Atlas Computer (Manchester)
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The Atlas Computer of the University of Manchester became operational in 1962, having been a joint development between the University, Ferranti and Plessey. It was arguably the fastest computer in the world until the release of the CDC 6600. It was said at the time that whenever it went offline half of the UK computer capacity was lost. It was a second-generation computer, using transistors.
Two other Atlas machines were built: one for British Petroleum (BP) and the University of London and one for the Atlas Computer Laboratory at Chilton near Oxford. A derivative system was built by Ferranti for Cambridge University, called the Titan, which had a different memory organisation, and ran a time-sharing operating system developed by Cambridge Computer Laboratory.
The University of Manchester's Atlas system was eventually decommissioned in 1971.
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Technical description
Hardware
The machine had many innovative features but the key operating parameters were:
- 48-bit word size
- 24-bit address space
- 16K words of core store (equivalent to 96 KB), featuring interleaving of odd/even addresses
- 96K words of drum store (eqv. to 576 KB), split across four drums but integrated with the core store using virtual memory and paging techniques
- Capability for the addition of (for the time) sophisticated new peripherals such as magnetic tape
- Addressing of peripherals through Vstore addresses and extracode routines
It did not use a synchronous clocking mechanism so performance measurements were not easy but as an example:
- Fixed-point register add — 1.59 microseconds
- Floating-point add, no modification — 1.61 microseconds
- Floating-point add, double modify — 2.61 microseconds
- Floating-point multiply, double modify — 4.97 microseconds
Extracode
One interesting feature of the Atlas was extracode, a system that allowed new instructions to be added in software.
The uppermost ten bits of an Atlas machine instruction denoted which operation should be performed. If the most significant bit was set to zero, this was an ordinary machine instruction executed directly by the hardware.
If the uppermost bit was set to one, this was an Extracode and was implemented as a special kind of subroutine jump to an address in the fixed store (ROM) where that address was determined by the other nine bits. Extracode mode had its own program address counter.
Many of the extracodes were what we would probably call microcodes nowadays; they were simple arithmetic procedures which would have been too inefficient to implement in hardware. However, about half of the codes were designated as Supervisor functions which invoked operating system procedures. Typical examples would be "Print the character specified on the stream specified" or "Read a block of 512 words from logical tape N"
Extracodes were the only means that a program could communicate with the Supervisor program.
Modern Intel-type CPUs have a mechanism which is similar in principle but termed 'interrupts'. This is a bad and unhelpful name, as it leads to confusion with hardware interrupts.
Software
A unique Supervisor software system managed the computer's processing time (as such it qualifies in modern terminology as an advanced job scheduler, or a simple operating system).
One of the first high level languages available on Atlas was named Atlas Autocode, which was an early forerunner to Algol. The Atlas also supported Algol 60, Fortran and COBOL. Being a university machine it was patronised by a large number of the student population who even had access to a protected machine code development environment.