asm in C “too many memory references for `mov'”

Question

I\'ve seen the post about the same error but i\'m still get error :

 too many memory references for `mov\'
 junk `hCPUIDmov buffer\' after expression


        

Answer 1

1. Because of how C works,

   __asm(
       "mov %eax, 01h"
       "CPUID"
       "mov buffer, edx"
   );
   

   is equivalent to

   __asm("mov %eax, 01h" "CPUID" "mov buffer, edx");
   

   which is equivalent to

   __asm("mov %eax, 01hCPUIDmov buffer, edx");
   

   which isn't what you want.

2. AT&T syntax (GAS's default) puts the destination register at the end.

3. AT&T syntax requires immediates to be prefixed with $.

4. You can't reference local variables like that; you need to pass them in as operands.

Wikipedia's article gives a working example that returns eax.

The following snippet might cover your use-cases (I'm not intricately familiar with GCC inline assembly or CPUID):

int eax, ebx, ecx, edx;
eax = 1;
__asm( "cpuid"
     : "+a" (eax), "+b" (ebx), "+c" (ecx), "+d" (edx));
buffer = edx

Answer 2

Your assembly code is not correctly formatted for gcc.

Firstly, gcc uses AT&T syntax (EDIT: by default, thanks nrz), so it needs a % added for each register reference and a $ for immediate operands. The destination operand is always on the right side.

Secondly, you'll need to pass a line separator (for example \n\t) for a new line. Since gcc passes your string straight to the assembler, it requires a particular syntax.

You should usually try hard to minimize your assembler since it may cause problems for the optimizer. Simplest way to minimize the assembler required would probably be to break the cpuid instruction out into a function, and reuse that.

void cpuid(int32_t *peax, int32_t *pebx, int32_t *pecx, int32_t *pedx)
{
    __asm(
         "CPUID"
          /* All outputs (eax, ebx, ecx, edx) */
        : "=a"(*peax), "=b"(*pebx), "=c"(*pecx), "=d"(*pedx)   
          /* All inputs (eax) */
        : "a"(*peax)                                           
    );
}

Then just simply call using;

int a=1, b, c, d;

cpuid(&a, &b, &c, &d);

Another possibly more elegant way is to do it using macros.