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.

An earlier logic chip used as a building block for CPUs. Bit slice processors used arithmetic logic units (ALUs) that typically came in 4-bit increments, although 1- and 2-bit devices were also made.