Computer Systems Laboratory Colloquium

4:15PM, Wednesday, February 04, 2003
NEC Auditorium, Gates Computer Science Building B03

The Architecture of Colossus, the first PC

Benjamin Wells
University of San Francisco
About the talk:

Colossus, the first electronic digital computer, was built by Tommy Flowers at the General Post Office Research Station in Dollis Hill, London. It was installed during December 1943 at Bletchley Park, the famous WWII British code-cracking enclave. Its purpose was to assist with the decryption of wireless traffic among German high-level commands encrypted using the Lorenz teletype cipher machine. Called Colossus because of its size, it could be run by a single operator --and often was. At least in that sense, it was also the world's first personal computer.

Bletchley had already developed a highly successful automated attack on the Enigma cipher system under the guidance and genius of Alan Turing. Built without direct input from Turing, Colossus was designed to support the cracking of the highest volume of German strategic code transmissions. These intelligence-rich messages were thousands of characters long, overshadowing the hand-encoded tactical traffic using Enigma. Because Colossus was kept secret until 1973, and full details of its use and construction were not released until 2000, it did not play a direct role in the evolution of digital computers. Of course, many who worked on it were involved with later computers.

With the release of previously classified documents, interest in Colossus has grown over the last three years. This accessible, multimedia talk will compare the architectural features of Colossus with those of modern PCs. Although it is tempting to assert that the former was a stored-program general purpose machine, as some have done in print, that analysis is less than promising. What is amazing is that Colossus introduced buffered I/O, branch decisions, biquinary representation, and bit masking, and anticipated some deeper modern features: parallelism, dual rail, hardware interrupt, shift register, asynchronous dataflow, and plug-ins. Moreover, recent results (AMS Abstracts 04T-68-2) show that a universal Turing machine could have been implemented on a cluster of the ten Colossi, proving the power of Colossus.

Background and links:

A reconstructed Colossus is up an running. See

About the speaker:

Benjamin Wells teaches both mathematics and computer science courses at the University of San Francisco, including freshman seminars that combine science and art. He holds degrees from MIT and UC Berkeley and has studied in four countries. The last student of noted logician Alfred Tarski, Wells works on the boundary of logic, algebra, and computing; he also contributes to computer graphics and visual communication. He won a John Templeton Foundation science and religion course prize in 1998 and held the USF Davies Professorship in 1989. He enjoys mysticism, cooking, computer-supported art, hiking, languages, dancing, tales, and family.

Contact information:

Benjamin Wells
Professor of Mathematics and Computer Science
University of San Francisco