cpuid - x86 CPUID access device
CPUID provides an interface for querying information about the x86 CPU.
This device is accessed by lseek
(2) or pread
(2) to the appropriate
CPUID level and reading in chunks of 16 bytes. A larger read size means
multiple reads of consecutive levels.
The lower 32 bits of the file position is used as the incoming %eax
the upper 32 bits of the file position as the incoming %ecx
, the latter
intended for "counting" eax
levels like eax=4
This driver uses /dev/cpu/CPUNUM/cpuid
, where CPUNUM
is the minor
number, and on an SMP box will direct the access to CPU CPUNUM
listed in /proc/cpuinfo
This file is protected so that it can be read only by the user root
members of the group root
The CPUID instruction can be directly executed by a program using inline
assembler. However this device allows convenient access to all CPUs without
changing process affinity.
Most of the information in cpuid
is reported by the kernel in cooked form
either in /proc/cpuinfo
or through subdirectories in
. Direct CPUID access through this device should
only be used in exceptional cases.
driver is not auto-loaded. On modular kernels you might need to
use the following command to load it explicitly before use:
$ modprobe cpuid
There is no support for CPUID functions that require additional input registers.
Very old x86 CPUs don't support CPUID.
Intel Corporation, Intel 64 and IA-32 Architectures Software Developer's Manual
Volume 2A: Instruction Set Reference, A-M, 3-180 CPUID reference.
Intel Corporation, Intel Processor Identification and the CPUID Instruction,
Application note 485.
This page is part of release 4.13 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at