Wikipedia
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.