[jeroenhd]

There's a nice guide by Ben Eater on Youtube about a breadboard computers: https://www.youtube.com/playlist?list=PLowKtXNTBypFbtuVMUVXN...

It doesn't sport any modern features like DMA, but builds up from the core basics: a 6502 chip, a clock, and a blinking LED, all hooked up on a breadboard. He also built a basic VGA card and explains protocols like PS/2, USB, and SPI. It's a great introduction or refresher into the low level hardware concepts behind computers. You can even buy kits to play along at home!

[porknubbins]

I was going to say, watching Ben Eater and building his 6502 machine myself filled in so many gaps in knowledge or fuzzy areas around basic computer hardware. If OP sees this he should definitely consider building some of these after Nand2Tetris.

[656565656565]

This! except I (slightly redesigned and) built a common z80 setup, no comparison for knowledge gained from this single effort.