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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 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

   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

   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

   This page is part of release 4.09 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/.



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