Proper Use of x86/x86_64 CPUID Instruction with Extended Assembler
The CPUID instruction always uses EAX/EBX/ECX/EDX, even on 64-bit platforms. On 64-bit platforms, the CPUID instruction sets the high 32-bit words of RAX/RBX/RCX/RDX equal to 0.
This is a generally useful C implementation that works on 32- and 64-bit systems:
#include <stdio.h>
int main() {
int i;
unsigned int index = 0;
unsigned int regs[4];
int sum;
__asm__ __volatile__(
#if defined(__x86_64__) || defined(_M_AMD64) || defined (_M_X64)
"pushq %%rbx \n\t" /* save %rbx */
#else
"pushl %%ebx \n\t" /* save %ebx */
#endif
"cpuid \n\t"
"movl %%ebx ,%[ebx] \n\t" /* write the result into output var */
#if defined(__x86_64__) || defined(_M_AMD64) || defined (_M_X64)
"popq %%rbx \n\t"
#else
"popl %%ebx \n\t"
#endif
: "=a"(regs[0]), [ebx] "=r"(regs[1]), "=c"(regs[2]), "=d"(regs[3])
: "a"(index));
for (i=4; i<8; i++) {
printf("%c" ,((char *)regs)[i]);
}
for (i=12; i<16; i++) {
printf("%c" ,((char *)regs)[i]);
}
for (i=8; i<12; i++) {
printf("%c" ,((char *)regs)[i]);
}
printf("\n");
}