Function:

Compares the accumulator (8,16 or 32 bit form) with op1 by internally
subtracting op1 from the accumulator and setting ZF according to the result.
If ZR, op2 is copied to op1, otherwise op1 is loaded into the accumulator.

On the A-step of the 486, this Mnemonic was coded using the opcodes for
the, discarded, A- to B0-step 386 instructions XBTS (a6) and IBTS (a7).
Because of software conflicts with software written for the early 386 DX the
opcodes for the 486 were changed to the ones above starting with the B step.

Note that some 386 software won't run on older 386es and some 486
software will not run on early 486es when using this instruction.




CPUID Identify CPU on 486 and higher CPUs
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