cpuid(4) — Linux manual page


cpuid(4)                Kernel Interfaces Manual                cpuid(4)

NAME         top

       cpuid - x86 CPUID access device

DESCRIPTION         top

       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

       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.

NOTES         top

       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.

       Very old x86 CPUs don't support CPUID.

SEE ALSO         top


       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.

Linux man-pages (unreleased)     (date)                         cpuid(4)