Add LETC core Stage W rd_mux and op access comparison

Add AND gates to validate write enable logic

rtl/letc/core/letc_core_stage_w.sv

@@ -4,6 +4,7 @@
  *
  * Copyright:
  *  Copyright (C) 2024 John Jekel
+ *  Copyright (C) 2024 Sam Graham
  * See the LICENSE file at the root of the project for licensing info.
  *
  * TODO longer description
@@ -19,15 +20,13 @@ module letc_core_stage_w
     import letc_core_pkg::*;
 (
     //Clock and reset
-    input logic i_clk,
-    input logic i_rst_n,
-
-    //TODO
+    input  logic        i_clk, // unused
+    input  logic        i_rst_n, // unused
 
     //Hazard/backpressure signals
-    output logic o_stage_ready,
-    input  logic i_stage_flush,
-    input  logic i_stage_stall,
+    output logic        o_stage_ready,
+    input  logic        i_stage_flush, // unused
+    input  logic        i_stage_stall, // unused
 
     //rd Write Port
     output reg_idx_t    o_rd_idx,
@@ -38,13 +37,31 @@ module letc_core_stage_w
     output logic        o_csr_explicit_wen,
     output csr_idx_t    o_csr_explicit_widx,
     output word_t       o_csr_explicit_wdata,
-    input  logic        i_csr_explicit_will,
+    input  logic        i_csr_explicit_will, // unused
 
     //From E2
     input  e2_to_w_s    i_e2_to_w
 );
 
-logic todo;
 assign o_stage_ready = 1'b1;
 
+assign o_csr_explicit_wen = (i_e2_to_w.csr_op == CSR_OP_ACCESS ? 1'b1 : 0) && i_e2_to_w.valid;
+assign o_csr_explicit_widx = i_e2_to_w.csr_idx;
+assign o_csr_explicit_wdata = i_e2_to_w.alu_result;
+
+assign o_rd_idx = i_e2_to_w.rd_idx;
+
+always_comb begin : rd_mux
+    unique case (i_e2_to_w.rd_src)
+        RD_SRC_ALU: o_rd_wdata = i_e2_to_w.alu_result;
+        RD_SRC_MEM: o_rd_wdata = i_e2_to_w.old_csr_value;
+        RD_SRC_CSR: o_rd_wdata = i_e2_to_w.memory_rdata;
+        default:    o_rd_wdata = 0;
+    endcase
+end : rd_mux
+
+assign o_rd_wen = i_e2_to_w.rd_we && i_e2_to_w.valid;
+
+// TODO handle exceptions
+
 endmodule : letc_core_stage_w