Only one LEO I computer ever existed, there were 10 LEO II computers and almost 40 standard LEO IIIs were installed around the UK and abroad. Another 19 LEO IIIs were produced with additional capabilities. 

Discover which LEO was purchased by which company here and find all the items in the archive that mention that particular LEO. 

Each generation of LEO was described using Roman numerals (LEO I, LEO II or LEO III) and then each individual machine was referred to in numerical sequence (e.g. LEO II/1). Click on the blue links to see all the items in the archive that relate to each LEO computer. (This is a work in progress. If no items relate to a LEO that you're interested in, please check back later.) With grateful thanks to the LEO Computers Society for the information contained in this page, particularly John Daines.

LEO I was a valve based machine and used mercury delay line storage.  Starting with the design of the Cambridge EDSAC, the LEO team made considerable changes to cater for (i) large data volumes both in and out and (ii) the requirement for the computer to be reliable, to work all day every day supporting a major business. 

John Daines of the LEO Computers Society comments: "It is inconceivable today that one computer could do all that work between late 1951 and the arrival of LEO II in 1957 WITH NO BACKUP!"

LEO II was also valve based and also used mercury delay line storage.  Much of the processor was similar but a big increase in overall speed was achieved by interleaving the bits in the delay lines that had the effect of making the store nearly 4 times faster.  It was also developed to support magnetic drums and magnetic tape storage.  The last 4 LEO II machines used core storage.  On both LEO I and II, programs were fed in by a bootstrap program called the Initial Orders from punched cards.

LEO III
LEO III was a new development, using new technology like transistors and core storage (that was much larger in capacity than LEO I and II).  It had no magnetic drums or discs.  It was a parallel machine with 40 bit adders, data paths and routes to autonomous input / output channels that avoided loading the central processor.  Programs were loaded from magnetic tapes and there was an operating system, called the Master Routine, to allocate resources to programs, control the key facility of multi-programming (several programs in store but only one active at any instant in time with each program protected from interfering with another) and to provide an interface for the operators to deal with job initiation and any error conditions.

LEO 326 and beyond
LEO III developed into LEO 326 with a much faster processor, larger main memory, multiple input / output channels and additional instructions.  The LEO III / 326 instruction set was implemented using microcode, a Wilkes invention that allowed the flexibility to add instructions e.g. double-length arithmetic, as requirements changed.