Re:Notes on the Pentium's Microcode Circuitry
by kens on Monday March 31, 2025 @05:24PM
(#43540148)
The Pentium's ROM appears to be slightly scrambled (see footnote 6 in my article). ROMs are often a bit permuted for electrical reasons. For example, instead of columns ordered ABABABAB..., they will be ordered ABBAABBA... and then the A and B select lines can be shared by two columns. But the columns in the Pentium appear to be permuted in an irregular way. I'm not sure if this was for obfuscation or if automated layout software decided this was better.
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Re:Notes on the Pentium's Microcode Circuitry
by mmastrac on Monday March 31, 2025 @05:40PM
(#43540290)
I'm curious if the register you see near the microcode ROM is potentially hooked up to MSRs -- it could potentially be a read or write buffer.
https://www.cs.cmu.edu/~ralf/papers/highmsr.html
> To the left of the MAR is a 32-bit register that is apparently unrelated to the microcode ROM, although I haven't determined its function.
That register could be a Model-Specific Register; I haven't looked at it closely enough to see what it does. The Pentium is very complicated with 3.1 million transistors, so my reverse-engineering of it is essentially bits and pieces here and there.