I recently designed a control unit for my computer. I had been debating about how to do this, and I've finally settled on a design. The crux of the issue was that I'm using EEPROM to store the control logic in a matrix, and the EEPROM has way fewer pins than I need to control. Leaving room to add later modules, I think I'll need between 40 and 100 control lines. Needless to say the 8-bit word from the EEPROM isn't enough, and using 5-13 EEPROMs instead of one isn't super tempting. I decided instead to use 16 8-bit registers to store a total of 128 control bits that are sequentially written by the EEPROM. This means the control unit will have to run about 16 times faster (18x IIRC when you account for register clocks and a reset signal), but that's fine. Assuming I can get the control unit to run at ~1MHz, that gives me a system clock speed around 100 KHz - that's good enough to do basic math in under a second.

While designing the control unit (CU), I had to tie together whole bunch of other systems and modules I had left alone for a while.  And while it solidified how a lot of the computer's core features would work, it also pointed out some glaring weaknesses. Among them, the keypad relies on interrupts to work - the CPU had no way of supporting this. I also want to be able to do floating point math in the future, and although I made draft designs for what would be required, it's too elaborate to implement now. It'll need its own bus, clock, registers, and hoards of EEPROM + glue logic. Frankly, the floating point math unit will be more complex than my entire CPU, so I'm holding off on it for now and designing into my CPU support for a floating point math co-processor in the distant future.

Now for the fun part - my new CPU will support keyboard interrupts, conditional instructions with the help of zero & carry flags, and options to run code either in flash memory, RAM, or an external drive, and of course a floating point math co-processor.

The CPU will be on two boards - one holds the CU, the other holds everything else. They'll both be 200x200 mm PCBs (huge sizes is one of the side effects of using THT parts :) ) and connect to each other, back to back. The CU board is ready for the board house and the 'eveything else' board should be done in the next week or so.