Enforce x0 write protection in decoder + add testbench

Author: Anubhav-30

dv/dv/tb_x0_protection.sv

@@ -0,0 +1,33 @@
+module tb_x0_protection;
+
+  logic [4:0] rf_waddr_o;
+  logic rf_we;
+  logic illegal_reg_rv32e;
+  logic rf_we_o;
+
+  assign rf_we_o = rf_we & ~illegal_reg_rv32e & (rf_waddr_o != 5'd0);
+
+  initial begin
+    $display("Testing x0 protection...");
+
+    rf_we = 1'b1;
+    illegal_reg_rv32e = 1'b0;
+
+    // x0 write
+    rf_waddr_o = 5'd0;
+    #1;
+    if (rf_we_o !== 0)
+      $error("FAIL: x0 write not blocked");
+
+    // ✅ valid write
+    rf_waddr_o = 5'd5;
+    #1;
+    if (rf_we_o !== 1)
+      $error("FAIL: valid write blocked");
+
+    $display("✅ PASS: x0 protection works");
+
+    $finish;
+  end
+
+endmodule

rtl/ibex_decoder.sv

@@ -1197,7 +1197,7 @@ module ibex_decoder #(
   assign illegal_insn_o = illegal_insn | illegal_reg_rv32e;
 
   // do not propagate regfile write enable if non-available registers are accessed in RV32E
-  assign rf_we_o = rf_we & ~illegal_reg_rv32e;
+  assign rf_we_o = rf_we & ~illegal_reg_rv32e  & (rf_waddr_o != 5'd0) ;
 
   // Not all bits are used
   assign unused_instr_alu = {instr_alu[19:15],instr_alu[11:7]};