diff --git a/nemu/src/isa/riscv32/inst.c b/nemu/src/isa/riscv32/inst.c
index 932775f..92a0081 100644
--- a/nemu/src/isa/riscv32/inst.c
+++ b/nemu/src/isa/riscv32/inst.c
@@ -131,7 +131,7 @@ static int decode_exec(Decode *s) {
 
   // "M"
   INSTPAT("0000001 ????? ????? 000 ????? 01100 11", mul    , R, R(rd) = src1 * src2);
-  INSTPAT("0000001 ????? ????? 001 ????? 01100 11", mulh   , R, R(rd) = (int64_t)src1 * (int64_t)src2 >> 32);
+  INSTPAT("0000001 ????? ????? 001 ????? 01100 11", mulh   , R, R(rd) = (int64_t)(sword_t)src1 * (int64_t)(sword_t)src2 >> 32);
   INSTPAT("0000001 ????? ????? 010 ????? 01100 11", mulhsu , R, R(rd) = (int64_t)src1 * (uint64_t)src2 >> 32);
   INSTPAT("0000001 ????? ????? 011 ????? 01100 11", mulhu  , R, R(rd) = (uint64_t)src1 * (uint64_t)src2 >> 32);
   INSTPAT("0000001 ????? ????? 100 ????? 01100 11", div    , R, R(rd) = (sword_t)src1 / (sword_t)src2);