(Or more recently, "Core Wars") A game played between assembly code programs running in the core of a simulated machine (and vicariously by their authors). The objective is to kill your opponents' programs by overwriting them.
The programs are written using an instruction set called "Redcode" and run on a virtual machine called "MARS" (Memory Array Redcode Simulator).
Core War was devised by Victor Vyssotsky, Robert Morris Sr., and Dennis Ritchie in the early 1960s (their original game was called "Darwin" and ran on a PDP-1 at Bell Labs). It was first described in the "Core War Guidelines" of March, 1984 by D. G. Jones and A. K. Dewdney of the Department of Computer Science at The University of Western Ontario (Canada).
Dewdney wrote several "Computer Recreations" articles in "Scientific American" which discussed Core War, starting with the May 1984 article. Those articles are contained in the two anthologies cited below. A.K. Dewdney's articles are still the most readable introduction to Core War, even though the Redcode dialect described in there is no longer current.
The International Core War Society (ICWS) creates and maintains Core War standards and the runs Core War tournaments. There have been six annual tournaments and two standards (ICWS'86 and ICWS'88).
["The Armchair Universe: An Exploration of Computer Worlds", A. K. Dewdney, W. H. Freeman, New York, 1988, ISBN 0-7167-1939-8, LCCN QA76.6 .D517 1988]
["The Magic Machine: A Handbook of Computer Sorcery", A. K. Dewdney, W. H. Freeman, New York, 1990, ISBN 0-7167-2125-2 (Hardcover), 0-7167-2144-9 (Paperback), LCCN QA76.6 .D5173 1990].