I built a 64k RAM 8080 computer on an S-100 bus. I started it out with CPM but transitioned to a UCSD Pascal environment so I could run a 6502 cross compiler.
I used the computer to learn how to program the Atari VCS game machine using a "Magic Card" that provided an interface to the Atari's 6502. I built a keyboard interface that emulated key presses on the Magic Card to load and test my game code. The Atari VCS had 128 bytes of RAM and the Magic Card provide around 1 or 2 K of RAM that served as a ROM emulator for the VCS. You really had to hoard RAM when all you had were 128 bytes to work with.
The whole rig cost around $2,000 in 1980 dollars but more than paid for itself in that I ended up working at Imagic, an Atari video game startup. That was one of the best jobs I had as the people I worked with were amazingly talented.