cpu.c

#include <stdio.h>

void getPSN(char *PSN)
{
    int varEAX, varEBX, varECX, varEDX;
    char str[9];
    //%eax=1 gives most significant 32 bits in eax 
    __asm__ __volatile__ ("cpuid"   : "=a" (varEAX), "=b" (varEBX), "=c" (varECX), "=d" (varEDX) : "a" (1));
    sprintf(str, "%08X", varEAX); //i.e. XXXX-XXXX-xxxx-xxxx-xxxx-xxxx
    sprintf(PSN, "%C%C%C%C-%C%C%C%C", str[0], str[1], str[2], str[3], str[4], str[5], str[6], str[7]);
    //%eax=3 gives least significant 64 bits in edx and ecx [if PN is enabled]
    __asm__ __volatile__ ("cpuid"   : "=a" (varEAX), "=b" (varEBX), "=c" (varECX), "=d" (varEDX) : "a" (3));
    sprintf(str, "%08X", varEDX); //i.e. xxxx-xxxx-XXXX-XXXX-xxxx-xxxx
    sprintf(PSN, "%s-%C%C%C%C-%C%C%C%C", PSN, str[0], str[1], str[2], str[3], str[4], str[5], str[6], str[7]);
    sprintf(str, "%08X", varECX); //i.e. xxxx-xxxx-xxxx-xxxx-XXXX-XXXX
    sprintf(PSN, "%s-%C%C%C%C-%C%C%C%C", PSN, str[0], str[1], str[2], str[3], str[4], str[5], str[6], str[7]);
}

int main()
{
     char PSN[30]; //24 Hex digits, 5 '-' separators, and a '\0'
     getPSN(PSN);
    printf("%s\n", PSN); //compare with: lshw | grep serial:
     return 0;
}