Add support for wide registers (where accesswidth < regwidth)

docs/limitations.rst

@@ -18,18 +18,41 @@ Registers instantiated using the ``alias`` keyword are not supported yet.
 
 Unaligned Registers
 -------------------
-All address offsets & strides shall be a multiple of the accesswidth used. Specifically:
+All address offsets & strides shall be a multiple of the cpuif bus width used. Specifically:
 
-* Each register's address and array stride shall be aligned to it's accesswidth.
-* Each regfile or addrmap shall use an offset and stride that is a multiple of the largest accesswidth it encloses.
+* Bus width is inferred by the maximum accesswidth used in the regblock.
+* Each component's address and array stride shall be aligned to the bus width.
 
 
-Register width, Access width and CPUIF bus width
-------------------------------------------------
-To keep the initial architecture simpler, currently ``regwidth``, ``accesswidth``
-and the resulting CPU bus width has some limitations:
+Uniform accesswidth
+-------------------
+All registers within a register block shall use the same accesswidth.
 
-* All registers shall have ``regwidth`` == ``accesswidth``
-* ``regwidth`` shall be the same across all registers within the block being exported.
+One exception is that registers with regwidth that is narrower than the cpuif
+bus width are permitted, provided that their regwidth is equal to their accesswidth.
 
-I have plans to remove these restrictions and allow for more flexibility in the future.
+For example:
+
+.. code-block:: systemrdl
+
+    // (Largest accesswidth used is 32, therefore the CPUIF bus width is 32)
+
+    reg {
+        regwidth = 32;
+        accesswidth = 32;
+    } reg_a @ 0x00; // OK. Regular 32-bit register
+
+    reg {
+        regwidth = 64;
+        accesswidth = 32;
+    } reg_b @ 0x08; // OK. "Wide" register of 64-bits, but is accessed using 32-bit subwords
+
+    reg {
+        regwidth = 8;
+        accesswidth = 8;
+    } reg_c @ 0x10; // OK. Is aligned to the cpuif bus width
+
+    reg {
+        regwidth = 32;
+        accesswidth = 8;
+    } bad_reg @ 0x14; // NOT OK. accesswidth conflicts with cpuif width