Computer Technology Limited

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Computer Technology Limited (CTL) was a British company founded in the late 1960s to manufacture and sell an innovative minicomputer system. It was roughly contemporary with Digital Equipment Corporation (DEC) in the United States, and its first computer (the Modular One) appeared for sale around 1970. 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 showed up in some US designs such as the aforementioned Intel 8086.

The Exec was known as E4. (E1, E2 and E3 were much simpler execs used only in the first few years of the company). E4 was based on crude object-oriented principles, though lacking most of what are now considered essential features of the paradigm. Objects included Activities (now more commonly known as tasks or processes), Segments (of memory), Files, Semaphores and Clocks. Another object type, the Sphere, was a run-time protection domain within which all other object types (including other Spheres) existed. There was some similarity to Unix in the use of serial byte oriented streams in the file system and interprocess communication, in contrast to the record-oriented file systems then dominant in commercial data processing. E4 also supported real-time priorities and virtual memory at the Segment level. It was a relatively elegant OS for its time but was never ported to other hardware, having been written entirely in assembler.

The Modular One was expensive and somewhat exotic, also somewhat unreliable. It never sold widely outside of the UK, and even in the UK it was surpassed in sales by DEC and Data General before the end of the 1970s. The systems were cost reduced with new technology over the mid '70s to mid '80s but never gained a significant market share. During the mid '80s, the company realised that the future lay in open systems and attempted to make the transition to Unix with re-badged systems from Motorola and Sequoia, however, sales of the proprietary systems fell off before the new systems could be ramped up to replace them, and the company was taken over by ACT in late 1989.