MMIX is an imaginary computer designed by DonaldKnuth.

Donald Knuth uses it to explain algorithms in a modern AssemblyLanguage in the series of book TheArtOfComputerProgramming.

http://www-cs-faculty.stanford.edu/~knuth/mmix.html

It is a RISC, with 256 registers and three-address form instructions. It has all the usual instructions (add), plus some trendy cache-conscious instructions (preload data), fused instructions (times 2 and add unsigned) and some rather weird ones (bitwise multiplex) plus a joke or two (sympathise with your machinery).

The books aren't quite done, and there is a mailing list for people who want to help write and debug MMIX programs that will go into the books.