diff --git a/hw/efinix_fpga/simulation/tbs/mapper_tb.sv b/hw/efinix_fpga/simulation/tbs/mapper_tb.sv
index 0f82882..f413529 100644
--- a/hw/efinix_fpga/simulation/tbs/mapper_tb.sv
+++ b/hw/efinix_fpga/simulation/tbs/mapper_tb.sv
@@ -63,6 +63,8 @@ int errors;
 
 int rnd_values [16];
 
+int rnd_addr;
+
 initial begin
     for (int i = 0; i < 16; i++) begin
         rnd_values[i] = $urandom();
@@ -95,6 +97,17 @@ initial begin
         end
     end
 
+    for (int i = 0; i < 16; i++) begin
+        rnd_addr = $urandom();
+        addr = i << 12 | rnd_addr[11:0];
+        #1 // Neccesary for this assertion to work
+        assert (map_addr == {rnd_values[i][12:0], rnd_addr[11:0]}) else begin
+            $error("Expected %x got %x", {rnd_values[i][12:0], rnd_addr[11:0]}, map_addr);
+        end
+
+        @(posedge r_clk_cpu);
+    end
+
     if (errors != 0) begin
         $finish_and_return(-1);
     end else begin
diff --git a/hw/efinix_fpga/src/mapper.sv b/hw/efinix_fpga/src/mapper.sv
index 83f7a07..8197032 100644
--- a/hw/efinix_fpga/src/mapper.sv
+++ b/hw/efinix_fpga/src/mapper.sv
@@ -13,12 +13,20 @@ logic [15:0] mm [16];
 
 logic [31:0] we;
 
+
+// TODO These have basically the same name.
 logic [15:0] mm_sel;
 
+logic [15:0] selected_mm;
+
 always_comb begin
     we = (i_we << i_cpu_addr[4:0]);
 
+    mm_sel = (1 << i_cpu_addr[4:1]);
     o_data = mm_sel[8*i_cpu_addr[0] +: 8];
+
+    selected_mm = mm[i_cpu_addr[15:12]];
+    o_mapped_addr = {selected_mm[12:0], i_cpu_addr[11:0]};
 end
 
 always_ff @(negedge i_clk or posedge i_reset) begin