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David Holdsworth: Reminiscence

I went to state schools in the then West Riding of Yorkshire, where the Director of Education was Alec Clegg, well-known for his left-wing views. As a result, I left a co-ed comprehensive school in 1961 and went on to read Physics at Oxford University following a few months working in the works laboratory of English Electric. I began my computing career as a physics research student writing Algol60 programs modelling quarks on a KDF9. 

After discovering that I might be better at computing than physics, I got a job at Leeds University in 1967 where we implemented the Eldon2 multi-access operating system on KDF9, which was still running at NPL in 1980. Leeds Universityís KDF9 was succeeded by an ICL 1906A where I was involved with George3 and George4. At Leeds I was an early champion of Amdahl and of UNIX.

I was often helping others with their computing issues. After completing the doctorate, I went to a job in the Electronic Computing Laboratory at Leeds University, where I worked in a variety of roles until 2004. Actually the thesis was written up while at Leeds. My developments on their KDF9 are documented in Resurrection, the journal of the Computer Conservation Society. Suffice to say that I was a key figure in implementing the Eldon2 multi-access system, which enabled us to offer interactive computing from March 1968, with 32 tele-types connected via a PDP-8.

I started resurrecting/preserving software in the late 1990s. By resurrecting/preserving I mean getting the software into a modern digital state and providing the ability to execute it. George3 was the first such rebirth, using the official George3 issue magnetic tapes, followed by the BBC Microís Domesday Project. The first software rescued from printer listings was KDF9ís Whetstone Algol. I was also involved in the preservation of digital material that is not computer software. An important principle of my work has been that emulation should work on a wide range of current hardware, with a view to working on future systems.

Sometime around new year 2013, John Daines asked if I could use my skills in software resurrection on the pile of listings of Leo III Intercode that had been collected by Colin Tully. After resurrecting software for 1900, BBC Micro and KDF9, I was keen to try rescuing software from a machine of which I had no prior knowledge, so as better to appreciate what we need to preserve if future generations can comprehend early machines. I was delighted when John Daines asked me if I could resurrect the Intercode system that he had obtained from Colin Tully's widow. I immediately put my work on the Edinburgh IMP system on the back burner, where it resides to this day.

I came to Leo III expecting to find an assembly language and set about implementing Intercode treating the printout as the source text of an assembly language. It soon dawned on me that there were no labels, and that really I was dealing with a machine code for a fictitious machine, a sort of Leo IV.

The raw machine code also came as something of a surprise, devoting all sorts of complexities to computing with a variable radix, and using sign-and-modulus for information in the store but converting to 2's complement in the A register (but not the B register). A step-by-step account of my voyage of discovery which led to a working Leo III emulation is here.

I am fascinated by Intercode, as I think it may offer a window onto the time when assembly languages were emerging, a time before my own entry into computing, perhaps via a privileged side entrance.

Date : Unknown

This exhibit has a reference ID of CH56463. Please quote this reference ID in any communication with the Centre for Computing History.

David Holdsworth: Reminiscence

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