CPUID: Difference between revisions

An Assembly-routine that checks whether CPUID is supported looks like the following:

 

; returns zero if CPUID is not supported. returns 1 otherwise.

pushfd

shr eax, 21

ret

 

The idea of the CPUID instruction is that you can call it with different values in EAX, and it will return different information about the processor. For example, if we want the Vendor ID String (see below), we should code something like that:

 

mov eax, 0x0

cpuid

 

There are differences between AMD and Intel. According to the Intel CPUID application note, we should first check the Vendor ID String for "GenuineIntel" before taking out information, such as the Processor Signature, Processor Feature Flags, etc.

When called with EAX = 0, CPUID returns the vendor ID string in EBX, EDX and ECX. Writing these to memory in this order results in a 12-character string. These can be tested against known Vendor ID strings:

 

/* Vendor-strings. */

#define CPUID_VENDOR_OLDAMD "AMDisbetter!" //early engineering samples of AMD K5 processor

#define CPUID_VENDOR_NSC "Geode by NSC"

#define CPUID_VENDOR_RISE "RiseRiseRise"

You already know that the Vendor ID String is returned in EBX, ECX, EDX. Let us take an Intel processor. It should return "GenuineIntel". Look at the following text to see how the string is placed in the registers:

Recent processors also use ECX for features (which form a different set), with which you should be very careful as some old CPUs return bogus information in this register.

 

enum {

CPUID_FEAT_ECX_SSE3 = 1 << 0,

CPUID_FEAT_EDX_PBE = 1 << 31

};

 

== Using CPUID from GCC ==

CPUID can be invoked with various request codes (in eax) and will return values in general registers (much as a built-in service interrupt). The following code is made Public Domain out of Clicker's x86/cpu.h

 

enum cpuid_requests {

CPUID_GETVENDORSTRING,

return highest;

}

 

== See Also ==