Bulls and cows

The game is played in turns by two opponents who aim to decipher the other's secret code by trial and error.

[citation needed] A version known as MOO was widely available for early mainframe computers, Unix and Multics systems, among others.

The first known version was written by Frank King at the Cambridge Computer Laboratory sometime before the summer of 1970.

[2] In 1972, the original Cambridge MOO was ported to the Multics operating system at MIT,[1] and reimplemented on early Unix at Bell Labs.

[3] A version called BASIC MOO was published in the DECUS Program Library for PDP computers and another was available through the DEC Users Society, both dating from 1971.

[4][5] A version written by Lane Hauck in the language FOCAL for the PDP-8 later served as the basis for the handheld game Comp IV by Milton Bradley.

*4digits* , an open source version of bulls and cows. This software uses "A" to denote "bulls" (digit in the correct position) and "B" to denote "cows" (digit in the wrong position).