man > cpuid(4)

cpuid(4)                              Kernel Interfaces 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 is 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.

       Early  i486  CPUs  do  not  support the CPUID instruction; opening this device for those CPUs
       fails with EIO.

SEE ALSO
       cpuid(1)

       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 6.7                          2024-01-05                                     cpuid(4)