Well, it turned out that there were more problems with the first revision of the ALU than I thought:

• I knew I'd left three pairs of inputs floating across two ICs in the comparison unit, but it turned out I'd also completely neglected to wire up power or ground. Both …

I finished soldering the remaining components onto the register boards last night, and started running a more comprehensive set of tests than I had before, and I've found some of the problems I was worried about.

Firstly, there's a simple mistake in the schematic, in which I swapped pins 14 …