Here are different ways to do the cpuid

Let the assembler/compiler do the register/memory copy for us

    asm("cpuid"
        : "=b" (first),
          "=c" (third),
          "=d" (second)
        : "a"  (input));

This generates code with this pointless copy via eax

   cpuid
   movl  %eax, -20(%ebp)
   movl  %ebx, %eax
   movl  %eax, -8(%ebp)
   movl  %ecx, %eax
   movl  %eax, -16(%ebp)
   movl  %edx, %eax
   movl  %eax, -12(%ebp)

Explictly show I want to do MOV operations, and use the =m constraint operator to tell it to copy to memory

    asm("mov $0,%%eax;
         cpuid;
         mov %%ebx,%0;
         mov %%edx,%1;
         mov %%ecx,%2;"
        :"=m"(first), "=m"(second), "=m"(third)
        :
        :"%eax");

This generates:

   mov $0,%eax;
         cpuid;
         mov %ebx,-8(%ebp);
         mov %edx,-12(%ebp);
         mov %ecx,-16(%ebp);

int main(void)
{
    unsigned int first,second,third;
    unsigned int feature;
    unsigned int input = 0;

    asm("cpuid"
        : "=b" (first),
          "=c" (third),
          "=d" (second)
        : "a"  (input));

    //make sure it's a real Intel processor
    if (first == 'uneG')
      if (second == 'Ieni')
        if (third == 'letn'){
            //it's a real Intel, so let's check the feature bit
       asm("mov $1,%%eax;
            cpuid;
            mov %%ecx,%0;"
           :"=r"(feature)
           :
           :"%eax");
       if(feature & EST_BIT){
         printf("This/these processors support EST.\n");
         printf("Returning 1\n");
         return 1;
       }
   }

    printf("This/these processors do not support EST.\n");
    printf("Returning 0\n");
    return 0;
}

-- MattWalsh - 06 Oct 2005