rv32g-core

commit 01e6e1ad0d0252ae4279a94cbf21deee8300965e
parent fe95a0356ebe488c2fa337db35a2b49f0535e40d
Author: Vineet Kumar <git@vineetk.net>
Date:   Mon,  1 Jun 2026 15:24:22 -0400

add basic exception signals

Diffstat:
M
rtl/stage_decode.sv
 | 
35
+++++++++++++++++++++--------------

1 file changed, 21 insertions(+), 14 deletions(-)
diff --git a/rtl/stage_decode.sv b/rtl/stage_decode.sv
@@ -62,20 +62,29 @@ typedef enum logic [2:0] {WB_NONE, // no register writeback
 			  WB_IMM   // LUI writes back an immediate value
 } wbsrc_t;
 
+// very rudimentary. privileged spec (chapter 3) defines this formally, but left to implementation
+// on unprivileged spec.
+typedef enum logic [1:0] {EXC_NONE,
+			  EXC_ILL_INST, // illegal instruction
+			  EXC_CALL,	// ECALL
+			  EXC_BKPT	// EBREAK
+} exception_t;
+
 typedef struct packed {
-  aluop_t aluop;
+  aluop_t     aluop;
   aluop1src_t aluop1src;
   aluop2src_t aluop2src;
-  memop_t memop;
-  pcsrc_t pcsrc;
-  branchop_t branchop;
-  wbsrc_t wbsrc;
+  memop_t     memop;
+  pcsrc_t     pcsrc;
+  branchop_t  branchop;
+  wbsrc_t     wbsrc;
+  exception_t exception;
 } ctrl_t;
 
 module stage_decode
   (input logic [31:0]  inst,
    input logic [31:0]  regfile [0:31],
-   output ctrl_t ctrl,
+   output ctrl_t       ctrl,
    output logic [31:0] imm,
    output logic [31:0] rs1_val,
    output logic [31:0] rs2_val,
@@ -279,7 +288,8 @@ module stage_decode
 	      memop:     MEM_NONE,
 	      pcsrc:     PC_SEQ,
 	      branchop:  BRANCH_NONE,
-	      wbsrc:     WB_NONE};
+	      wbsrc:     WB_NONE,
+	      exception: EXC_NONE};
      unique case (name)
        // rv32i
        LUI: ctrl.wbsrc = WB_IMM;
@@ -509,17 +519,14 @@ module stage_decode
 	 ctrl.aluop2src = ALUOP2_REG;
 	 ctrl.wbsrc = WB_ALU;
        end
-       // MISC-MEM instructions are basically NOPs
+       // MISC-MEM instructions are basically NOPs on single-hart system
        FENCE: ;
        FENCE_TSO: ;
        PAUSE: ;
        // SYSTEM instructions do traps
-       ECALL: begin
-         ctrl.aluop = ALU_NONE;
-       end
-       EBREAK: begin
-         ctrl.aluop = ALU_NONE;
-       end
+       ECALL: ctrl.exception = EXC_CALL;
+       EBREAK: ctrl.exception = EXC_BKPT;
+       default: ctrl.exception = EXC_ILL_INST;
      endcase
    end