There is some uncertainty here. Either it is the acronym for Millions of Instructions Per Second, or a RISC computer architecture - http://en.wikipedia.org/wiki/MIPS_architecture.
A quick google search turns up this handy reference for the opcodes and RTL descriptions of the MIPS functions: http://www.student.cs.uwaterloo.ca/~isg/res/mips/opcodes
If you want hard copy text, I'd suggest "MIPS Assembly Language Programming" by Robert L. Britton. Of all the CS books I have ever used (or rather didn't use), I used this one the most.