error: invalid operand for instruction (inl)

I have a small snippet of code for accessing the VMware backdoor:

$ cat vmw_backdoor.cpp


#include <stdint.h>

#include <stdio.h>

#include <string.h>

#define MAX_UINT32 UINT32_MAX

#define VMW_MAGIC 0x564d5868

#define VMW_CMD_IOPORT 0x5658

#define VMW_CMD_GET_VER 0xa

struct registers


uint32_t eax;

uint32_t ebx;

uint32_t ecx;

uint32_t edx;

uint32_t ebp;

uint32_t edi;

uint32_t esi;


static void vmw_backdoor(unsigned cmd, registers* regs)


asm volatile(

“inl (%%dx)”

: “=a” (regs->eax), “=c” (regs->ecx), “=d” (regs->edx), “=b” (regs->ebx)

: “0” (VMW_MAGIC), “1” (cmd), “2” (VMW_CMD_IOPORT), “3” (MAX_UINT32)

: “memory”);


int main(void)


registers regs;

memset(&regs, 0, sizeof regs);

vmw_backdoor(VMW_CMD_GET_VER, &regs);

printf(“VMware version: %d.\n”, regs.eax);

return 0;



This works in g++:

$ g++ vmw_backdoor.cpp

$ ./a.out

VMware version: 6.

It fails to compile in clang:

$ clang vmw_backdoor.cpp

vmw_backdoor.cpp:26:9: error: invalid operand for instruction

“inl (%%dx)”


:1:6: note: instantiated into assembly here

inl (%dx)


1 error generated.

Any ideas how to make this work? Or is this a clang bug – I know the inline assembler is probably the last component anyone really cares about, but any help would be much appreciated.


John Calcote