I finally got tired of wondering about what really goes on inside a CPU that I decided to build my own 4 bit CPU project. I chose a 4 bit design simply to keep the level of wiring sane as the initial hardware version will be on a bread board and a permanent version wirewrapped.
The design has been successfully tested in Logisim with a couple of simple programs. First program simple counts from 0 to 10 then starts again and the other one counts from 10 to 0 and starts again, so obviously I have kept pretty simple.
I’m using around 17 74x TTL series chips throughout. I could have cut the chip count by using a 74181 4 bit ALU but have decided to build the ALU from discreet gates. The program RAM will consist of 2 x 2114 SRAM that I happen to have had on hand. The microcode is going to be hardwired into a diode ROM as a hat-tip to the rope memory used in the Apollo Guidance Computer.
I’ve started breadboarding the design (schematics created with Kicad) using cut lengths of solid core wire in an attempt to try and keep it tidy, as some of the other bread-boarded designs using standard jumpers gets messy and in the end all you can see is a mass of wiring.
I’ll keep this project up to date as it progresses.