Corewar

demo:corewar$ ls *.cor
lucien.cor  quentin.cor
demo:corewar$ ./corewar lucien.cor quentin.cor | tail -4
The player 2(Quentin) is alive.
The player 2(Quentin) is alive.
The player 2(Quentin) is alive.
The player 2(Quentin) has won.
demo:corewar$ ./corewar -dump 400 lucien.cor quentin.cor | grep -v alive | head -6 | cut -c1-80
00000000      02 90 00 00  00 41 01 02  90 00 00 00  00 02 00 00  00 41 00 02  0
--------
00000c00      0b 68 01 00  1f 00 01 0c  00 18 02 90  00 00 00 4c  01 02 90 00  0
00000c30      00 00 00 41  08 03 70 08  00 64 04 54  06 07 07 0b  54 07 07 01  0
The player 2(Quentin) has won.
demo:corewar$ 

Corewar is a programming game from the 1980s. Small programs, the champions, are loaded into the same circular block of memory and run in turns by a virtual machine. They can rewrite memory, including each other’s, and copy themselves. The last one still proving it is alive wins.

How it works

Champions are written in a tiny assembly language and compiled to bytes; the virtual machine reads those bytes and runs them, all sixteen instructions of them. Memory wraps around like a ring, so running off the end starts again at the beginning. A dump option stops a match at any moment and prints the whole memory, so a run can be compared byte by byte with the reference. I also wrote a champion of my own, ChampLucien, and the team added a 3D viewer to watch the fight.

What I took from it

• With a virtual machine, tiny details (byte order, wrap-around, a jump off by one) decide whether anything works, so comparing dumps earned its keep.
• Building the machine taught me to read machine code the way a processor does.