The CPUID opcode is a processor supplementary instruction (its name derived from CPU IDentification) for the x86 architecture. 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, 0A2h (as two bytes, or 0A20Fh 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 sometimes 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 often provide on-chip registers which can be read to obtain the same sorts of information provided by this instruction.
Read more about CPUID: Calling CPUID, Accessing The Id From Other Languages, Uptake of CPUID Instructions Outside X86