CPUID

The CPUID opcode is a processor supplementary instruction (its name derived from CPU IDentification) for the x86 architecture allowing software to discover details of the processor. It was introduced by Intel in 1993 when it introduced the Pentium and SL-enhanced 486 processors.

By using the CPUID opcode, software can determine processor type and the presence of features (like MMX/SSE). The CPUID opcode is 0Fh, A2h (as two bytes, or A20Fh as a single word) and the value in the EAX register, and in some cases the ECX register, specifies what information to return.

Prior to the general availability of the CPUID instruction, programmers would write esoteric machine code which exploited minor differences in CPU behavior in order to determine the processor make and model.

Outside the x86 family, developers are mostly still required to use esoteric processes to determine the variations in CPU design that are present. While the CPUID instruction is specific to the x86 architecture, other architectures (like ARM) often provide on-chip registers which can be read to obtain the same sorts of information provided by this instruction.

In assembly language the CPUID instruction takes no parameters as CPUID implicitly uses the EAX register to determine the main category of information returned. In Intel's more recent terminology, this is called the CPUID leaf. CPUID should be called with EAX = 0 first, as this will return in the EAX register the highest EAX calling parameter (leaf) that the CPU supports.

To obtain extended function information CPUID should be called with the most significant bit of EAX set. To determine the highest extended function calling parameter, call CPUID with EAX = 80000000h.

CPUID leaves greater than 3 but less than 80000000 are accessible only when the model-specific registers have IA32_MISC_ENABLE.BOOT_NT4 [bit 22] = 0 (which is so by default). As the name suggests, Windows NT4 did not boot properly unless this bit was set, but later versions of Windows do not need it, so basic leaves greater than 4 can be assumed visible on current Windows systems. As of July 2014^[update], basic valid leaves go up to 14h, but the information returned by some leaves are not disclosed in publicly available documentation, i.e. they are "reserved".

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