3 ms·
From note #13: > The silicon implementation of the lower eight bits of the ALU / registers is flipped compared to the upper eight bits. The motivation is to pu
by jasonzemos 6y ago
From note #13:
> The silicon implementation of the lower eight bits of the ALU / registers is flipped compared to the upper eight bits. The motivation is to put the ALU signals next to the flag circuitry that needs these signals.
This caught me out because I was expecting some more obvious rotation or discernible mirroring of the upper half and lower half. Is the layout of bits 15:8 entirely different yet logically equivalent? Looking closely though I do see some similarities in the outer circuits at the very top and very bottom, yet there is a large gap across the lower bank.
- kens 6y agoThe top and bottom have almost identical circuitry, even though the layouts look entirely different. The layout of the ALU is highly optimized; since everything is repeated 16 times, squeezing out a bit of space makes a big difference. I assume the layouts are different due to the surroundings: The top stages have two bus wires next to them, while the bottom stage just has one bus wire, so there is a bit more room. Some of the horizontal metal wiring is different (like the wiring in the gap you saw), and power and ground may be offset slightly. The top stage needs to fit with the register interface circuitry just above it. My suspicion is that these relatively small changes resulted in the layouts being visibly fairly different. Maybe I'll do a gate-by-gate comparison at some point to check.