cpuid(4) Kernel Interfaces Manual cpuid(4)
NAME
cpuid - x86 CPUID access device
DESCRIPTION
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 mul-
tiple 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 num-
ber, 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.
NOTES
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/de-
vices/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 de-
vice for those CPUs fails with EIO.
SEE ALSO
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 Instruc-
tion, Application note 485.
Linux man-pages 6.9.1 2024-05-02 cpuid(4)
Generated by dwww version 1.16 on Sat Oct 3 05:59:35 CEST 2026.