CPUID

CPUID is an assembler instruction to identify Intel compatible CPUs. Calling that instruction with register EAX set to 1 returns information about the CPU model in register EBX.

Processor Info and Feature Bits

EAX	31..28	15..14	13..12	27..20	11..8	19..16	7..4	3..0	Model	Hosts
	res.	res.	P.Type	Ext.Family	Family	Ext.Model	Model	Stepping
000106a5				00000000	0110	0001	1010	0101	Intel(R) Xeon(R) CPU E5504	hejne, skepp
000106a5				00000000	0110	0001	1010	0101	Intel(R) Xeon(R) CPU E5506	frosta, smarta
000106a5				00000000	0110	0001	1010	0101	Intel(R) Xeon(R) CPU E5520	utby
000206c2				00000000	0110	0010	1010	0010	Intel(R) Xeon(R) CPU E5606	xen1
000206d7				00000000	0110	0010	1101	0111	Intel(R) Xeon(R) CPU E5-2620	krus
000306e4				00000000	0110	0011	1110	0100	Intel(R) Xeon(R) CPU E5-2650 v2	spoka
000306f2				00000000	0110	0011	1111	0010	Intel(R) Xeon(R) CPU E5-2620	lattjo, sparka
000806e9				00000000	0110	1000	1110	1001	Intel(R) Core(TM) i5-7200U CPU	ivar

Extended Family is used instead of Family only if the later one if 15. Extended Model provides 4 additional upper bits for Model only if Family is 6 or 15.

The Linux kernel shows this in /proc/cpuinfo:

vendor_id	: GenuineIntel
cpu family	: 6
model		: 142
model name	: Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
stepping	: 9
microcode	: 0x8e

Here is some C example code to decode the CPUID information on Linux:

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <cpuid.h>

static void proc(void) {
  char buf[4 << 10];
  int fd = open("/proc/cpuinfo", O_RDONLY);
  if (fd < 0) {
    perror("open() failed");
    return;
  }
  while (1) {
    ssize_t nread = read(fd, buf, sizeof buf);
    if (nread == 0)
      break;
    if (nread < 0) {
      perror("read() failed");
      goto out;
    }
    char *begin, *end;
    for (begin = end = buf; end < buf + nread && *end; end++) {
      if (*end == '\n') {
        *end = '\0';
        if (!strncmp(begin, "model name  : ", 13)) {
          printf("%s\n", begin + 13);
          goto out;
        }
        begin = end + 1;
      }
    }
  }
out:
  if (close(fd)) {
    perror("close() failed");
    return;
  }
}

static void cpuid(void) {
  union {
    struct {
      uint8_t step: 4;
      uint8_t model: 4;
      uint8_t family: 4;
      uint8_t type: 2;
      uint8_t pad1: 2;
      uint8_t emodel: 4;
      uint8_t efamily: 8;
      uint8_t pad2: 4;
    };
    uint32_t reg;
  } eax;
  uint32_t ebx, ecx, edx;

  if (!__get_cpuid (1 /* Processor Info and Feature Bits */, &eax.reg, &ebx, &ecx, &edx))
    return;

  printf("EAX: %08x\nEBX: %08x\nECX: %08x\nEDX: %08x\n", eax.reg, ebx, ecx, edx);
  printf("stepping=%d model=%d|%d family=%d|%d ptype=%d\n", eax.step, eax.emodel, eax.model, eax.efamily, eax.family, eax.type);

  int model = eax.model, family = eax.family;
  if (eax.family >= 6)
      model |= eax.emodel << 4;
  if (eax.family == 15)
    family += eax.efamily;
  printf("model=%d family=%d\n", model, family);
}

int main(void) {
  cpuid();
  proc();
  return EXIT_SUCCESS;
}

http://www.sandpile.org/x86/cpuid.htm

Written on October 24, 2018