cpuid(4) - phpMan

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 is intended for "counting" eax lev-
els 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.