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Notes on the Pentium's microcode circuitry

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180 points leotravis10 | 1 comments | 31 Mar 25 18:35 UTC | HN request time: 0.399s | source
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dev_john15 ◴[01 Apr 25 09:02 UTC] No.43544457[source]▶
Fascinating deep dive into the Pentium microcode ROM circuitry! It's incredible to see the clever tricks Intel used, like the pseudo-random counter, to pack so much logic into such a constrained space. Articles like this give us a rare glimpse into the unsung engineering heroics behind these landmark chips.
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Aardwolf ◴[01 Apr 25 09:16 UTC] No.43544547[source]▶
I would love to know how multiplication and division work in modern chips to have such low cycle count compared to addition, since in theory the addition complexity is linear in the amount of bits but multiplication and division are quadratic, or loglinear for large inputs. Part of that is solved by surface area rather than time I guess, but that's also true for the adders already with the carry logic
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1. RiverCrochet ◴[01 Apr 25 17:19 UTC] No.43549294[source]▶
I remember reading somewhere--memory is hazy--that at least division uses a partial look up table, kinda like how you'd do it in 6502 assembly back in the day. E.g., if you have to multiply something by 5, and you can get the range of inputs down to something reasonable, then you can just have a table of x*5 for that range and just look it up.

Also I'm not sure multiplication/division are quadratic if your algorithm is not "add X to itself Y times." Look at this for 6502 16-bit multiply - https://www.llx.com/Neil/a2/mult.html - it's dependent on the bit width, not the value of the multiplier/cand. Of course this is for integers, not floating point.