Several of the instructions in the current design need a bit of a rethink. In particular, I've been thinking about how this CPU could eventually be pipelined, and as part of that, how to simplify the control unit. I've probably made too much use of the flexibility that microcoding instructions …

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 …

The register board is way more complicated than my first PCB, so I'm a little reluctant to solder everything on it until I'm given it a bit of testing, so I just assembled the control logic and two of the 13 non-zero registers. And it works! I'm very pleased and …

My register boards arrived from JLPCB! Overall, pretty happy with them, but the quality is definitely lower than PCBWay. Three of the five boards have defects in the solder mask (none of which look fatal, though the bare copper you can see in the second photo above might be a …