Ken Thompson

Programming Techniques: Regular expression search algorithm

A method for locating specific character strings embedded in character text is described and an implementation of this method in the form of a compiler is discussed. The compiler accepts a regular expression as source language and produces an IBM 7094 program as object language. The object program then accepts the text to be searched as input and produces a signal every time an embedded string in the text matches the given regular expression. Examples, problems, and solutions are also presented.

Password security: a case history

This paper describes the history of the design of the password security scheme on a remotely accessed time-sharing system. The present design was the result of countering observed attempts to penetrate the system. The result is a compromise between extreme security and ease of use.

Reflections on trusting trust

To what extent should one trust a statement that a program is free of Trojan horses? Perhaps it is more important to trust the people who wrote the software.

The use of name spaces in Plan 9

Plan 9 is a distributed system built at the Computing Sciences Research Center of AT&T Bell Laboratories over the last few years. Its goal is to provide a production-quality system for software development and general computation using heterogeneous hardware and minimal software. A Plan 9 system comprises CPU and file servers in a central location connected together by fast networks. Slower networks fan out to workstation-class machines that serve as user terminals. Plan 9 argues that given a few carefully implemented abstractions it is possible to produce a small operating system that provides support for the largest systems on a variety of architectures and networks. The foundations of the system are built on two ideas: a per-process name space and a simple message-oriented file system protocol.

BELLE: chess hardware

The computer chess program BELLE is currently the World Computer Chess Champion and the North American Computer Chess Champion. In human play, BELLE has consistently obtained master performance ratings. This paper describes the special-purpose hardware that gives BELLE its advantage: speed.

Computer chess: Master level play in 1981?

Two papers will be presented and a general discussion period will then follow. The panel members are all members of the ACM Computer Chess Committee. The first paper, which appears elsewhere in the Proceedings, is the work of Tony Marsland. It is entitled “A survey of enhancements to the alpha-beta algorithm.” The paper reviews move ordering and search reduction techniques which improve the effectiveness of the alpha-beta algorithm when used in a chess program. A critical review of these search modifications is provided, and a recursive formula to estimate the search time is proposed, one which reflects the characteristics of the strongly ordered trees produced through improved use of the alpha-beta algorithm.