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CPUID(4)                   Linux Programmer's Manual                  CPUID(4)



NAME
       cpuid - x86 CPUID access device

DESCRIPTIONS
       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 only be read 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 Ref-
       erence, 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-03-31                          CPUID(4)