OXO

EDSAC (Electronic Delay Storage Automatic Calculator), the first "practical stored-program electronic computer", was built at the University of Cambridge Mathematical Laboratory and ran its first program on May 6, 1949.

Among its small software library (small not least because the early computer was unique, as were most contemporary computing machines) was a program written in 1952 called OXO, an implementation of tic-tac-toe which EDSAC plays perfectly, and which briefly took the title of "oldest documented computer game" from Tennis for Two (1958) before relinquishing it in turn to NIMROD (1951, not unchallenged itself).

In its emulated EDSAC environment the player interacts with the game by entering numbers into the machine's simulated dial; 0 and 1 are used for binary choices (determining who plays first, the human or computer player), and locations on the tic-tac-toe grid are indicated according to a horizontally-flipped keypad scheme where 9 is in the top left corner and 1 in the bottom right. The game board is depicted graphically in EDSAC's 35x16 cathode ray screen.