diff --git a/i386-asm.c b/i386-asm.c
index 2ffa53b..a036e49 100644
--- a/i386-asm.c
+++ b/i386-asm.c
@@ -893,6 +893,15 @@ ST_FUNC void asm_opcode(TCCState *s1, int opcode)
 
     pc = 0;
     if (pa->instr_type & OPC_MODRM) {
+	if (!nb_ops) {
+	    /* A modrm opcode without operands is a special case (e.g. mfence).
+	       It has a group and acts as if there's an register operand 0
+	       (ax).  */
+	    i = 0;
+	    ops[i].type = OP_REG;
+	    ops[i].reg = 0;
+	    goto modrm_found;
+	}
         /* first look for an ea operand */
         for(i = 0;i < nb_ops; i++) {
             if (op_type[i] & OP_EA)
diff --git a/tests/asmtest.S b/tests/asmtest.S
index 79419b9..bf51c44 100644
--- a/tests/asmtest.S
+++ b/tests/asmtest.S
@@ -607,6 +607,13 @@ int $0x10
     rdmsr
     rdpmc
     ud2
+#ifdef __x86_64__
+    syscall
+    sysret
+    lfence
+    mfence
+    sfence
+#endif
 
     emms
     movd %edx, %mm3
diff --git a/x86_64-asm.h b/x86_64-asm.h
index cf67e69..a40b697 100644
--- a/x86_64-asm.h
+++ b/x86_64-asm.h
@@ -88,6 +88,9 @@ ALT(DEF_ASM_OP2(btcw, 0x0fba, 7, OPC_MODRM | OPC_WLX, OPT_IM8, OPT_REGW | OPT_EA
      DEF_ASM_OP0(rdtsc, 0x0f31)
      DEF_ASM_OP0(rdmsr, 0x0f32)
      DEF_ASM_OP0(rdpmc, 0x0f33)
+
+     DEF_ASM_OP0(syscall, 0x0f05)
+     DEF_ASM_OP0(sysret, 0x0f07)
      DEF_ASM_OP0(ud2, 0x0f0b)
 
      /* NOTE: we took the same order as gas opcode definition order */
@@ -482,6 +485,9 @@ ALT(DEF_ASM_OP2(movhps, 0x0f17, 0, OPC_MODRM, OPT_SSE, OPT_EA | OPT_REG32 ))
     DEF_ASM_OP2(sqrtps, 0x0f51, 0, OPC_MODRM, OPT_EA | OPT_SSE, OPT_SSE )
     DEF_ASM_OP2(subps, 0x0f5c, 0, OPC_MODRM, OPT_EA | OPT_SSE, OPT_SSE )
 
+    DEF_ASM_OP0L(lfence, 0x0fae, 5, OPC_MODRM)
+    DEF_ASM_OP0L(mfence, 0x0fae, 6, OPC_MODRM)
+    DEF_ASM_OP0L(sfence, 0x0fae, 7, OPC_MODRM)
 #undef ALT
 #undef DEF_ASM_OP0
 #undef DEF_ASM_OP0L