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Belle for Chess
There is a sideline to the career of Ken Thompson which has nothing to do with Unix, C or any of the types of program that he is usually associated with.
As a boy he enjoyed playing chess as well as electronics. What could be more natural then than to create a chess playing programs and machines.
In 1972 Thompson started work on Belle, a chess playing program that used traditional search techniques combined with a database of end games.
Ken Thompson and Joe Condon studying chess moves
Later (1976) he and Joe Condon developed a hardware prototype for a move generator for Belle, possibly the first dedicated chess playing hardware. This developed from a 200-move per second machine to 120,000 moves per second by 1980. This fast machine used 1700 chips and did all of the work necessary to play top class chess. Earlier versions had used a PDP 11 as a host but the 1980 version of Belle did everything and won three ACM computer chess championships (1980, 81 and 82).
It also made history in being the first program to be awarded the title of master (US). The Fredkin Foundation awarded Belle's creators with a prize of $5000 for the first software chess master.
Thompson even managed to get the rules of chess changed! The 50-move rule said that after 50 moves in which no pawn advanced and no piece was captured the game should be declared a stalemate. As the result of using his endgame database to analyse simple endings he managed to show that there were games that could be won if allowed to go on beyond the 50 moves. As a result the 50-move rule was changed - but then found to be unworkable and so changed back!