Mac Hack, also known as Mac Hac and the Greenblatt Chess Program, is a computer chess program written by Richard D. Greenblatt at the Massachusetts Institute of Technology (MIT). Developed in the mid-1960s, it was a pioneering effort in Artificial intelligence and computer game playing. Mac Hack VI, the version running on a PDP-6 computer, holds several historical firsts: it was the first chess program to compete under human tournament conditions, the first to receive a chess rating from a recognized chess federation, and the first to defeat a human player in a tournament game. The program's name derives from Project MAC (Multi-Level Access Computer or Machine-Aided Cognition), a major sponsored research program at MIT, with the Roman numeral VI referring to the PDP-6 machine for which it was written.
Development
Greenblatt, a skilled chess player, was inspired to create Mac Hack after reading MIT Artificial Intelligence Memo 41, which described the earlier Kotok-McCarthy chess program, during a visit to Stanford University in 1965. He subsequently developed the program at MIT over 1965 and 1966. The software was written in MIDAS macro assembly language for the PDP-6, a computer donated to MIT by DEC (serial number 2, the first working unit of its kind). During its development, the program was compiled approximately two hundred times, reflecting an iterative and hands-on approach to debugging and refinement.
Greenblatt incorporated fifty heuristics into Mac Hack, drawing on his own chess knowledge to guide the program's search and evaluation. In later years, around 2004, he reflected on the search strategy of the earlier Kotok-McCarthy program, suggesting that searching the seven best moves at each of the first two plies with a depth limit of two would have outperformed its default widths of "4 3 2 2 1 1 1 1" across eight plies. This insight highlighted differences in search philosophy between the two programs.
Tournament Play
By the time Mac Hack was published in 1969, it had participated in eighteen tournaments and played hundreds of complete games. To compete in official events, the PDP-6 was made an honorary member of the Massachusetts State Chess Association and the United States Chess Federation (USCF). In 1966, the program received a rating of 1243 after taking part in the Massachusetts Amateur Championship. The following year, in 1967, it played in four tournaments, achieving three wins, twelve losses, and three draws. A notable milestone occurred in 1967 when Mac Hack VI defeated Ben Landy, a human player with a USCF rating of 1510, in game 3 of tournament 2 in the Massachusetts State Championship. This win marked the first time a computer had beaten a human in a tournament setting.
The program was documented in MIT Artificial Intelligence Memo 174, co-authored by Greenblatt with Donald E. Eastlake III and Stephen D. Crocker, which included recorded games and technical details of the implementation.
Influence
Mac Hack was notable for its accessibility and distribution. It operated via teletype, was later ported to the PDP-10, and became the first computer chess program to be widely distributed beyond its original institution. Technically, it was the first chess program to employ a transposition table, a data structure that stores previously evaluated positions to avoid redundant computation. This optimization became a vital component of game tree search in subsequent chess programs and other game-playing systems.
The program's success also influenced the broader field of Artificial intelligence. Greenblatt, along with Tom Knight, went on to advance AI research at MIT and built the Lisp machine in 1973, a dedicated computer architecture for the Lisp programming language that became influential in AI development. Mac Hack's achievements demonstrated the feasibility of computers competing in human intellectual activities, contributing to the early foundations of computer chess and AI research.
Legacy
Mac Hack's legacy lies in its demonstration that computers could perform complex, strategic reasoning in a competitive environment. Its tournament participation and rating set precedents for later chess computers, such as Deep Blue, and its technical innovations, like the transposition table, became standard practice. The program also exemplified the collaborative and exploratory spirit of early AI research at MIT, where projects like Project MAC fostered interdisciplinary work. While modern chess engines far surpass Mac Hack in strength, its historical role as a pioneer in computer chess and AI remains significant.