Add CDB arbiter module and integrate into tomasulo_wrapper

  Implement priority-based CDB arbiter (Week 8) that selects one FU
  completion per cycle for broadcast to ROB and all RS instances.
  Priority order is longest-latency-first: FP_DIV > DIV > FP_MUL >
  MUL > FP_ADD > MEM > ALU.

  - New RTL: cdb_arbiter.sv (combinational fixed-priority encoder)
    with inline formal assertions and cover properties
  - New cocotb tests: 16 unit tests for priority, propagation, stress
  - Wrapper integration: replace i_cdb_write/i_cdb ports with
    i_fu_complete[NumFus]/o_cdb_grant/o_cdb, instantiate arbiter
    internally wired to ROB and all 6 RS instances
  - Update wrapper test infrastructure with fu_complete interface
    and backward-compat aliases for existing CDB helpers
  - Fix two wrapper tests that used non-allocated ROB tags for CDB
    broadcasts (now allocate proper producer entries first)

Author: adambagley

formal/cdb_arbiter.sby

@@ -0,0 +1,21 @@
+[tasks]
+bmc
+cover
+
+[options]
+bmc: mode bmc
+bmc: depth 4
+cover: mode cover
+cover: depth 8
+
+[engines]
+smtbmc boolector
+
+[script]
+read -formal -sv riscv_pkg.sv
+read -formal -sv cdb_arbiter.sv
+prep -top cdb_arbiter
+
+[files]
+../hw/rtl/cpu_and_mem/cpu/riscv_pkg.sv
+../hw/rtl/cpu_and_mem/cpu/tomasulo/cdb_arbiter/cdb_arbiter.sv

formal/tomasulo_wrapper.sby

@@ -18,6 +18,7 @@ read -formal -sv mwp_dist_ram.sv
 read -sv reorder_buffer.sv
 read -sv register_alias_table.sv
 read -sv reservation_station.sv
+read -sv cdb_arbiter.sv
 read -formal -sv tomasulo_wrapper.sv
 prep -top tomasulo_wrapper
 
@@ -28,4 +29,5 @@ prep -top tomasulo_wrapper
 ../hw/rtl/cpu_and_mem/cpu/tomasulo/reorder_buffer/reorder_buffer.sv
 ../hw/rtl/cpu_and_mem/cpu/tomasulo/register_alias_table/register_alias_table.sv
 ../hw/rtl/cpu_and_mem/cpu/tomasulo/reservation_station/reservation_station.sv
+../hw/rtl/cpu_and_mem/cpu/tomasulo/cdb_arbiter/cdb_arbiter.sv
 ../hw/rtl/cpu_and_mem/cpu/tomasulo/tomasulo_wrapper/tomasulo_wrapper.sv

hw/rtl/cpu_and_mem/cpu/tomasulo/cdb_arbiter/README.md

@@ -0,0 +1,42 @@
+# CDB Arbiter
+
+Priority-based multiplexer that selects one functional unit completion per cycle
+for broadcast on the Common Data Bus (CDB).
+
+## Overview
+
+The CDB arbiter receives completion requests from all 7 functional units and
+grants the CDB to exactly one per cycle using fixed-priority arbitration.
+Longer-latency FUs get higher priority to avoid further stalling.
+
+## Interface
+
+| Port | Dir | Width | Description |
+|------|-----|-------|-------------|
+| `i_clk` | in | 1 | Clock (for formal only) |
+| `i_rst_n` | in | 1 | Reset (for formal only) |
+| `i_fu_complete[7]` | in | 7×81 | FU completion requests |
+| `o_cdb` | out | 84 | CDB broadcast (to RS + ROB) |
+| `o_grant[6:0]` | out | 7 | Per-FU grant signals |
+
+## Priority Order
+
+| Priority | FU Type | Latency |
+|----------|---------|---------|
+| 1 (highest) | FP_DIV | ~32-35 cycles |
+| 2 | DIV | 17 cycles |
+| 3 | FP_MUL | ~8-9 cycles |
+| 4 | MUL | 4 cycles |
+| 5 | FP_ADD | ~4-5 cycles |
+| 6 | MEM | variable |
+| 7 (lowest) | ALU | combinational |
+
+## Verification
+
+- **Formal**: BMC (depth 4) + cover (depth 8) via `formal/cdb_arbiter.sby`
+- **Cocotb**: 16 unit tests in `verif/cocotb_tests/tomasulo/cdb_arbiter/`
+
+## Files
+
+- `cdb_arbiter.sv` — RTL + formal assertions
+- `cdb_arbiter.f` — filelist

hw/rtl/cpu_and_mem/cpu/tomasulo/cdb_arbiter/cdb_arbiter.f

@@ -0,0 +1,8 @@
+# CDB Arbiter file list
+# Priority-based mux for FU completion → CDB broadcast
+
+# Package dependency
+$(ROOT)/hw/rtl/cpu_and_mem/cpu/riscv_pkg.sv
+
+# Module
+$(ROOT)/hw/rtl/cpu_and_mem/cpu/tomasulo/cdb_arbiter/cdb_arbiter.sv