Modular One User Manual
Comprehensive Modular One manual.
Computer Technology Limited or CTL, was a British computer company founded slightly later than Digital Equipment Corporation (DEC) in the United States.
Founder Iann Barron had worked for Elliot Computing but left to form CTL when he couldn't persuade Elliot to incorporate his ideas in their next generation of computers. He left CTL in 1971, going on to form Inmos and develop the transputer.
The first CTL computer (the Modular One) appeared for sale in 1969.
The Modular One was a 16-bit computer built with Emitter Coupled Logic (ECL) and was competitive with other first generation minicomputers. Its most distinctive hardware features were memory-mapped I/O, and an early version of segmented memory (similar to the later Intel 8086 but having both base and limit). The latter, together with two execution states (Normal State and Special State) made possible the implementation of a self-protecting operating system kernel (known as the Executive, or Exec). Such ideas were popular in British computer academia at the time and later were adopted by some US designs such as the aforementioned Intel 8086. Also, the power system was set up as a peripheral with interrupt capabilities which gave the machine the ability to power down gracefully in an emergency.
An important idea in Modular One was that the main memory was much like another peripheral, for instance a printer, but was both input and output. When an instruction (or data) was gotten from memory, the request went out over a cable one or two metres long to another (memory) box, also about one metre cube. It was thought that a voltage edge was faster than a pulse, so a request was represented by a single voltage transition. The word being read would travel the one or two metres, and then (because magnetic core memory was destructive read)it would be sent back and written back to where it had been. Thus, the core operating sistem was travelling to and fro that distance, leading to corruption. Iann Barron wanted to sell the core software as well as the hardware, so the customer was not given the wherewithal to restore the crucial operaitng system, which had to be restored by a travelling maintenance engineer. When our biggest, enraged customer who had spent £300,000 on us got hold of my name, and asked me; "What is your earthing policy?", rather than telling him I said I would visit him. I worked during the nights to ameliorate earthing, leaving them to work during the day. I was fired a year or so after. - Ivor Catt
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