diff --git a/x86_64-asm.h b/x86_64-asm.h
index c2df8223..69cd77ca 100644
--- a/x86_64-asm.h
+++ b/x86_64-asm.h
@@ -26,6 +26,7 @@
      DEF_ASM_OP0(cwtl, 0x98)
      DEF_ASM_OP0(cwtd, 0x6699)
      DEF_ASM_OP0(cltd, 0x99)
+     DEF_ASM_OP0(cqto, 0x4899)
      DEF_ASM_OP0(int3, 0xcc)
      DEF_ASM_OP0(into, 0xce)
      DEF_ASM_OP0(iret, 0xcf)
diff --git a/x86_64-gen.c b/x86_64-gen.c
index e22de5c5..052bf874 100644
--- a/x86_64-gen.c
+++ b/x86_64-gen.c
@@ -1137,7 +1137,13 @@ void gen_opi(int op)
             r = TREG_RAX;
         } else {
             if (op == TOK_UDIV || op == TOK_UMOD) {
-                o(0xf7d231); /* xor %edx, %edx, div fr, %eax */
+                if ((vtop->type.t & VT_BTYPE) & VT_LLONG) {
+                    o(0xd23148); /* xor %rdx, %rdx */
+                    o(0x48 + REX_BASE(fr));
+                } else {
+                    o(0xd231); /* xor %edx, %edx */
+                }
+                o(0xf7); /* div fr, %eax */
                 o(0xf0 + fr);
             } else {
                 if ((vtop->type.t & VT_BTYPE) & VT_LLONG) {