHandle(dut, f"{port_prefix}_PRDATA"),
+                "PREADY": SignalHandle(dut, f"{port_prefix}_PREADY"),
+                "PSLVERR": SignalHandle(dut, f"{port_prefix}_PSLVERR"),
+            },
+        }
+        table[master["inst_name"]] = entry
+    return table
+
+
+def _all_index_pairs(table: dict[str, dict[str, Any]]):
+    for name, entry in table.items():
+        for idx in entry["indices"]:
+            yield name, idx
+
+
+def _write_pattern(address: int, width: int) -> int:
+    mask = (1 << width) - 1
+    return ((address * 0x1021) ^ 0x1357_9BDF) & mask
+
+
+def _read_pattern(address: int, width: int) -> int:
+    mask = (1 << width) - 1
+    return ((address ^ 0xDEAD_BEE5) + width) & mask
 
 
 @cocotb.test()
-async def test_apb4_read_write_paths(dut):
-    """Drive APB4 slave signals and observe master activity."""
-    s_apb = _apb4_slave(dut)
-    masters = {
-        "reg1": _apb4_master(dut, "m_apb_reg1"),
-        "reg2": _apb4_master(dut, "m_apb_reg2"),
-        "reg3": _apb4_master(dut, "m_apb_reg3"),
-    }
+async def test_apb4_address_decoding(dut) -> None:
+    """Drive the APB4 slave interface and verify master fanout across all sampled registers."""
+    config = _load_config()
+    slave = _Apb4SlaveShim(dut)
+    masters = _build_master_table(dut, config["masters"])
 
-    # Default slave side inputs
-    s_apb.PSEL.value = 0
-    s_apb.PENABLE.value = 0
-    s_apb.PWRITE.value = 0
-    s_apb.PADDR.value = 0
-    s_apb.PWDATA.value = 0
-    s_apb.PPROT.value = 0
-    s_apb.PSTRB.value = 0
+    slave.PSEL.value = 0
+    slave.PENABLE.value = 0
+    slave.PWRITE.value = 0
+    slave.PADDR.value = 0
+    slave.PPROT.value = 0
+    slave.PWDATA.value = 0
+    slave.PSTRB.value = 0
 
-    for master in masters.values():
-        master.PRDATA.value = 0
-        master.PREADY.value = 0
-        master.PSLVERR.value = 0
+    for master_name, idx in _all_index_pairs(masters):
+        entry = masters[master_name]
+        _set_value(entry["inputs"]["PRDATA"], idx, 0)
+        _set_value(entry["inputs"]["PREADY"], idx, 0)
+        _set_value(entry["inputs"]["PSLVERR"], idx, 0)
 
     await Timer(1, units="ns")
 
-    # ------------------------------------------------------------------
-    # Write transfer to reg2
-    # ------------------------------------------------------------------
-    masters["reg2"].PREADY.value = 1
-    s_apb.PADDR.value = WRITE_ADDR
-    s_apb.PWDATA.value = WRITE_DATA
-    s_apb.PSTRB.value = 0xF
-    s_apb.PPROT.value = 0
-    s_apb.PWRITE.value = 1
-    s_apb.PSEL.value = 1
-    s_apb.PENABLE.value = 1
+    addr_mask = (1 << config["address_width"]) - 1
+    strobe_mask = (1 << config["byte_width"]) - 1
 
-    await Timer(1, units="ns")
+    for txn in config["transactions"]:
+        master_name = txn["master"]
+        index = tuple(txn["index"])
+        entry = masters[master_name]
 
-    assert int(masters["reg2"].PSEL.value) == 1, "reg2 must be selected for write"
-    assert int(masters["reg2"].PWRITE.value) == 1, "Write strobes should propagate"
-    assert int(masters["reg2"].PADDR.value) == WRITE_ADDR, "Address should fan out"
-    assert int(masters["reg2"].PWDATA.value) == WRITE_DATA, "Write data should fan out"
+        address = txn["address"] & addr_mask
+        write_data = _write_pattern(address, config["data_width"])
 
-    for name, master in masters.items():
-        if name != "reg2":
-            assert int(master.PSEL.value) == 0, f"{name} should remain idle on write"
+        # Prime master-side inputs for the write phase
+        _set_value(entry["inputs"]["PREADY"], index, 1)
+        _set_value(entry["inputs"]["PSLVERR"], index, 0)
 
-    assert int(s_apb.PREADY.value) == 1, "Ready should mirror selected master"
-    assert int(s_apb.PSLVERR.value) == 0, "No error expected on successful write"
+        slave.PADDR.value = address
+        slave.PWDATA.value = write_data
+        slave.PSTRB.value = strobe_mask
+        slave.PPROT.value = 0
+        slave.PWRITE.value = 1
+        slave.PSEL.value = 1
+        slave.PENABLE.value = 1
 
-    # Return to idle
-    s_apb.PSEL.value = 0
-    s_apb.PENABLE.value = 0
-    s_apb.PWRITE.value = 0
-    masters["reg2"].PREADY.value = 0
-    await Timer(1, units="ns")
+        await Timer(1, units="ns")
 
-    # ------------------------------------------------------------------
-    # Read transfer from reg3
-    # ------------------------------------------------------------------
-    masters["reg3"].PRDATA.value = READ_DATA
-    masters["reg3"].PREADY.value = 1
-    masters["reg3"].PSLVERR.value = 0
+        assert _get_int(entry["outputs"]["PSEL"], index) == 1, f"{master_name} should assert PSEL for write"
+        assert _get_int(entry["outputs"]["PWRITE"], index) == 1, f"{master_name} should see write intent"
+        assert _get_int(entry["outputs"]["PADDR"], index) == address, f"{master_name} must receive write address"
+        assert _get_int(entry["outputs"]["PWDATA"], index) == write_data, f"{master_name} must receive write data"
+        assert _get_int(entry["outputs"]["PSTRB"], index) == strobe_mask, f"{master_name} must receive full strobes"
 
-    s_apb.PADDR.value = READ_ADDR
-    s_apb.PSEL.value = 1
-    s_apb.PENABLE.value = 1
-    s_apb.PWRITE.value = 0
+        for other_name, other_idx in _all_index_pairs(masters):
+            if other_name == master_name and other_idx == index:
+                continue
+            other_entry = masters[other_name]
+            assert (
+                _get_int(other_entry["outputs"]["PSEL"], other_idx) == 0
+            ), f"{other_name}{other_idx} should remain idle during {txn['label']}"
 
-    await Timer(1, units="ns")
+        assert int(slave.PREADY.value) == 1, "Slave ready should reflect selected master"
+        assert int(slave.PSLVERR.value) == 0, "No error expected during write"
 
-    assert int(masters["reg3"].PSEL.value) == 1, "reg3 must be selected for read"
-    assert int(masters["reg3"].PWRITE.value) == 0, "Read should deassert write"
-    assert int(masters["reg3"].PADDR.value) == READ_ADDR, "Read address should propagate"
+        # Return to idle for next transaction
+        slave.PSEL.value = 0
+        slave.PENABLE.value = 0
+        slave.PWRITE.value = 0
+        _set_value(entry["inputs"]["PREADY"], index, 0)
+        await Timer(1, units="ns")
 
-    for name, master in masters.items():
-        if name != "reg3":
-            assert int(master.PSEL.value) == 0, f"{name} should remain idle on read"
+        # ------------------------------------------------------------------
+        # Read phase
+        # ------------------------------------------------------------------
+        read_data = _read_pattern(address, config["data_width"])
+        _set_value(entry["inputs"]["PRDATA"], index, read_data)
+        _set_value(entry["inputs"]["PREADY"], index, 1)
+        _set_value(entry["inputs"]["PSLVERR"], index, 0)
 
-    assert int(s_apb.PRDATA.value) == READ_DATA, "Read data should return from master"
-    assert int(s_apb.PREADY.value) == 1, "Ready must follow selected master"
-    assert int(s_apb.PSLVERR.value) == 0, "No error expected on successful read"
+        slave.PADDR.value = address
+        slave.PWRITE.value = 0
+        slave.PSEL.value = 1
+        slave.PENABLE.value = 1
 
-    # Back to idle
-    s_apb.PSEL.value = 0
-    s_apb.PENABLE.value = 0
-    masters["reg3"].PREADY.value = 0
-    await Timer(1, units="ns")
+        await Timer(1, units="ns")
+
+        assert _get_int(entry["outputs"]["PSEL"], index) == 1, f"{master_name} must assert PSEL for read"
+        assert _get_int(entry["outputs"]["PWRITE"], index) == 0, f"{master_name} should deassert write for reads"
+        assert _get_int(entry["outputs"]["PADDR"], index) == address, f"{master_name} must receive read address"
+
+        for other_name, other_idx in _all_index_pairs(masters):
+            if other_name == master_name and other_idx == index:
+                continue
+            other_entry = masters[other_name]
+            assert (
+                _get_int(other_entry["outputs"]["PSEL"], other_idx) == 0
+            ), f"{other_name}{other_idx} must stay idle during read of {txn['label']}"
+
+        assert int(slave.PRDATA.value) == read_data, "Slave should observe readback data from master"
+        assert int(slave.PREADY.value) == 1, "Slave ready should follow responding master"
+        assert int(slave.PSLVERR.value) == 0, "Read should complete without error"
+
+        # Reset to idle before progressing
+        slave.PSEL.value = 0
+        slave.PENABLE.value = 0
+        _set_value(entry["inputs"]["PREADY"], index, 0)
+        _set_value(entry["inputs"]["PRDATA"], index, 0)
+        await Timer(1, units="ns")

tests/cocotb/apb4/smoke/test_runner.py

@@ -1,7 +1,10 @@
-"""Pytest wrapper launching the APB4 cocotb smoke test."""
+"""Pytest wrapper launching the APB4 cocotb smoke tests."""
 
+from __future__ import annotations
+
+import json
 from pathlib import Path
-
+import logging
 import pytest
 
 from peakrdl_busdecoder.cpuif.apb4.apb4_cpuif_flat import APB4CpuifFlat
@@ -11,20 +14,25 @@ try:  # pragma: no cover - optional dependency shim
 except ImportError:  # pragma: no cover
     from cocotb_tools.runner import get_runner
 
-from tests.cocotb_lib.utils import compile_rdl_and_export, get_verilog_sources
+from tests.cocotb_lib import RDL_CASES
+from tests.cocotb_lib.utils import get_verilog_sources, prepare_cpuif_case
 
 
 @pytest.mark.simulation
 @pytest.mark.verilator
-def test_apb4_smoke(tmp_path: Path) -> None:
-    """Compile the APB4 design and execute the cocotb smoke test."""
+@pytest.mark.parametrize(("rdl_file", "top_name"), RDL_CASES, ids=[case[1] for case in RDL_CASES])
+def test_apb4_smoke(tmp_path: Path, rdl_file: str, top_name: str) -> None:
+    """Compile each APB4 design variant and execute the cocotb smoke test."""
     repo_root = Path(__file__).resolve().parents[4]
+    rdl_path = repo_root / "tests" / "cocotb_lib" / "rdl" / rdl_file
+    build_root = tmp_path / top_name
 
-    module_path, package_path = compile_rdl_and_export(
-        str(repo_root / "tests" / "cocotb_lib" / "multiple_reg.rdl"),
-        "multi_reg",
-        tmp_path,
+    module_path, package_path, config = prepare_cpuif_case(
+        str(rdl_path),
+        top_name,
+        build_root,
         cpuif_cls=APB4CpuifFlat,
+        control_signal="PSEL",
     )
 
     sources = get_verilog_sources(
@@ -34,17 +42,39 @@ def test_apb4_smoke(tmp_path: Path) -> None:
     )
 
     runner = get_runner("verilator")
-    build_dir = tmp_path / "sim_build"
+    sim_build = build_root / "sim_build"
+    
+    try:
+        runner.build(
+            sources=sources,
+            hdl_toplevel=module_path.stem,
+            build_dir=sim_build,
+            log_file=str(build_root / "build.log"),
+        )
+    except SystemExit as e:
+        # Print build log on failure for easier debugging
+        log_path = build_root / "build.log"
+        if log_path.exists():
+            logging.error("\n\n=== Build Log ===\n")
+            logging.error(log_path.read_text())
+            logging.error("\n=== End Build Log ===\n")
+        if e.code != 0:
+            raise
 
-    runner.build(
-        sources=sources,
-        hdl_toplevel=module_path.stem,
-        build_dir=build_dir,
-    )
-
-    runner.test(
-        hdl_toplevel=module_path.stem,
-        test_module="tests.cocotb.apb4.smoke.test_register_access",
-        build_dir=build_dir,
-        log_file=str(tmp_path / "sim.log"),
-    )
+    try:
+        runner.test(
+            hdl_toplevel=module_path.stem,
+            test_module="tests.cocotb.apb4.smoke.test_register_access",
+            build_dir=sim_build,
+            log_file=str(build_root / "simulation.log"),
+            extra_env={"RDL_TEST_CONFIG": json.dumps(config)},
+        )
+    except SystemExit as e:
+        # Print simulation log on failure for easier debugging
+        log_path = build_root / "simulation.log"
+        if log_path.exists():
+            logging.error("\n\n=== Simulation Log ===\n")
+            logging.error(log_path.read_text())
+            logging.error("\n=== End Simulation Log ===\n")
+        if e.code != 0:
+            raise

tests/cocotb/apb4/smoke/test_variable_depth.py

@@ -0,0 +1,227 @@
+"""APB4 smoke tests for variable depth design testing max_decode_depth parameter."""
+
+import cocotb
+from cocotb.triggers import Timer
+
+
+class _Apb4SlaveShim:
+    def __init__(self, dut):
+        prefix = "s_apb"
+        self.PSEL = getattr(dut, f"{prefix}_PSEL")
+        self.PENABLE = getattr(dut, f"{prefix}_PENABLE")
+        self.PWRITE = getattr(dut, f"{prefix}_PWRITE")
+        self.PADDR = getattr(dut, f"{prefix}_PADDR")
+        self.PPROT = getattr(dut, f"{prefix}_PPROT")
+        self.PWDATA = getattr(dut, f"{prefix}_PWDATA")
+        self.PSTRB = getattr(dut, f"{prefix}_PSTRB")
+        self.PRDATA = getattr(dut, f"{prefix}_PRDATA")
+        self.PREADY = getattr(dut, f"{prefix}_PREADY")
+        self.PSLVERR = getattr(dut, f"{prefix}_PSLVERR")
+
+
+class _Apb4MasterShim:
+    def __init__(self, dut, base: str):
+        self.PSEL = getattr(dut, f"{base}_PSEL")
+        self.PENABLE = getattr(dut, f"{base}_PENABLE")
+        self.PWRITE = getattr(dut, f"{base}_PWRITE")
+        self.PADDR = getattr(dut, f"{base}_PADDR")
+        self.PPROT = getattr(dut, f"{base}_PPROT")
+        self.PWDATA = getattr(dut, f"{base}_PWDATA")
+        self.PSTRB = getattr(dut, f"{base}_PSTRB")
+        self.PRDATA = getattr(dut, f"{base}_PRDATA")
+        self.PREADY = getattr(dut, f"{base}_PREADY")
+        self.PSLVERR = getattr(dut, f"{base}_PSLVERR")
+
+
+def _apb4_slave(dut):
+    return getattr(dut, "s_apb", None) or _Apb4SlaveShim(dut)
+
+
+def _apb4_master(dut, base: str):
+    return getattr(dut, base, None) or _Apb4MasterShim(dut, base)
+
+
+@cocotb.test()
+async def test_depth_1(dut):
+    """Test max_decode_depth=1 - should have interface for inner1 only."""
+    s_apb = _apb4_slave(dut)
+    
+    # At depth 1, we should have m_apb_inner1 but not deeper interfaces
+    inner1 = _apb4_master(dut, "m_apb_inner1")
+    
+    # Default slave side inputs
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    s_apb.PWRITE.value = 0
+    s_apb.PADDR.value = 0
+    s_apb.PWDATA.value = 0
+    s_apb.PPROT.value = 0
+    s_apb.PSTRB.value = 0
+    
+    inner1.PRDATA.value = 0
+    inner1.PREADY.value = 0
+    inner1.PSLVERR.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select inner1)
+    inner1.PREADY.value = 1
+    s_apb.PADDR.value = 0x0
+    s_apb.PWDATA.value = 0x12345678
+    s_apb.PSTRB.value = 0xF
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(inner1.PSEL.value) == 1, "inner1 must be selected"
+    assert int(inner1.PWRITE.value) == 1, "Write should propagate"
+    assert int(s_apb.PREADY.value) == 1, "Ready should mirror master"
+
+
+@cocotb.test()
+async def test_depth_2(dut):
+    """Test max_decode_depth=2 - should have interfaces for reg1 and inner2."""
+    s_apb = _apb4_slave(dut)
+    
+    # At depth 2, we should have m_apb_reg1 and m_apb_inner2
+    reg1 = _apb4_master(dut, "m_apb_reg1")
+    inner2 = _apb4_master(dut, "m_apb_inner2")
+    
+    # Default slave side inputs
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    s_apb.PWRITE.value = 0
+    s_apb.PADDR.value = 0
+    s_apb.PWDATA.value = 0
+    s_apb.PPROT.value = 0
+    s_apb.PSTRB.value = 0
+    
+    reg1.PRDATA.value = 0
+    reg1.PREADY.value = 0
+    reg1.PSLVERR.value = 0
+    
+    inner2.PRDATA.value = 0
+    inner2.PREADY.value = 0
+    inner2.PSLVERR.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select reg1)
+    reg1.PREADY.value = 1
+    s_apb.PADDR.value = 0x0
+    s_apb.PWDATA.value = 0xABCDEF01
+    s_apb.PSTRB.value = 0xF
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg1.PSEL.value) == 1, "reg1 must be selected for address 0x0"
+    assert int(inner2.PSEL.value) == 0, "inner2 should not be selected"
+    
+    # Reset
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    reg1.PREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x10 (should select inner2)
+    inner2.PREADY.value = 1
+    s_apb.PADDR.value = 0x10
+    s_apb.PWDATA.value = 0x23456789
+    s_apb.PSTRB.value = 0xF
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(inner2.PSEL.value) == 1, "inner2 must be selected for address 0x10"
+    assert int(reg1.PSEL.value) == 0, "reg1 should not be selected"
+
+
+@cocotb.test()
+async def test_depth_0(dut):
+    """Test max_decode_depth=0 - should have interfaces for all leaf registers."""
+    s_apb = _apb4_slave(dut)
+    
+    # At depth 0, we should have all leaf registers: reg1, reg2, reg2b
+    reg1 = _apb4_master(dut, "m_apb_reg1")
+    reg2 = _apb4_master(dut, "m_apb_reg2")
+    reg2b = _apb4_master(dut, "m_apb_reg2b")
+    
+    # Default slave side inputs
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    s_apb.PWRITE.value = 0
+    s_apb.PADDR.value = 0
+    s_apb.PWDATA.value = 0
+    s_apb.PPROT.value = 0
+    s_apb.PSTRB.value = 0
+    
+    for master in [reg1, reg2, reg2b]:
+        master.PRDATA.value = 0
+        master.PREADY.value = 0
+        master.PSLVERR.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select reg1)
+    reg1.PREADY.value = 1
+    s_apb.PADDR.value = 0x0
+    s_apb.PWDATA.value = 0x11111111
+    s_apb.PSTRB.value = 0xF
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg1.PSEL.value) == 1, "reg1 must be selected for address 0x0"
+    assert int(reg2.PSEL.value) == 0, "reg2 should not be selected"
+    assert int(reg2b.PSEL.value) == 0, "reg2b should not be selected"
+    
+    # Reset
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    reg1.PREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x10 (should select reg2)
+    reg2.PREADY.value = 1
+    s_apb.PADDR.value = 0x10
+    s_apb.PWDATA.value = 0x22222222
+    s_apb.PSTRB.value = 0xF
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg2.PSEL.value) == 1, "reg2 must be selected for address 0x10"
+    assert int(reg1.PSEL.value) == 0, "reg1 should not be selected"
+    assert int(reg2b.PSEL.value) == 0, "reg2b should not be selected"
+    
+    # Reset
+    s_apb.PSEL.value = 0
+    s_apb.PENABLE.value = 0
+    reg2.PREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x14 (should select reg2b)
+    reg2b.PREADY.value = 1
+    s_apb.PADDR.value = 0x14
+    s_apb.PWDATA.value = 0x33333333
+    s_apb.PSTRB.value = 0xF
+    s_apb.PWRITE.value = 1
+    s_apb.PSEL.value = 1
+    s_apb.PENABLE.value = 1
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg2b.PSEL.value) == 1, "reg2b must be selected for address 0x14"
+    assert int(reg1.PSEL.value) == 0, "reg1 should not be selected"
+    assert int(reg2.PSEL.value) == 0, "reg2 should not be selected"

tests/cocotb/apb4/smoke/test_variable_depth_runner.py

@@ -0,0 +1,131 @@
+"""Pytest wrapper launching the APB4 cocotb smoke test for variable depth."""
+
+from pathlib import Path
+
+import pytest
+
+from peakrdl_busdecoder.cpuif.apb4.apb4_cpuif_flat import APB4CpuifFlat
+
+try:  # pragma: no cover - optional dependency shim
+    from cocotb.runner import get_runner
+except ImportError:  # pragma: no cover
+    from cocotb_tools.runner import get_runner
+
+from tests.cocotb_lib.utils import compile_rdl_and_export, get_verilog_sources
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_apb4_variable_depth_1(tmp_path: Path) -> None:
+    """Test APB4 design with max_decode_depth=1."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=APB4CpuifFlat,
+        max_decode_depth=1,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "apb4_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+        log_file=str(tmp_path / "build_depth_1.log"),
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.apb4.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth1.log"),
+        testcase="test_depth_1",
+    )
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_apb4_variable_depth_2(tmp_path: Path) -> None:
+    """Test APB4 design with max_decode_depth=2."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=APB4CpuifFlat,
+        max_decode_depth=2,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "apb4_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+        log_file=str(tmp_path / "build_depth_2.log"),
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.apb4.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth_2.log"),
+        testcase="test_depth_2",
+    )
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_apb4_variable_depth_0(tmp_path: Path) -> None:
+    """Test APB4 design with max_decode_depth=0 (all levels)."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=APB4CpuifFlat,
+        max_decode_depth=0,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "apb4_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+        log_file=str(tmp_path / "build_depth_0.log"),
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.apb4.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth_0.log"),
+        testcase="test_depth_0",
+    )

tests/cocotb/axi4lite/smoke/test_register_access.py

@@ -1,15 +1,19 @@
-"""AXI4-Lite smoke test ensuring address decode fanout works."""
+"""AXI4-Lite smoke test driven from SystemRDL-generated register maps."""
+
+from __future__ import annotations
+
+import json
+import os
+from typing import Any, Iterable
 
 import cocotb
 from cocotb.triggers import Timer
 
-WRITE_ADDR = 0x4
-READ_ADDR = 0x8
-WRITE_DATA = 0x1357_9BDF
-READ_DATA = 0x2468_ACED
-
+from tests.cocotb_lib.handle_utils import SignalHandle, resolve_handle
 
 class _AxilSlaveShim:
+    """Accessor for AXI4-Lite slave ports on the DUT."""
+
     def __init__(self, dut):
         prefix = "s_axil"
         self.AWREADY = getattr(dut, f"{prefix}_AWREADY")
@@ -33,129 +37,177 @@ class _AxilSlaveShim:
         self.RRESP = getattr(dut, f"{prefix}_RRESP")
 
 
-class _AxilMasterShim:
-    def __init__(self, dut, base: str):
-        self.AWREADY = getattr(dut, f"{base}_AWREADY")
-        self.AWVALID = getattr(dut, f"{base}_AWVALID")
-        self.AWADDR = getattr(dut, f"{base}_AWADDR")
-        self.AWPROT = getattr(dut, f"{base}_AWPROT")
-        self.WREADY = getattr(dut, f"{base}_WREADY")
-        self.WVALID = getattr(dut, f"{base}_WVALID")
-        self.WDATA = getattr(dut, f"{base}_WDATA")
-        self.WSTRB = getattr(dut, f"{base}_WSTRB")
-        self.BREADY = getattr(dut, f"{base}_BREADY")
-        self.BVALID = getattr(dut, f"{base}_BVALID")
-        self.BRESP = getattr(dut, f"{base}_BRESP")
-        self.ARREADY = getattr(dut, f"{base}_ARREADY")
-        self.ARVALID = getattr(dut, f"{base}_ARVALID")
-        self.ARADDR = getattr(dut, f"{base}_ARADDR")
-        self.ARPROT = getattr(dut, f"{base}_ARPROT")
-        self.RREADY = getattr(dut, f"{base}_RREADY")
-        self.RVALID = getattr(dut, f"{base}_RVALID")
-        self.RDATA = getattr(dut, f"{base}_RDATA")
-        self.RRESP = getattr(dut, f"{base}_RRESP")
+def _load_config() -> dict[str, Any]:
+    payload = os.environ.get("RDL_TEST_CONFIG")
+    if payload is None:
+        raise RuntimeError("RDL_TEST_CONFIG environment variable was not provided")
+    return json.loads(payload)
 
 
-def _axil_slave(dut):
-    return getattr(dut, "s_axil", None) or _AxilSlaveShim(dut)
+def _resolve(handle, indices: Iterable[int]):
+    return resolve_handle(handle, indices)
 
 
-def _axil_master(dut, base: str):
-    return getattr(dut, base, None) or _AxilMasterShim(dut, base)
+def _set_value(handle, indices: Iterable[int], value: int) -> None:
+    _resolve(handle, indices).value = value
+
+
+def _get_int(handle, indices: Iterable[int]) -> int:
+    return int(_resolve(handle, indices).value)
+
+
+def _build_master_table(dut, masters_cfg: list[dict[str, Any]]) -> dict[str, dict[str, Any]]:
+    table: dict[str, dict[str, Any]] = {}
+    for master in masters_cfg:
+        prefix = master["port_prefix"]
+        entry = {
+            "indices": [tuple(idx) for idx in master["indices"]] or [tuple()],
+            "outputs": {
+                "AWVALID": SignalHandle(dut, f"{prefix}_AWVALID"),
+                "AWADDR": SignalHandle(dut, f"{prefix}_AWADDR"),
+                "AWPROT": SignalHandle(dut, f"{prefix}_AWPROT"),
+                "WVALID": SignalHandle(dut, f"{prefix}_WVALID"),
+                "WDATA": SignalHandle(dut, f"{prefix}_WDATA"),
+                "WSTRB": SignalHandle(dut, f"{prefix}_WSTRB"),
+                "ARVALID": SignalHandle(dut, f"{prefix}_ARVALID"),
+                "ARADDR": SignalHandle(dut, f"{prefix}_ARADDR"),
+                "ARPROT": SignalHandle(dut, f"{prefix}_ARPROT"),
+            },
+            "inputs": {
+                "AWREADY": SignalHandle(dut, f"{prefix}_AWREADY"),
+                "WREADY": SignalHandle(dut, f"{prefix}_WREADY"),
+                "BVALID": SignalHandle(dut, f"{prefix}_BVALID"),
+                "BRESP": SignalHandle(dut, f"{prefix}_BRESP"),
+                "ARREADY": SignalHandle(dut, f"{prefix}_ARREADY"),
+                "RVALID": SignalHandle(dut, f"{prefix}_RVALID"),
+                "RDATA": SignalHandle(dut, f"{prefix}_RDATA"),
+                "RRESP": SignalHandle(dut, f"{prefix}_RRESP"),
+            },
+        }
+        table[master["inst_name"]] = entry
+    return table
+
+
+def _all_index_pairs(table: dict[str, dict[str, Any]]):
+    for name, entry in table.items():
+        for idx in entry["indices"]:
+            yield name, idx
+
+
+def _write_pattern(address: int, width: int) -> int:
+    mask = (1 << width) - 1
+    return ((address * 0x3105) ^ 0x1357_9BDF) & mask
+
+
+def _read_pattern(address: int, width: int) -> int:
+    mask = (1 << width) - 1
+    return ((address ^ 0x2468_ACED) + width) & mask
 
 
 @cocotb.test()
-async def test_axi4lite_read_write_paths(dut):
-    """Drive AXI4-Lite slave channels and validate master side wiring."""
-    s_axil = _axil_slave(dut)
-    masters = {
-        "reg1": _axil_master(dut, "m_axil_reg1"),
-        "reg2": _axil_master(dut, "m_axil_reg2"),
-        "reg3": _axil_master(dut, "m_axil_reg3"),
-    }
+async def test_axi4lite_address_decoding(dut) -> None:
+    """Stimulate AXI4-Lite slave channels and verify master port selection."""
+    config = _load_config()
+    slave = _AxilSlaveShim(dut)
+    masters = _build_master_table(dut, config["masters"])
 
-    # Default slave-side inputs
-    s_axil.AWVALID.value = 0
-    s_axil.AWADDR.value = 0
-    s_axil.AWPROT.value = 0
-    s_axil.WVALID.value = 0
-    s_axil.WDATA.value = 0
-    s_axil.WSTRB.value = 0
-    s_axil.BREADY.value = 0
-    s_axil.ARVALID.value = 0
-    s_axil.ARADDR.value = 0
-    s_axil.ARPROT.value = 0
-    s_axil.RREADY.value = 0
+    slave.AWVALID.value = 0
+    slave.AWADDR.value = 0
+    slave.AWPROT.value = 0
+    slave.WVALID.value = 0
+    slave.WDATA.value = 0
+    slave.WSTRB.value = 0
+    slave.BREADY.value = 0
+    slave.ARVALID.value = 0
+    slave.ARADDR.value = 0
+    slave.ARPROT.value = 0
+    slave.RREADY.value = 0
 
-    for master in masters.values():
-        master.AWREADY.value = 0
-        master.WREADY.value = 0
-        master.BVALID.value = 0
-        master.BRESP.value = 0
-        master.ARREADY.value = 0
-        master.RVALID.value = 0
-        master.RDATA.value = 0
-        master.RRESP.value = 0
+    for master_name, idx in _all_index_pairs(masters):
+        entry = masters[master_name]
+        _set_value(entry["inputs"]["AWREADY"], idx, 0)
+        _set_value(entry["inputs"]["WREADY"], idx, 0)
+        _set_value(entry["inputs"]["BVALID"], idx, 0)
+        _set_value(entry["inputs"]["BRESP"], idx, 0)
+        _set_value(entry["inputs"]["ARREADY"], idx, 0)
+        _set_value(entry["inputs"]["RVALID"], idx, 0)
+        _set_value(entry["inputs"]["RDATA"], idx, 0)
+        _set_value(entry["inputs"]["RRESP"], idx, 0)
 
     await Timer(1, units="ns")
 
-    # --------------------------------------------------------------
-    # Write transaction targeting reg2
-    # --------------------------------------------------------------
-    s_axil.AWADDR.value = WRITE_ADDR
-    s_axil.AWPROT.value = 0
-    s_axil.AWVALID.value = 1
-    s_axil.WDATA.value = WRITE_DATA
-    s_axil.WSTRB.value = 0xF
-    s_axil.WVALID.value = 1
-    s_axil.BREADY.value = 1
+    addr_mask = (1 << config["address_width"]) - 1
+    strobe_mask = (1 << config["byte_width"]) - 1
 
-    await Timer(1, units="ns")
+    for txn in config["transactions"]:
+        master_name = txn["master"]
+        index = tuple(txn["index"])
+        entry = masters[master_name]
 
-    assert int(masters["reg2"].AWVALID.value) == 1, "reg2 AWVALID should follow slave"
-    assert int(masters["reg2"].WVALID.value) == 1, "reg2 WVALID should follow slave"
-    assert int(masters["reg2"].AWADDR.value) == WRITE_ADDR, "AWADDR should fan out"
-    assert int(masters["reg2"].WDATA.value) == WRITE_DATA, "WDATA should fan out"
-    assert int(masters["reg2"].WSTRB.value) == 0xF, "WSTRB should propagate"
+        address = txn["address"] & addr_mask
+        write_data = _write_pattern(address, config["data_width"])
 
-    for name, master in masters.items():
-        if name != "reg2":
-            assert int(master.AWVALID.value) == 0, f"{name} AWVALID should stay low"
-            assert int(master.WVALID.value) == 0, f"{name} WVALID should stay low"
+        slave.AWADDR.value = address
+        slave.AWPROT.value = 0
+        slave.AWVALID.value = 1
+        slave.WDATA.value = write_data
+        slave.WSTRB.value = strobe_mask
+        slave.WVALID.value = 1
+        slave.BREADY.value = 1
 
-    # Release write channel
-    s_axil.AWVALID.value = 0
-    s_axil.WVALID.value = 0
-    s_axil.BREADY.value = 0
-    await Timer(1, units="ns")
+        await Timer(1, units="ns")
 
-    # --------------------------------------------------------------
-    # Read transaction targeting reg3
-    # --------------------------------------------------------------
-    masters["reg3"].RVALID.value = 1
-    masters["reg3"].RDATA.value = READ_DATA
-    masters["reg3"].RRESP.value = 0
+        assert _get_int(entry["outputs"]["AWVALID"], index) == 1, f"{master_name} should see AWVALID asserted"
+        assert _get_int(entry["outputs"]["AWADDR"], index) == address, f"{master_name} must receive AWADDR"
+        assert _get_int(entry["outputs"]["WVALID"], index) == 1, f"{master_name} should see WVALID asserted"
+        assert _get_int(entry["outputs"]["WDATA"], index) == write_data, f"{master_name} must receive WDATA"
+        assert _get_int(entry["outputs"]["WSTRB"], index) == strobe_mask, f"{master_name} must receive WSTRB"
 
-    s_axil.ARADDR.value = READ_ADDR
-    s_axil.ARPROT.value = 0
-    s_axil.ARVALID.value = 1
-    s_axil.RREADY.value = 1
+        for other_name, other_idx in _all_index_pairs(masters):
+            if other_name == master_name and other_idx == index:
+                continue
+            other_entry = masters[other_name]
+            assert (
+                _get_int(other_entry["outputs"]["AWVALID"], other_idx) == 0
+            ), f"{other_name}{other_idx} AWVALID should remain low during {txn['label']}"
+            assert (
+                _get_int(other_entry["outputs"]["WVALID"], other_idx) == 0
+            ), f"{other_name}{other_idx} WVALID should remain low during {txn['label']}"
 
-    await Timer(1, units="ns")
+        slave.AWVALID.value = 0
+        slave.WVALID.value = 0
+        slave.BREADY.value = 0
+        await Timer(1, units="ns")
 
-    assert int(masters["reg3"].ARVALID.value) == 1, "reg3 ARVALID should follow slave"
-    assert int(masters["reg3"].ARADDR.value) == READ_ADDR, "ARADDR should fan out"
+        read_data = _read_pattern(address, config["data_width"])
+        _set_value(entry["inputs"]["RVALID"], index, 1)
+        _set_value(entry["inputs"]["RDATA"], index, read_data)
+        _set_value(entry["inputs"]["RRESP"], index, 0)
 
-    for name, master in masters.items():
-        if name != "reg3":
-            assert int(master.ARVALID.value) == 0, f"{name} ARVALID should stay low"
+        slave.ARADDR.value = address
+        slave.ARPROT.value = 0
+        slave.ARVALID.value = 1
+        slave.RREADY.value = 1
 
-    assert int(s_axil.RVALID.value) == 1, "Slave should raise RVALID when master responds"
-    assert int(s_axil.RDATA.value) == READ_DATA, "Read data should return to slave"
-    assert int(s_axil.RRESP.value) == 0, "No error expected for read"
+        await Timer(1, units="ns")
 
-    # Return to idle
-    s_axil.ARVALID.value = 0
-    s_axil.RREADY.value = 0
-    masters["reg3"].RVALID.value = 0
-    await Timer(1, units="ns")
+        assert _get_int(entry["outputs"]["ARVALID"], index) == 1, f"{master_name} should assert ARVALID"
+        assert _get_int(entry["outputs"]["ARADDR"], index) == address, f"{master_name} must receive ARADDR"
+
+        for other_name, other_idx in _all_index_pairs(masters):
+            if other_name == master_name and other_idx == index:
+                continue
+            other_entry = masters[other_name]
+            assert (
+                _get_int(other_entry["outputs"]["ARVALID"], other_idx) == 0
+            ), f"{other_name}{other_idx} ARVALID should remain low during read of {txn['label']}"
+
+        assert int(slave.RVALID.value) == 1, "Slave should observe RVALID when master responds"
+        assert int(slave.RDATA.value) == read_data, "Read data must fold back to slave"
+        assert int(slave.RRESP.value) == 0, "Read response should indicate success"
+
+        slave.ARVALID.value = 0
+        slave.RREADY.value = 0
+        _set_value(entry["inputs"]["RVALID"], index, 0)
+        _set_value(entry["inputs"]["RDATA"], index, 0)
+        await Timer(1, units="ns")

tests/cocotb/axi4lite/smoke/test_runner.py

@@ -1,5 +1,8 @@
-"""Pytest wrapper launching the AXI4-Lite cocotb smoke test."""
+"""Pytest wrapper launching the AXI4-Lite cocotb smoke tests."""
 
+from __future__ import annotations
+
+import json
 from pathlib import Path
 
 import pytest
@@ -11,20 +14,25 @@ try:  # pragma: no cover - optional dependency shim
 except ImportError:  # pragma: no cover
     from cocotb_tools.runner import get_runner
 
-from tests.cocotb_lib.utils import compile_rdl_and_export, get_verilog_sources
+from tests.cocotb_lib import RDL_CASES
+from tests.cocotb_lib.utils import get_verilog_sources, prepare_cpuif_case
 
 
 @pytest.mark.simulation
 @pytest.mark.verilator
-def test_axi4lite_smoke(tmp_path: Path) -> None:
-    """Compile the AXI4-Lite design and execute the cocotb smoke test."""
+@pytest.mark.parametrize(("rdl_file", "top_name"), RDL_CASES, ids=[case[1] for case in RDL_CASES])
+def test_axi4lite_smoke(tmp_path: Path, rdl_file: str, top_name: str) -> None:
+    """Compile each AXI4-Lite design variant and execute the cocotb smoke test."""
     repo_root = Path(__file__).resolve().parents[4]
+    rdl_path = repo_root / "tests" / "cocotb_lib" / "rdl" / rdl_file
+    build_root = tmp_path / top_name
 
-    module_path, package_path = compile_rdl_and_export(
-        str(repo_root / "tests" / "cocotb_lib" / "multiple_reg.rdl"),
-        "multi_reg",
-        tmp_path,
+    module_path, package_path, config = prepare_cpuif_case(
+        str(rdl_path),
+        top_name,
+        build_root,
         cpuif_cls=AXI4LiteCpuifFlat,
+        control_signal="AWVALID",
     )
 
     sources = get_verilog_sources(
@@ -34,17 +42,18 @@ def test_axi4lite_smoke(tmp_path: Path) -> None:
     )
 
     runner = get_runner("verilator")
-    build_dir = tmp_path / "sim_build"
+    sim_build = build_root / "sim_build"
 
     runner.build(
         sources=sources,
         hdl_toplevel=module_path.stem,
-        build_dir=build_dir,
+        build_dir=sim_build,
     )
 
     runner.test(
         hdl_toplevel=module_path.stem,
         test_module="tests.cocotb.axi4lite.smoke.test_register_access",
-        build_dir=build_dir,
-        log_file=str(tmp_path / "sim.log"),
+        build_dir=sim_build,
+        log_file=str(build_root / "simulation.log"),
+        extra_env={"RDL_TEST_CONFIG": json.dumps(config)},
     )

tests/cocotb/axi4lite/smoke/test_variable_depth.py

@@ -0,0 +1,271 @@
+"""AXI4-Lite smoke tests for variable depth design testing max_decode_depth parameter."""
+
+import cocotb
+from cocotb.triggers import Timer
+
+
+class _AxilSlaveShim:
+    def __init__(self, dut):
+        prefix = "s_axil"
+        self.AWREADY = getattr(dut, f"{prefix}_AWREADY")
+        self.AWVALID = getattr(dut, f"{prefix}_AWVALID")
+        self.AWADDR = getattr(dut, f"{prefix}_AWADDR")
+        self.AWPROT = getattr(dut, f"{prefix}_AWPROT")
+        self.WREADY = getattr(dut, f"{prefix}_WREADY")
+        self.WVALID = getattr(dut, f"{prefix}_WVALID")
+        self.WDATA = getattr(dut, f"{prefix}_WDATA")
+        self.WSTRB = getattr(dut, f"{prefix}_WSTRB")
+        self.BREADY = getattr(dut, f"{prefix}_BREADY")
+        self.BVALID = getattr(dut, f"{prefix}_BVALID")
+        self.BRESP = getattr(dut, f"{prefix}_BRESP")
+        self.ARREADY = getattr(dut, f"{prefix}_ARREADY")
+        self.ARVALID = getattr(dut, f"{prefix}_ARVALID")
+        self.ARADDR = getattr(dut, f"{prefix}_ARADDR")
+        self.ARPROT = getattr(dut, f"{prefix}_ARPROT")
+        self.RREADY = getattr(dut, f"{prefix}_RREADY")
+        self.RVALID = getattr(dut, f"{prefix}_RVALID")
+        self.RDATA = getattr(dut, f"{prefix}_RDATA")
+        self.RRESP = getattr(dut, f"{prefix}_RRESP")
+
+
+class _AxilMasterShim:
+    def __init__(self, dut, base: str):
+        self.AWREADY = getattr(dut, f"{base}_AWREADY")
+        self.AWVALID = getattr(dut, f"{base}_AWVALID")
+        self.AWADDR = getattr(dut, f"{base}_AWADDR")
+        self.AWPROT = getattr(dut, f"{base}_AWPROT")
+        self.WREADY = getattr(dut, f"{base}_WREADY")
+        self.WVALID = getattr(dut, f"{base}_WVALID")
+        self.WDATA = getattr(dut, f"{base}_WDATA")
+        self.WSTRB = getattr(dut, f"{base}_WSTRB")
+        self.BREADY = getattr(dut, f"{base}_BREADY")
+        self.BVALID = getattr(dut, f"{base}_BVALID")
+        self.BRESP = getattr(dut, f"{base}_BRESP")
+        self.ARREADY = getattr(dut, f"{base}_ARREADY")
+        self.ARVALID = getattr(dut, f"{base}_ARVALID")
+        self.ARADDR = getattr(dut, f"{base}_ARADDR")
+        self.ARPROT = getattr(dut, f"{base}_ARPROT")
+        self.RREADY = getattr(dut, f"{base}_RREADY")
+        self.RVALID = getattr(dut, f"{base}_RVALID")
+        self.RDATA = getattr(dut, f"{base}_RDATA")
+        self.RRESP = getattr(dut, f"{base}_RRESP")
+
+
+def _axil_slave(dut):
+    return getattr(dut, "s_axil", None) or _AxilSlaveShim(dut)
+
+
+def _axil_master(dut, base: str):
+    return getattr(dut, base, None) or _AxilMasterShim(dut, base)
+
+
+@cocotb.test()
+async def test_depth_1(dut):
+    """Test max_decode_depth=1 - should have interface for inner1 only."""
+    s_axil = _axil_slave(dut)
+    
+    # At depth 1, we should have m_axil_inner1 but not deeper interfaces
+    inner1 = _axil_master(dut, "m_axil_inner1")
+    
+    # Default slave side inputs
+    s_axil.AWVALID.value = 0
+    s_axil.AWADDR.value = 0
+    s_axil.AWPROT.value = 0
+    s_axil.WVALID.value = 0
+    s_axil.WDATA.value = 0
+    s_axil.WSTRB.value = 0
+    s_axil.BREADY.value = 0
+    s_axil.ARVALID.value = 0
+    s_axil.ARADDR.value = 0
+    s_axil.ARPROT.value = 0
+    s_axil.RREADY.value = 0
+    
+    inner1.AWREADY.value = 0
+    inner1.WREADY.value = 0
+    inner1.BVALID.value = 0
+    inner1.BRESP.value = 0
+    inner1.ARREADY.value = 0
+    inner1.RVALID.value = 0
+    inner1.RDATA.value = 0
+    inner1.RRESP.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select inner1)
+    inner1.AWREADY.value = 1
+    inner1.WREADY.value = 1
+    s_axil.AWVALID.value = 1
+    s_axil.AWADDR.value = 0x0
+    s_axil.WVALID.value = 1
+    s_axil.WDATA.value = 0x12345678
+    s_axil.WSTRB.value = 0xF
+    
+    await Timer(1, units="ns")
+    
+    assert int(inner1.AWVALID.value) == 1, "inner1 write address valid must be set"
+    assert int(inner1.WVALID.value) == 1, "inner1 write data valid must be set"
+
+
+@cocotb.test()
+async def test_depth_2(dut):
+    """Test max_decode_depth=2 - should have interfaces for reg1 and inner2."""
+    s_axil = _axil_slave(dut)
+    
+    # At depth 2, we should have m_axil_reg1 and m_axil_inner2
+    reg1 = _axil_master(dut, "m_axil_reg1")
+    inner2 = _axil_master(dut, "m_axil_inner2")
+    
+    # Default slave side inputs
+    s_axil.AWVALID.value = 0
+    s_axil.AWADDR.value = 0
+    s_axil.AWPROT.value = 0
+    s_axil.WVALID.value = 0
+    s_axil.WDATA.value = 0
+    s_axil.WSTRB.value = 0
+    s_axil.BREADY.value = 0
+    s_axil.ARVALID.value = 0
+    s_axil.ARADDR.value = 0
+    s_axil.ARPROT.value = 0
+    s_axil.RREADY.value = 0
+    
+    for master in [reg1, inner2]:
+        master.AWREADY.value = 0
+        master.WREADY.value = 0
+        master.BVALID.value = 0
+        master.BRESP.value = 0
+        master.ARREADY.value = 0
+        master.RVALID.value = 0
+        master.RDATA.value = 0
+        master.RRESP.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select reg1)
+    reg1.AWREADY.value = 1
+    reg1.WREADY.value = 1
+    s_axil.AWVALID.value = 1
+    s_axil.AWADDR.value = 0x0
+    s_axil.WVALID.value = 1
+    s_axil.WDATA.value = 0xABCDEF01
+    s_axil.WSTRB.value = 0xF
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg1.AWVALID.value) == 1, "reg1 must be selected for address 0x0"
+    assert int(inner2.AWVALID.value) == 0, "inner2 should not be selected"
+    
+    # Reset
+    s_axil.AWVALID.value = 0
+    s_axil.WVALID.value = 0
+    reg1.AWREADY.value = 0
+    reg1.WREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x10 (should select inner2)
+    inner2.AWREADY.value = 1
+    inner2.WREADY.value = 1
+    s_axil.AWVALID.value = 1
+    s_axil.AWADDR.value = 0x10
+    s_axil.WVALID.value = 1
+    s_axil.WDATA.value = 0x23456789
+    s_axil.WSTRB.value = 0xF
+    
+    await Timer(1, units="ns")
+    
+    assert int(inner2.AWVALID.value) == 1, "inner2 must be selected for address 0x10"
+    assert int(reg1.AWVALID.value) == 0, "reg1 should not be selected"
+
+
+@cocotb.test()
+async def test_depth_0(dut):
+    """Test max_decode_depth=0 - should have interfaces for all leaf registers."""
+    s_axil = _axil_slave(dut)
+    
+    # At depth 0, we should have all leaf registers: reg1, reg2, reg2b
+    reg1 = _axil_master(dut, "m_axil_reg1")
+    reg2 = _axil_master(dut, "m_axil_reg2")
+    reg2b = _axil_master(dut, "m_axil_reg2b")
+    
+    # Default slave side inputs
+    s_axil.AWVALID.value = 0
+    s_axil.AWADDR.value = 0
+    s_axil.AWPROT.value = 0
+    s_axil.WVALID.value = 0
+    s_axil.WDATA.value = 0
+    s_axil.WSTRB.value = 0
+    s_axil.BREADY.value = 0
+    s_axil.ARVALID.value = 0
+    s_axil.ARADDR.value = 0
+    s_axil.ARPROT.value = 0
+    s_axil.RREADY.value = 0
+    
+    for master in [reg1, reg2, reg2b]:
+        master.AWREADY.value = 0
+        master.WREADY.value = 0
+        master.BVALID.value = 0
+        master.BRESP.value = 0
+        master.ARREADY.value = 0
+        master.RVALID.value = 0
+        master.RDATA.value = 0
+        master.RRESP.value = 0
+    
+    await Timer(1, units="ns")
+    
+    # Write to address 0x0 (should select reg1)
+    reg1.AWREADY.value = 1
+    reg1.WREADY.value = 1
+    s_axil.AWVALID.value = 1
+    s_axil.AWADDR.value = 0x0
+    s_axil.WVALID.value = 1
+    s_axil.WDATA.value = 0x11111111
+    s_axil.WSTRB.value = 0xF
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg1.AWVALID.value) == 1, "reg1 must be selected for address 0x0"
+    assert int(reg2.AWVALID.value) == 0, "reg2 should not be selected"
+    assert int(reg2b.AWVALID.value) == 0, "reg2b should not be selected"
+    
+    # Reset
+    s_axil.AWVALID.value = 0
+    s_axil.WVALID.value = 0
+    reg1.AWREADY.value = 0
+    reg1.WREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x10 (should select reg2)
+    reg2.AWREADY.value = 1
+    reg2.WREADY.value = 1
+    s_axil.AWVALID.value = 1
+    s_axil.AWADDR.value = 0x10
+    s_axil.WVALID.value = 1
+    s_axil.WDATA.value = 0x22222222
+    s_axil.WSTRB.value = 0xF
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg2.AWVALID.value) == 1, "reg2 must be selected for address 0x10"
+    assert int(reg1.AWVALID.value) == 0, "reg1 should not be selected"
+    assert int(reg2b.AWVALID.value) == 0, "reg2b should not be selected"
+    
+    # Reset
+    s_axil.AWVALID.value = 0
+    s_axil.WVALID.value = 0
+    reg2.AWREADY.value = 0
+    reg2.WREADY.value = 0
+    await Timer(1, units="ns")
+    
+    # Write to address 0x14 (should select reg2b)
+    reg2b.AWREADY.value = 1
+    reg2b.WREADY.value = 1
+    s_axil.AWVALID.value = 1
+    s_axil.AWADDR.value = 0x14
+    s_axil.WVALID.value = 1
+    s_axil.WDATA.value = 0x33333333
+    s_axil.WSTRB.value = 0xF
+    
+    await Timer(1, units="ns")
+    
+    assert int(reg2b.AWVALID.value) == 1, "reg2b must be selected for address 0x14"
+    assert int(reg1.AWVALID.value) == 0, "reg1 should not be selected"
+    assert int(reg2.AWVALID.value) == 0, "reg2 should not be selected"

tests/cocotb/axi4lite/smoke/test_variable_depth_runner.py

@@ -0,0 +1,128 @@
+"""Pytest wrapper launching the AXI4-Lite cocotb smoke test for variable depth."""
+
+from pathlib import Path
+
+import pytest
+
+from peakrdl_busdecoder.cpuif.axi4lite.axi4_lite_cpuif_flat import AXI4LiteCpuifFlat
+
+try:  # pragma: no cover - optional dependency shim
+    from cocotb.runner import get_runner
+except ImportError:  # pragma: no cover
+    from cocotb_tools.runner import get_runner
+
+from tests.cocotb_lib.utils import compile_rdl_and_export, get_verilog_sources
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_axi4lite_variable_depth_1(tmp_path: Path) -> None:
+    """Test AXI4-Lite design with max_decode_depth=1."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=AXI4LiteCpuifFlat,
+        max_decode_depth=1,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "axi4lite_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.axi4lite.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth1.log"),
+        testcase="test_depth_1",
+    )
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_axi4lite_variable_depth_2(tmp_path: Path) -> None:
+    """Test AXI4-Lite design with max_decode_depth=2."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=AXI4LiteCpuifFlat,
+        max_decode_depth=2,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "axi4lite_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.axi4lite.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth2.log"),
+        testcase="test_depth_2",
+    )
+
+
+@pytest.mark.simulation
+@pytest.mark.verilator
+def test_axi4lite_variable_depth_0(tmp_path: Path) -> None:
+    """Test AXI4-Lite design with max_decode_depth=0 (all levels)."""
+    repo_root = Path(__file__).resolve().parents[4]
+
+    module_path, package_path = compile_rdl_and_export(
+        str(repo_root / "tests" / "cocotb_lib" / "variable_depth.rdl"),
+        "variable_depth",
+        tmp_path,
+        cpuif_cls=AXI4LiteCpuifFlat,
+        max_decode_depth=0,
+    )
+
+    sources = get_verilog_sources(
+        module_path,
+        package_path,
+        [repo_root / "hdl-src" / "axi4lite_intf.sv"],
+    )
+
+    runner = get_runner("verilator")
+    build_dir = tmp_path / "sim_build"
+
+    runner.build(
+        sources=sources,
+        hdl_toplevel=module_path.stem,
+        build_dir=build_dir,
+    )
+
+    runner.test(
+        hdl_toplevel=module_path.stem,
+        test_module="tests.cocotb.axi4lite.smoke.test_variable_depth",
+        build_dir=build_dir,
+        log_file=str(tmp_path / "sim_depth0.log"),
+        testcase="test_depth_0",
+    )

tests/cocotb_lib/__init__.py

@@ -1,3 +1,10 @@
-from pathlib import Path
+"""Manifest of SystemRDL sources used by the cocotb simulations."""
 
-rdls = map(Path, ["simple.rdl", "multiple_reg.rdl"])
+RDL_CASES: list[tuple[str, str]] = [
+    ("simple.rdl", "simple_test"),
+    ("multiple_reg.rdl", "multi_reg"),
+    ("deep_hierarchy.rdl", "deep_hierarchy"),
+    ("wide_status.rdl", "wide_status"),
+    ("variable_layout.rdl", "variable_layout"),
+    ("asymmetric_bus.rdl", "asymmetric_bus"),
+]

tests/cocotb_lib/handle_utils.py

@@ -0,0 +1,69 @@
+"""Utilities for resolving cocotb signal handles across simulators."""
+
+from __future__ import annotations
+
+from typing import Any, Iterable
+
+
+class SignalHandle:
+    """
+    Wrapper that resolves array elements even when the simulator does not expose
+    unpacked arrays through ``handle[idx]``.
+    """
+
+    def __init__(self, dut, name: str) -> None:
+        self._dut = dut
+        self._name = name
+        self._base = getattr(dut, name, None)
+        self._cache: dict[tuple[int, ...], Any] = {}
+
+    def resolve(self, indices: tuple[int, ...]):
+        if not indices:
+            return self._base if self._base is not None else self._lookup(tuple())
+
+        if indices not in self._cache:
+            self._cache[indices] = self._direct_or_lookup(indices)
+        return self._cache[indices]
+
+    def _direct_or_lookup(self, indices: tuple[int, ...]):
+        if self._base is not None:
+            ref = self._base
+            try:
+                for idx in indices:
+                    ref = ref[idx]
+                return ref
+            except (IndexError, TypeError, AttributeError):
+                pass
+
+        return self._lookup(indices)
+
+    def _lookup(self, indices: tuple[int, ...]):
+        suffix = "".join(f"[{idx}]" for idx in indices)
+        path = f"{self._name}{suffix}"
+
+        try:
+            return getattr(self._dut, path)
+        except AttributeError:
+            pass
+
+        errors: list[Exception] = []
+        for extended in (False, True):
+            try:
+                return self._dut._id(path, extended=extended)
+            except (AttributeError, ValueError) as exc:
+                errors.append(exc)
+
+        raise AttributeError(f"Unable to resolve handle '{path}' via dut._id") from errors[-1]
+
+
+def resolve_handle(handle, indices: Iterable[int]):
+    """Resolve either a regular cocotb handle or a ``SignalHandle`` wrapper."""
+    index_tuple = tuple(indices)
+
+    if isinstance(handle, SignalHandle):
+        return handle.resolve(index_tuple)
+
+    ref = handle
+    for idx in index_tuple:
+        ref = ref[idx]
+    return ref

tests/cocotb_lib/rdl/asymmetric_bus.rdl

@@ -0,0 +1,105 @@
+regfile port_rf {
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } port_enable[0:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } port_speed[3:1];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } port_width[8:4];
+    } control @ 0x0;
+
+    reg {
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } error_count[15:0];
+
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } retry_count[31:16];
+    } counters @ 0x4;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } qos[7:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } virtual_channel[9:8];
+    } qos @ 0x8;
+};
+
+addrmap asymmetric_bus {
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } control[3:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } id[15:4];
+    } control @ 0x0;
+
+    reg {
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } status_flags[19:0];
+    } status @ 0x4;
+
+    reg {
+        regwidth = 64;
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x00abcdef;
+        } timestamp_low[31:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x00123456;
+        } timestamp_high[55:32];
+    } timestamp @ 0x8;
+
+    reg {
+        regwidth = 128;
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } extended_id[63:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x1;
+        } parity[64:64];
+    } extended @ 0x10;
+
+    port_rf port[6] @ 0x40 += 0x20;
+};

tests/cocotb_lib/rdl/deep_hierarchy.rdl

@@ -0,0 +1,115 @@
+addrmap deep_hierarchy {
+    regfile context_rf {
+        reg {
+            field {
+                sw = rw;
+                hw = r;
+                reset = 0x1;
+            } enable[7:0];
+
+            field {
+                sw = r;
+                hw = w;
+                onread = rclr;
+                reset = 0x0;
+            } status[15:8];
+
+            field {
+                sw = rw;
+                hw = rw;
+                reset = 0x55;
+            } mode[23:16];
+        } command @ 0x0;
+
+        reg {
+            field {
+                sw = rw;
+                hw = rw;
+                reset = 0x1234;
+            } threshold[15:0];
+        } threshold @ 0x4;
+
+        reg {
+            field {
+                sw = rw;
+                hw = rw;
+                reset = 0x0;
+            } counter[31:0];
+        } counter @ 0x8;
+    };
+
+    regfile engine_rf {
+        context_rf context[3] @ 0x0;
+
+        reg {
+            field {
+                sw = rw;
+                hw = rw;
+                reset = 0x0;
+            } timeout[15:0];
+
+            field {
+                sw = rw;
+                hw = rw;
+                reset = 0x1;
+            } priority[19:16];
+        } config @ 0x30;
+
+        reg {
+            field {
+                sw = r;
+                hw = w;
+                onread = rclr;
+                reset = 0x0;
+            } error[31:0];
+        } error_log @ 0x34;
+    };
+
+    addrmap fabric_slice {
+        engine_rf engines[4] @ 0x0;
+
+        regfile monitor_rf {
+            reg {
+                field {
+                    sw = r;
+                    hw = w;
+                    reset = 0x0;
+                } perf_count[31:0];
+            } perf @ 0x0;
+
+            reg {
+                field {
+                    sw = r;
+                    hw = w;
+                    reset = 0x0;
+                } last_error[31:0];
+            } last_error @ 0x4;
+        };
+
+        monitor_rf monitor @ 0x400;
+
+        reg {
+            field {
+                sw = rw;
+                hw = rw;
+                reset = 0xdeadbeef;
+            } fabric_ctrl[31:0];
+        } fabric_ctrl @ 0x500;
+    };
+
+    fabric_slice slices[2] @ 0x0 += 0x800;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x1;
+        } global_enable[0:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x4;
+        } debug_level[3:1];
+    } global_control @ 0x1000;
+};

tests/cocotb_lib/rdl/variable_layout.rdl

@@ -0,0 +1,156 @@
+reg ctrl_reg_t {
+    desc = "Control register shared across channels.";
+
+    field {
+        sw = rw;
+        hw = rw;
+        reset = 0x1;
+    } enable[0:0];
+
+    field {
+        sw = rw;
+        hw = rw;
+        reset = 0x2;
+    } mode[3:1];
+
+    field {
+        sw = rw;
+        hw = rw;
+        reset = 0x0;
+    } prescale[11:4];
+};
+
+regfile channel_rf {
+    ctrl_reg_t control @ 0x0;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } gain[11:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x200;
+        } offset[23:12];
+    } calibrate @ 0x4;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } sample_count[15:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } error_count[31:16];
+    } counters @ 0x8;
+};
+
+regfile slice_rf {
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } slope[15:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } intercept[31:16];
+    } calibration @ 0x0;
+
+    reg {
+        regwidth = 64;
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } min_val[31:0];
+
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } max_val[63:32];
+    } range @ 0x4;
+};
+
+regfile tile_rf {
+    channel_rf channel[3] @ 0x0;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } tile_mode[1:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } tile_enable[2:2];
+    } tile_ctrl @ 0x100;
+
+    slice_rf slice[2] @ 0x200;
+};
+
+regfile summary_rf {
+    reg {
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } total_errors[31:0];
+    } errors @ 0x0;
+
+    reg {
+        field {
+            sw = r;
+            hw = w;
+            reset = 0x0;
+        } total_samples[31:0];
+    } samples @ 0x4;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } interrupt_enable[7:0];
+    } interrupt_enable @ 0x8;
+};
+
+addrmap variable_layout {
+    tile_rf tiles[2] @ 0x0;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } watchdog_enable[0:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x100;
+        } watchdog_timeout[16:1];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } watchdog_mode[18:17];
+    } watchdog @ 0x2000;
+
+    summary_rf summary @ 0x3000;
+};

tests/cocotb_lib/rdl/wide_status.rdl

@@ -0,0 +1,69 @@
+reg status_reg_t {
+    regwidth = 64;
+    desc = "Status register capturing wide flags and sticky bits.";
+
+    field {
+        sw = r;
+        hw = w;
+        onread = rclr;
+        reset = 0x0;
+    } flags[62:0];
+
+    field {
+        sw = rw;
+        hw = r;
+        reset = 0x1;
+    } sticky_bit[63:63];
+};
+
+reg metrics_reg_t {
+    regwidth = 64;
+    desc = "Metrics register pairing counters with thresholds.";
+
+    field {
+        sw = r;
+        hw = w;
+        reset = 0x0;
+    } count[31:0];
+
+    field {
+        sw = rw;
+        hw = rw;
+        reset = 0x0;
+    } threshold[63:32];
+};
+
+addrmap wide_status {
+    status_reg_t status_blocks[16] @ 0x0;
+
+    metrics_reg_t metrics[4] @ 0x400;
+
+    reg {
+        regwidth = 128;
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } configuration[127:0];
+    } configuration @ 0x800;
+
+    reg {
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0;
+        } version_major[7:0];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x1;
+        } version_minor[15:8];
+
+        field {
+            sw = rw;
+            hw = rw;
+            reset = 0x0100;
+        } build[31:16];
+    } version @ 0x900;
+};

tests/cocotb_lib/utils.py

@@ -1,9 +1,13 @@
 """Common utilities for cocotb testbenches."""
 
+from __future__ import annotations
+
+from collections import defaultdict
 from pathlib import Path
 from typing import Any
 
 from systemrdl import RDLCompiler
+from systemrdl.node import AddressableNode, AddrmapNode, RegNode
 
 from peakrdl_busdecoder.cpuif.base_cpuif import BaseCpuif
 from peakrdl_busdecoder.exporter import BusDecoderExporter
@@ -65,3 +69,206 @@ def get_verilog_sources(module_path: Path, package_path: Path, intf_files: list[
     # Add module file
     sources.append(str(module_path))
     return sources
+
+
+def prepare_cpuif_case(
+    rdl_source: str,
+    top_name: str,
+    output_dir: Path,
+    cpuif_cls: type[BaseCpuif],
+    *,
+    control_signal: str,
+    max_samples_per_master: int = 3,
+    exporter_kwargs: dict[str, Any] | None = None,
+) -> tuple[Path, Path, dict[str, Any]]:
+    """
+    Compile SystemRDL, export the CPUIF, and build a configuration payload for cocotb tests.
+
+    Parameters
+    ----------
+    rdl_source:
+        Path to the SystemRDL source file.
+    top_name:
+        Name of the top-level addrmap to elaborate.
+    output_dir:
+        Directory where generated HDL will be written.
+    cpuif_cls:
+        CPUIF implementation class to use during export.
+    control_signal:
+        Name of the control signal used to identify master ports
+        (``"PSEL"`` for APB, ``"AWVALID"`` for AXI4-Lite, etc.).
+    max_samples_per_master:
+        Limit for the number of register addresses sampled per master in the test matrix.
+    exporter_kwargs:
+        Optional keyword overrides passed through to :class:`BusDecoderExporter`.
+
+    Returns
+    -------
+    tuple
+        ``(module_path, package_path, config_dict)``, where the configuration dictionary
+        is JSON-serializable and describes masters, indices, and sampled transactions.
+    """
+    compiler = RDLCompiler()
+    compiler.compile_file(rdl_source)
+    root = compiler.elaborate(top_name)
+    top_node = root.top  # type: ignore[assignment]
+
+    export_kwargs: dict[str, Any] = {"cpuif_cls": cpuif_cls}
+    if exporter_kwargs:
+        export_kwargs.update(exporter_kwargs)
+
+    exporter = BusDecoderExporter()
+    exporter.export(root, str(output_dir), **export_kwargs)
+
+    module_name = export_kwargs.get("module_name", top_name)
+    package_name = export_kwargs.get("package_name", f"{top_name}_pkg")
+
+    module_path = Path(output_dir) / f"{module_name}.sv"
+    package_path = Path(output_dir) / f"{package_name}.sv"
+
+    config = _build_case_config(
+        top_node,
+        exporter.cpuif,
+        control_signal,
+        max_samples_per_master=max_samples_per_master,
+    )
+
+    config["address_width"] = exporter.cpuif.addr_width
+    config["data_width"] = exporter.cpuif.data_width
+    config["byte_width"] = exporter.cpuif.data_width // 8
+
+    return module_path, package_path, config
+
+
+def _build_case_config(
+    top_node: AddrmapNode,
+    cpuif: BaseCpuif,
+    control_signal: str,
+    *,
+    max_samples_per_master: int,
+) -> dict[str, Any]:
+    master_entries: dict[str, dict[str, Any]] = {}
+
+    for child in cpuif.addressable_children:
+        signal = cpuif.signal(control_signal, child)
+        # Example: m_apb_tiles_PSEL[N_TILESS] -> m_apb_tiles
+        base = signal.split("[", 1)[0]
+        suffix = f"_{control_signal}"
+        if not base.endswith(suffix):
+            raise ValueError(f"Unable to derive port prefix from '{signal}'")
+        port_prefix = base[: -len(suffix)]
+
+        master_entries[child.inst_name] = {
+            "inst_name": child.inst_name,
+            "port_prefix": port_prefix,
+            "is_array": bool(child.is_array),
+            "dimensions": list(child.array_dimensions or []),
+            "indices": set(),
+        }
+
+    # Map each register to its top-level master and collect addresses
+    groups: dict[tuple[str, tuple[int, ...]], list[tuple[int, str]]] = defaultdict(list)
+
+    def visit(node: AddressableNode) -> None:
+        if isinstance(node, RegNode):
+            master = node  # type: AddressableNode
+            while master.parent is not top_node:
+                parent = master.parent
+                if not isinstance(parent, AddressableNode):
+                    raise RuntimeError("Encountered unexpected hierarchy while resolving master node")
+                master = parent
+
+            inst_name = master.inst_name
+            if inst_name not in master_entries:
+                # Handles cases where the register itself is the master (direct child of top)
+                signal = cpuif.signal(control_signal, master)
+                base = signal.split("[", 1)[0]
+                suffix = f"_{control_signal}"
+                if not base.endswith(suffix):
+                    raise ValueError(f"Unable to derive port prefix from '{signal}'")
+                port_prefix = base[: -len(suffix)]
+                master_entries[inst_name] = {
+                    "inst_name": inst_name,
+                    "port_prefix": port_prefix,
+                    "is_array": bool(master.is_array),
+                    "dimensions": list(master.array_dimensions or []),
+                    "indices": set(),
+                }
+
+            idx_tuple = tuple(master.current_idx or [])
+            master_entries[inst_name]["indices"].add(idx_tuple)
+
+            relative_addr = int(node.absolute_address) - int(top_node.absolute_address)
+            full_path = node.get_path()
+            label = full_path.split(".", 1)[1] if "." in full_path else full_path
+            groups[(inst_name, idx_tuple)].append((relative_addr, label))
+
+        for child in node.children(unroll=True):
+            if isinstance(child, AddressableNode):
+                visit(child)
+
+    visit(top_node)
+
+    masters_list = []
+    for entry in master_entries.values():
+        indices = entry["indices"] or {()}
+        entry["indices"] = [list(idx) for idx in sorted(indices)]
+        masters_list.append(
+            {
+                "inst_name": entry["inst_name"],
+                "port_prefix": entry["port_prefix"],
+                "is_array": entry["is_array"],
+                "dimensions": entry["dimensions"],
+                "indices": entry["indices"],
+            }
+        )
+
+    transactions = []
+    for (inst_name, idx_tuple), items in groups.items():
+        addresses = sorted({addr for addr, _ in items})
+        samples = _sample_addresses(addresses, max_samples_per_master)
+        for addr in samples:
+            label = next(lbl for candidate, lbl in items if candidate == addr)
+            transactions.append(
+                {
+                    "address": addr,
+                    "master": inst_name,
+                    "index": list(idx_tuple),
+                    "label": label,
+                }
+            )
+
+    transactions.sort(key=lambda item: (item["master"], item["index"], item["address"]))
+
+    masters_list.sort(key=lambda item: item["inst_name"])
+
+    return {
+        "masters": masters_list,
+        "transactions": transactions,
+    }
+
+
+def _sample_addresses(addresses: list[int], max_samples: int) -> list[int]:
+    if len(addresses) <= max_samples:
+        return addresses
+
+    samples: list[int] = []
+    samples.append(addresses[0])
+    if len(addresses) > 1:
+        samples.append(addresses[-1])
+
+    if len(addresses) > 2:
+        mid = addresses[len(addresses) // 2]
+        if mid not in samples:
+            samples.append(mid)
+
+    idx = 1
+    while len(samples) < max_samples:
+        pos = (len(addresses) * idx) // max_samples
+        candidate = addresses[min(pos, len(addresses) - 1)]
+        if candidate not in samples:
+            samples.append(candidate)
+        idx += 1
+
+    samples.sort()
+    return samples

tests/cocotb_lib/variable_depth.rdl

@@ -0,0 +1,31 @@
+// Variable depth register hierarchy for testing max_decode_depth parameter
+addrmap level2 {
+    reg {
+        field {
+            sw=rw;
+            hw=r;
+        } data2[31:0];
+    } reg2 @ 0x0;
+    
+    reg {
+        field {
+            sw=rw;
+            hw=r;
+        } data2b[31:0];
+    } reg2b @ 0x4;
+};
+
+addrmap level1 {
+    reg {
+        field {
+            sw=rw;
+            hw=r;
+        } data1[31:0];
+    } reg1 @ 0x0;
+    
+    level2 inner2 @ 0x10;
+};
+
+addrmap variable_depth {
+    level1 inner1 @ 0x0;
+};

tests/conftest.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-collect_ignore_glob = ["cocotb/*/smoke/test_register_access.py"]
+collect_ignore_glob = ["cocotb/*/smoke/test_register_access.py", "cocotb/*/smoke/test_variable_depth.py"]
 
 import os
 from collections.abc import Callable

tests/exporter/test_bus_decoder_exporter.py

@@ -89,7 +89,8 @@ class TestBusDecoderExporter:
 
         exporter = BusDecoderExporter()
         output_dir = str(tmp_path)
-        exporter.export(top, output_dir, cpuif_cls=APB4Cpuif)
+        # Use depth=0 to descend all the way to registers
+        exporter.export(top, output_dir, cpuif_cls=APB4Cpuif, max_decode_depth=0)
 
         # Check that output files are created
         module_file = tmp_path / "outer_block.sv"

tests/generator/test_questa_compatibility.py

@@ -0,0 +1,131 @@
+"""Test Questa simulator compatibility for instance arrays."""
+
+from collections.abc import Callable
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+from systemrdl.node import AddrmapNode
+
+from peakrdl_busdecoder import BusDecoderExporter
+from peakrdl_busdecoder.cpuif.apb4 import APB4Cpuif
+
+
+def test_instance_array_questa_compatibility(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that instance arrays generate Questa-compatible code.
+    
+    This test ensures that:
+    - Struct members for arrays use unpacked array syntax (name[dim])
+    - NOT packed bit-vector syntax ([dim-1:0]name)
+    - Struct is unpacked (not packed)
+    - Array indexing with loop variables works correctly
+    
+    This fixes the error: "Nonconstant index into instance array"
+    """
+    rdl_source = """
+    addrmap test_map {
+        reg {
+            field {
+                sw=rw;
+                hw=r;
+            } data[31:0];
+        } my_reg[4] @ 0x0 += 0x10;
+    };
+    """
+    top = compile_rdl(rdl_source, top="test_map")
+    
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif)
+        
+        # Read the generated module
+        module_file = Path(tmpdir) / "test_map.sv"
+        content = module_file.read_text()
+        
+        # Should use unpacked struct
+        assert "typedef struct {" in content
+        assert "typedef struct packed" not in content
+        
+        # Should use unpacked array syntax for array members
+        assert "logic my_reg[4];" in content
+        
+        # Should NOT use packed bit-vector syntax
+        assert "[3:0]my_reg" not in content
+        
+        # Should have proper array indexing in decode logic
+        assert "cpuif_wr_sel.my_reg[i0] = 1'b1;" in content
+        assert "cpuif_rd_sel.my_reg[i0] = 1'b1;" in content
+        
+        # Should have proper array indexing in fanout/fanin logic
+        assert "cpuif_wr_sel.my_reg[gi0]" in content or "cpuif_rd_sel.my_reg[gi0]" in content
+        assert "cpuif_wr_sel.my_reg[i0]" in content or "cpuif_rd_sel.my_reg[i0]" in content
+
+
+def test_multidimensional_array_questa_compatibility(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that multidimensional instance arrays generate Questa-compatible code."""
+    rdl_source = """
+    addrmap test_map {
+        reg {
+            field {
+                sw=rw;
+                hw=r;
+            } data[31:0];
+        } my_reg[2][3] @ 0x0 += 0x10;
+    };
+    """
+    top = compile_rdl(rdl_source, top="test_map")
+    
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif)
+        
+        # Read the generated module
+        module_file = Path(tmpdir) / "test_map.sv"
+        content = module_file.read_text()
+        
+        # Should use unpacked struct with multidimensional array
+        assert "typedef struct {" in content
+        
+        # Should use unpacked array syntax for multidimensional arrays
+        assert "logic my_reg[2][3];" in content
+        
+        # Should NOT use packed bit-vector syntax
+        assert "[1:0][2:0]my_reg" not in content
+        assert "[5:0]my_reg" not in content
+
+
+def test_nested_instance_array_questa_compatibility(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that nested instance arrays generate Questa-compatible code."""
+    rdl_source = """
+    addrmap inner_map {
+        reg {
+            field {
+                sw=rw;
+                hw=r;
+            } data[31:0];
+        } inner_reg[2] @ 0x0 += 0x10;
+    };
+    
+    addrmap outer_map {
+        inner_map inner[3] @ 0x0 += 0x100;
+    };
+    """
+    top = compile_rdl(rdl_source, top="outer_map")
+    
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif)
+        
+        # Read the generated module
+        module_file = Path(tmpdir) / "outer_map.sv"
+        content = module_file.read_text()
+        
+        # Should use unpacked struct
+        assert "typedef struct {" in content
+        
+        # Inner should be an array
+        # The exact syntax may vary, but it should be unpacked
+        # Look for the pattern of unpacked arrays, not packed bit-vectors
+        assert "inner[3]" in content or "logic inner" in content
+        
+        # Should NOT use packed bit-vector syntax like [2:0]inner
+        assert "[2:0]inner" not in content

tests/generator/test_struct_generator.py

@@ -96,3 +96,10 @@ class TestStructGenerator:
         assert "[" in result and "]" in result
         # Should reference the register
         assert "my_regs" in result
+        # Should use unpacked array syntax (name[size]), not packed bit-vector ([size:0]name)
+        assert "my_regs[4]" in result
+        # Should NOT use packed bit-vector syntax
+        assert "[3:0]my_regs" not in result
+        # Should be unpacked struct, not packed
+        assert "typedef struct {" in result
+        assert "typedef struct packed" not in result

tests/unit/test_external_nested.py

@@ -41,9 +41,9 @@ def test_external_nested_components_generate_correct_decoder(external_nested_rdl
         assert "cpuif_rd_sel.multicast.response" not in content
         
         # Verify struct is flat (no nested structs for external children)
-        assert "typedef struct packed" in content
+        assert "typedef struct" in content
         assert "logic multicast;" in content
-        assert "logic [15:0]port;" in content
+        assert "logic port[16];" in content
 
 
 def test_external_nested_components_generate_correct_interfaces(external_nested_rdl: AddrmapNode) -> None:
@@ -101,7 +101,8 @@ def test_non_external_nested_components_are_descended(compile_rdl: Callable[...,
     
     with TemporaryDirectory() as tmpdir:
         exporter = BusDecoderExporter()
-        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif)
+        # Use depth=0 to descend all the way down to registers
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=0)
         
         # Read the generated module
         module_file = Path(tmpdir) / "outer_block.sv"

tests/unit/test_max_decode_depth.py

@@ -0,0 +1,256 @@
+"""Test max_decode_depth parameter behavior."""
+
+from collections.abc import Callable
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+from systemrdl.node import AddrmapNode
+
+from peakrdl_busdecoder import BusDecoderExporter
+from peakrdl_busdecoder.cpuif.apb4 import APB4Cpuif
+
+
+def test_depth_1_generates_top_level_interface_only(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that depth=1 generates interface only for top-level children."""
+    rdl_source = """
+    addrmap level1 {
+        reg {
+            field { sw=rw; hw=r; } data1[31:0];
+        } reg1 @ 0x0;
+    };
+
+    addrmap level0 {
+        level1 inner1 @ 0x0;
+    };
+    """
+    top = compile_rdl(rdl_source, top="level0")
+
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=1)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        # Should have interface for inner1 only
+        assert "m_apb_inner1" in content
+        # Should NOT have interface for reg1
+        assert "m_apb_reg1" not in content
+        
+        # Struct should have inner1 but not nested structure
+        assert "logic inner1;" in content
+
+
+def test_depth_2_generates_second_level_interfaces(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that depth=2 generates interfaces for second-level children."""
+    rdl_source = """
+    addrmap level2 {
+        reg {
+            field { sw=rw; hw=r; } data2[31:0];
+        } reg2 @ 0x0;
+    };
+
+    addrmap level1 {
+        reg {
+            field { sw=rw; hw=r; } data1[31:0];
+        } reg1 @ 0x0;
+        
+        level2 inner2 @ 0x10;
+    };
+
+    addrmap level0 {
+        level1 inner1 @ 0x0;
+    };
+    """
+    top = compile_rdl(rdl_source, top="level0")
+
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=2)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        # Should have interfaces for reg1 and inner2
+        assert "m_apb_reg1" in content
+        assert "m_apb_inner2" in content
+        # Should NOT have interface for inner1 or reg2
+        assert "m_apb_inner1" not in content
+        assert "m_apb_reg2" not in content
+        
+        # Struct should be hierarchical with inner1.reg1 and inner1.inner2
+        assert "cpuif_sel_inner1_t" in content
+        assert "logic reg1;" in content
+        assert "logic inner2;" in content
+
+
+def test_depth_0_decodes_all_levels(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that depth=0 decodes all the way down to registers."""
+    rdl_source = """
+    addrmap level2 {
+        reg {
+            field { sw=rw; hw=r; } data2[31:0];
+        } reg2 @ 0x0;
+        
+        reg {
+            field { sw=rw; hw=r; } data2b[31:0];
+        } reg2b @ 0x4;
+    };
+
+    addrmap level1 {
+        reg {
+            field { sw=rw; hw=r; } data1[31:0];
+        } reg1 @ 0x0;
+        
+        level2 inner2 @ 0x10;
+    };
+
+    addrmap level0 {
+        level1 inner1 @ 0x0;
+    };
+    """
+    top = compile_rdl(rdl_source, top="level0")
+
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=0)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        # Should have interfaces for all leaf registers
+        assert "m_apb_reg1" in content
+        assert "m_apb_reg2" in content
+        assert "m_apb_reg2b" in content
+        # Should NOT have interfaces for addrmaps
+        assert "m_apb_inner1" not in content
+        assert "m_apb_inner2" not in content
+        
+        # Struct should be fully hierarchical
+        assert "cpuif_sel_inner1_t" in content
+        assert "cpuif_sel_inner2_t" in content
+
+
+def test_depth_affects_decode_logic(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that decode logic changes based on max_decode_depth."""
+    rdl_source = """
+    addrmap level1 {
+        reg {
+            field { sw=rw; hw=r; } data1[31:0];
+        } reg1 @ 0x0;
+    };
+
+    addrmap level0 {
+        level1 inner1 @ 0x0;
+    };
+    """
+    top = compile_rdl(rdl_source, top="level0")
+
+    # Test depth=1: should set cpuif_wr_sel.inner1
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=1)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        assert "cpuif_wr_sel.inner1 = 1'b1;" in content
+        assert "cpuif_wr_sel.inner1.reg1" not in content
+
+    # Test depth=2: should set cpuif_wr_sel.inner1.reg1
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=2)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        assert "cpuif_wr_sel.inner1.reg1 = 1'b1;" in content
+        assert "cpuif_wr_sel.inner1 = 1'b1;" not in content
+
+
+def test_depth_affects_fanout_fanin(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test that fanout/fanin logic changes based on max_decode_depth."""
+    rdl_source = """
+    addrmap level1 {
+        reg {
+            field { sw=rw; hw=r; } data1[31:0];
+        } reg1 @ 0x0;
+    };
+
+    addrmap level0 {
+        level1 inner1 @ 0x0;
+    };
+    """
+    top = compile_rdl(rdl_source, top="level0")
+
+    # Test depth=1: should have fanout for inner1
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=1)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        assert "m_apb_inner1.PSEL" in content
+        assert "m_apb_reg1.PSEL" not in content
+
+    # Test depth=2: should have fanout for reg1
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=2)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        assert "m_apb_reg1.PSEL" in content
+        assert "m_apb_inner1.PSEL" not in content
+
+
+def test_depth_3_with_deep_hierarchy(compile_rdl: Callable[..., AddrmapNode]) -> None:
+    """Test depth=3 with a 4-level deep hierarchy."""
+    rdl_source = """
+    addrmap level3 {
+        reg {
+            field { sw=rw; hw=r; } data3[31:0];
+        } reg3 @ 0x0;
+    };
+
+    addrmap level2 {
+        reg {
+            field { sw=rw; hw=r; } data2[31:0];
+        } reg2 @ 0x0;
+        
+        level3 inner3 @ 0x10;
+    };
+
+    addrmap level1 {
+        reg {
+            field { sw=rw; hw=r; } data1[31:0];
+        } reg1 @ 0x0;
+        
+        level2 inner2 @ 0x10;
+    };
+
+    addrmap level0 {
+        level1 inner1 @ 0x0;
+    };
+    """
+    top = compile_rdl(rdl_source, top="level0")
+
+    with TemporaryDirectory() as tmpdir:
+        exporter = BusDecoderExporter()
+        exporter.export(top, tmpdir, cpuif_cls=APB4Cpuif, max_decode_depth=3)
+
+        module_file = Path(tmpdir) / "level0.sv"
+        content = module_file.read_text()
+
+        # Should have interfaces at depth 3: reg2, inner3
+        # (reg1 is at depth 2, not 3)
+        assert "m_apb_reg2" in content
+        assert "m_apb_inner3" in content
+        # Should NOT have interfaces at other depths
+        assert "m_apb_inner1" not in content
+        assert "m_apb_inner2" not in content
+        assert "m_apb_reg1" not in content
+        assert "m_apb_reg3" not in content

tools/shims/xargs

@@ -9,7 +9,6 @@ invocation constructed from stdin tokens.
 
 from __future__ import annotations
 
-import os
 import subprocess
 import sys
 

uv.lock

@@ -608,7 +608,7 @@ wheels = [
 
 [[package]]
 name = "peakrdl-busdecoder"
-version = "0.2.0"
+version = "0.5.0"
 source = { editable = "." }
 dependencies = [
     { name = "jinja2" },