Add `add` instructions for ALU and disable write

risc-v_shell.tlv

@@ -103,7 +103,15 @@
    *passed = 1'b0;
    *failed = *cyc_cnt > M4_MAX_CYC;
 
+   // Arithmetic Logic Unit
+   $result[31:0] =
+      $is_addi ? $src1_value + $imm :
+      $is_add ? $src1_value + $src2_value :
+      32'b0;
+
    // Register file
+   // TODO: this is for debugging only, REMOVE ME
+   $rd_valid = 1'b0;
    m4+rf(32, 32, $reset, $rd_valid, $rd[4:0], $wr_data[31:0], $rs1_valid, $rs1[4:0], $src1_value, $rs2_valid, $rs2[4:0], $src2_value)
    //m4+dmem(32, 32, $reset, $addr[4:0], $wr_en, $wr_data[31:0], $rd_en, $rd_data)
    m4+cpu_viz()