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NAME | DESCRIPTION | NOTES | SEE ALSO | COLOPHON |
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cpuid(4) Kernel Interfaces Manual cpuid(4)
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, and the upper 32 bits of the file position as the incoming
%ecx, the latter is 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 as
listed in /proc/cpuinfo.
This file is protected so that it can be read only by the user
root, or 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
/sys/devices/system/cpu. Direct CPUID access through this device
should only be used in exceptional cases.
The cpuid 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.
Early i486 CPUs do not support the CPUID instruction; opening this
device for those CPUs fails with EIO.
cpuid(1)
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 cpuid(4)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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