CPUID(4)                            Linux Programmer's 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 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.   How‐
       ever 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 follow‐
       ing 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 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2009-03-31                                   CPUID(4)