refactor(core): address WB stage architectural and naming review feedback

- Renamed `wb_reg_write_data` and `wb_reg_write_addr` to `wb_rd_wdata` and `wb_rd_addr` respectively to comply with `docs/naming_convention.md`.
- Removed the 64-bit adder for `mem_wb_pc + 64'd4` from the WB stage to improve timing. Introduced a new `mem_wb_pc_plus_4` pipeline input instead.
- Added a `has_trap` signal to gate architectural state updates (`wb_reg_write_en` and `csr_req`) upon exceptions, ensuring precise exceptions without corrupting the register file or CSRs.

Co-authored-by: Richard-Qin-001 <69390919+Richard-Qin-001@users.noreply.github.com>

1 files changed, 17 insertions, 12 deletions

diff --git a/rv64cpu/rtl/core/wb_stage.sv b/rv64cpu/rtl/core/wb_stage.sv
index 13dd417..26c9c7b 100644
--- a/rv64cpu/rtl/core/wb_stage.sv
+++ b/rv64cpu/rtl/core/wb_stage.sv
@@ -25,6 +25,7 @@ module wb_stage
     input  logic [XLEN-1:0]        mem_wb_alu_result,
     input  logic [XLEN-1:0]        mem_wb_mem_rdata,
     input  logic [XLEN-1:0]        mem_wb_pc,
+    input  logic [XLEN-1:0]        mem_wb_pc_plus_4,
     
     input  logic                   mem_wb_reg_write_en,
     input  logic [4:0]             mem_wb_rd_addr,
@@ -46,8 +47,8 @@ module wb_stage
     input  logic [XLEN-1:0]        csr_rdata,         // Data read from CSR unit
 
     // Outputs to Register File
-    output logic [XLEN-1:0]        wb_reg_write_data,
-    output logic [4:0]             wb_reg_write_addr,
+    output logic [XLEN-1:0]        wb_rd_wdata,
+    output logic [4:0]             wb_rd_addr,
     output logic                   wb_reg_write_en,
 
     // Outputs to CSR Unit
@@ -66,9 +67,15 @@ module wb_stage
 );
 
     // --------------------------
+    // Exception/Trap Detection
+    // --------------------------
+    logic has_trap;
+    assign has_trap = mem_wb_illegal_instr | mem_wb_is_ecall | mem_wb_is_ebreak;
+
+    // --------------------------
     // CSR Signal Routing
     // --------------------------
-    assign csr_req  = mem_wb_is_csr;
+    assign csr_req  = mem_wb_is_csr & ~has_trap;
     assign csr_op   = mem_wb_csr_op;
     assign csr_addr = mem_wb_imm[11:0];
     
@@ -83,13 +90,13 @@ module wb_stage
     always_comb begin
         if (mem_wb_is_csr) begin
             // CSR reads write the previous CSR value to the destination register
-            wb_reg_write_data = csr_rdata;
+            wb_rd_wdata = csr_rdata;
         end else begin
             case (mem_wb_wb_sel)
-                2'b00: wb_reg_write_data = mem_wb_alu_result;
-                2'b01: wb_reg_write_data = mem_wb_mem_rdata;
-                2'b10: wb_reg_write_data = mem_wb_pc + 64'd4;
-                default: wb_reg_write_data = '0;
+                2'b00: wb_rd_wdata = mem_wb_alu_result;
+                2'b01: wb_rd_wdata = mem_wb_mem_rdata;
+                2'b10: wb_rd_wdata = mem_wb_pc_plus_4;
+                default: wb_rd_wdata = '0;
             endcase
         end
     end
@@ -98,10 +105,8 @@ module wb_stage
     // Register File Write Control
     // --------------------------
     // We only write back if the instruction didn't trap.
-    // For now, assume pure write-enable propagation.
-    // A future trap controller might kill wb_reg_write_en if taking a trap.
-    assign wb_reg_write_addr  = mem_wb_rd_addr;
-    assign wb_reg_write_en    = mem_wb_reg_write_en;
+    assign wb_rd_addr      = mem_wb_rd_addr;
+    assign wb_reg_write_en = mem_wb_reg_write_en & ~has_trap;
 
     // --------------------------
     // Trap / Exception Signals