Concept
The design they settled on for the new machine, the ‘HARRIAC’, had been specified in England by one of Ferranti’s UK salesmen, Harry Johnson, as a commercial data-processing machine. HARRIAC had much in common with the Ferranti Argus series of process control computers, which date from 1958. The Argus, in turn, bore a strong family resemblance to the Ferranti Pegasus, whose architecture was strongly influenced by Christopher Strachey. The FP-6000 was implemented with discrete transistor logic circuits that had been developed by Maurice Gribble at Ferranti UK’s Wythenshawe plant, where they were known as ‘griblons’.
In order to be successful the machine would have to differentiate itself from the rest of the "seven dwarfs" in the US computer industry, which were having problems of their own given IBMs overwhelming presence. After some study they decided one up-and-coming area was multitasking (then known as multiprogramming), and started looking into ways for their computer to directly support it.
The key problem in supporting multiprogramming was the need for programs to be loaded into different locations in memory, so that more than one could run at the same time. Without multiprogramming a program was normally loaded into the "base" of memory, its notional location zero. In order to provide this environment for several programs, each program was assigned a fixed amount of the core memory, its base location being known as the datum and last location known as the limit. Every store operation by the CPU automatically offset the effective address by the datum. Most of these concepts had originally been developed for Ferranti UK's Ferranti Orion project, which several members of the FP-6000 team had worked on.
In order to prevent fragmentation of memory, each time a program terminated the FP-6000's operating system, known as Executive, would temporarily stop the other programs and recopy them to the lowest end of core. This way the available memory was always at the "top". Although this technique eliminated the need for storing a list of memory blocks, it was at the cost of expensive copies every time a program ended. This would make the system unsuitable for running an operating system such as Unix, which is "made up" of a series of tiny programs that are frequently started and stopped, but Unix did not exist at the time and the model for most operating systems was a sort of "extended batch mode", running long-lived programs.
The machine was also designed from the outset to allow it to scale across a wide variety of needs. The system included 64 hardware channels that could be connected to peripherals of any sort, and could run with a wide variety of core memory sizes. In other ways the machine was fairly similar to the ReserVec's Gemini machine, using a 24-bit word with a 25-bit for parity checking and a simple machine language. One change was the lack of a memory drum, as the advances in core allowed them to replace the drum entirely.
Read more about this topic: Ferranti-Packard 6000
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