David Given, who posts on here sometimes, did a great video about the process of coding a game on an unexpanded ZX81: http://cowlark.com/2018-09-26-zx81-programming/index.html

That's recently been ported to work as a lichess AI

https://fosdem.org/2022/schedule/event/retro_chess/

(by the same author as the book 20 GOTO 10)

Yes, and it is our lot that decries software bloat. We know what can be done with K bytes, why do we need K * K * K to simply boot?

Ever written anything in assembly? We trade cycles and bytes for programmer convenience. Assembly code heavily optimized for size will do things that are absolutely horrifying from a maintenance or general "architectural cleanliness" angle. Terribleness you might encounter:

* Reading instructions as constants because the values you need happen to already be in the code.

* Self-modification, including writing variables as constants into the instruction stream, generating instructions at run-time, and reusing code space as data space for one-once code.

* Using bits in the opcodes to store data, since a single bitflip doesn't affect the interpretation of some instructions. E.g. on the Z80 if Z and C are both always 1 at a particular point, you can use either jrz (0010 1000) or jrc (0011 1000) and use the 5th bit in that instruction as a flag. Free storage!

I do find this sort of thing fun. As a hobby. I wouldn't want to write an air traffic control system that way.