I'm working on the multiplication circuit in the Pentium; I've done a partial writeup: https://www.righto.com/2025/03/pentium-multiplier-adder-reve... The short answer is that multiplication uses a large tree of adders so it can add up all the long-division terms at once. It also uses base-8 for the multiplier to reduce the number of terms. The adders are 4:2 carry-save compressors that take four numbers as inputs and produce two numbers as outputs.

I also wrote about the Pentium's division circuitry and the infamous FDIV bug: https://www.righto.com/2024/12/this-die-photo-of-pentium-sho... The short answer is that the Pentium used base-4 SRT division, similar to long division but generating two bits of result per cycle. It used a lookup table to determine the two quotient bits; an error in this table resulted in the bug.