Redcode is the languaged used to write warriors for the game Corewar.
mov 0, 1
In order to load your warrior into the game, the recode must be processed by a Parser. The Parsed output of the above instruction looks like this:
MOV.I $0, $1
You can (but don't have to) specify the modifier and addressing modes in your source redcode. Using different modifiers and addressing modes allow you to change the behaviour of opcodes in interesting ways.
Redcode is specified according to a number of standards. The most recent (current) standard is ICWS'94. This document provides a very detailed description of redcode and the game of corewar.
Older, legacy standards were created in 1986 and 1988. ICWS'94 is a superset of the functionality from the older standards and was designed to be backwards compatible. A warrior written to the 1988 spec should parse and execute correctly under the ICWS'94 standard.