CPUID: Difference between revisions

This assembly routine checks if CPUID is supported:

pushfd ;Save EFLAGS

pushfd ;Store EFLAGS

and eax,0x00200000 ;eax = zero if ID bit can't be changed, else non-zero

ret

Note 1: There are some old CPUs where CPUID is supported but the ID bit in EFLAGS is not (NexGen). There are also CPUs that support CPUID if and only if it has to be enabled first (Cyrix M1).

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 from CPUs.

#define CPUID_VENDOR_AMD "AuthenticAMD"

#define CPUID_VENDOR_BHYVE "bhyve bhyve "

#define CPUID_VENDOR_QNX " QNXQVMBSQG "

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: