Electronics give birth to the computer

Considerable flexibility was built into Colossus. Data streams could be routed through different logical operations; and parameters, such as the changeable patterns of noughts and ones used by the Lorenz wheels, could be altered using wired links.

The first model, Colossus 1, was a proof of concept, though it went straight into operation.

Colossus 2, which was built just in time for the 1944 D-Day landings, made the first known use of shift registers.

Five of them read five successive characters from the message tape, allowing five different start positions to be tried at the same time by five processing units - parallel processing that pushed the throughput up to 25,000 characters a second.

It also pushed the valve count up from 1,500 to 2,400. But Colossus was smaller than you might think from its name, amounting to two ranks of electronics about the size of a couple of large bookshelves.

By hook or by crook, Colossus could perform all the Boolean operations required of a modern computer. Harry Fensome, who helped build Colossus, writes in Jack Copeland’s book, Colossus: The Secret of Bletchley Park’s Codebreaking Computers: “In Colossus we had all the elements to make a general-purpose device.”

Colossus probably shortened the war. The big question was whether a bankrupt and bewildered Britain could capitalise on it when the war was over.

Find out more
Much of the writing about Colossus and its work is obscure, mixing old and new jargon. Jack Copeland's book Colossus: The Secret of Bletchley Park's Codebreaking Computers (ISBN 0-19-284055-X OUP £18.99) is invaluable, containing papers from many of those involved.

Tony Sale of the National Computing Museum has posted a simulation at www.codesandciphers.org.uk. There's more at the museum's website.

For last month’s article go here.

For the final piece in the series, go here.