His Nibbler implements a 4-bit CPU using 7400 logic chips. Because he’s come up with the architecture himself he’s taking a lot of steps to check all of his work before committing to a PCB.

The DDL-4 is a project to build a modular 4-bit CPU using bitslice methods ... each on its own colourful PCB and labeled on the silkscreen. These modules can then be configured and plugged ...