I've enjoyed playing the Steam version of this. Really good puzzles and even has a story line. Print out the manual for a proper old-school experience!
They are very simple, imaginary machines. They only have one general purpose register, a single non-addressable register to which you can copy or swap the value in the main register and four I/O registers for communicating with adjacent machines. To get anything complex done, you need to use multiple machines and much of the puzzling is spent figuring out how to divide the responsibilities.
It is an imaginary machine. There are only about a dozen different instructions which makes learning it easier. It also makes solving some of the puzzles more interesting when you run into limits on the number of instructions that you have space for.
It is an imaginary machine, intended to have restrictions that most real hardware wouldn't have (like not being able to access the BAK register directly), but the aim of this is to make the game more of a challenge, and thus more fun.
