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CPUID

Instruction for x86 microprocessors

  • 44 min read
  • 47 chapters

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A program can use the CPUID to determine processor type and whether features such as MMX/SSE are implemented.

History

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. With the introduction of the 80386 processor, EDX on reset indicated the revision but this was only readable after reset and there was no standard way for applications to read the value.

Outside the x86 family, developers are mostly still required to use esoteric processes (involving instruction timing or CPU fault triggers) to determine the variations in CPU design that are present.

For example, in the Motorola 68000 series — which never had a CPUID instruction of any kind — certain specific instructions required elevated privileges. These could be used to tell various CPU family members apart. In the Motorola 68010 the instruction MOVE from SR became privileged. Because the 68000 offered an unprivileged MOVE from SR the two different CPUs could be told apart by a CPU error condition being triggered.

While the CPUID instruction is specific to the x86 architecture, other architectures (like ARM) often provide on-chip registers which can be read in prescribed ways to obtain the same sorts of information provided by the x86 CPUID instruction.

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