Core War is a programming game created by D. G. Jones and A. K. Dewdney in which two or more battle programs (called “warriors”) compete for control of a virtual computer. These battle programs are written in an abstract assembly language called Redcode. At the beginning of a game, each battle program is loaded into memory at a random location, after which each program executes one instruction in turn. The object of the game is to cause the processes of opposing programs to terminate (which happens if they execute an invalid instruction), leaving the victorious program in sole possession of the machine.
There are several simulators for writing a program, debugging it, and making it fight against opponents. The official simulator is pMars which runs on many different systems, and the screenshot below shows pMars compiled with the ncurses library.