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Notes on the Pentium's microcode circuitry
Most people think of machine instructions as the fundamental steps that a computer performs. However, many processors have another layer of ...
However, carefully tracing the signals from the standard-cell logic shows that is connected to the Microcode Address Register, not the branch predictor. The Pentium has this testing circuitry for many ROMs and PLAs, including the division PLA that caused the infamous FDIV bug. Specifically, a pseudo-random number generator runs through each address, while another circuit computes a checksum of the ROM output, forming a "signature" word.
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