CPUID(4)                  Linux Programmer's 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 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.

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.

COLOPHON         top

       This page is part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2009-03-31                         CPUID(4)

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