Comment by hnkain

https://en.wikipedia.org/wiki/Little_man_computer is a simple computer model for education, from the 1960's. I built an emulator for it, which you can find here: http://paulhankin.github.io/lmc/lmc.html

It's not a game, but I found it fun to write simple programs; the machine code is so limited you need to find tricks to do anything non-trivial. For a challenge, try to write 1: a program that multiples two inputs, 2: a sieve of erastothenes, and 3: a program that can sort input.

The spec for the assembler is on the wikipedia page.