Elliott 803

The Elliott 803 was a small computer manufactured by the British company Elliott Brothers in the 1960s. About 250 were built and most British universities and colleges bought one.

History

The 800 series started with the 801, a one-off test machine built in 1957. The 802 was a production model but only seven were sold between 1958 and 1961. The short-lived 803A was built in 1959 and first delivered in 1960; the 803B was built in 1960 and first delivered in 1961. Elliott subsequently developed the much faster Elliott 503 computer to be software compatible.

Over 200 Elliott 803 computers were delivered to customers, at a price of about £29,000 in 1960 [http://www.ourcomputerheritage.org/wp/upload/CCS-E3X1.pdf] , the majority of sales being the 803B version (with more parallel paths internally, larger memory and hardware floating-point operations). In 2007, at least three working Elliott 803 computers survive. There is one in the Science Museum (London) and the Computer Conservation Society have two, one in London and one at Bletchley Park.

Hardware description

The 803 was a transistorised bit-serial machine; the 803B had more parallel paths internally. It used ferrite core memory in 4096 or 8192 words of 40 bits, comprising 39 bits of data with parity. The CPU was housed in a single cabinet about 66 inches long, 16 inches deep and 56 inches high. Circuitry was based on printed circuit boards with the printed circuits being rather simple and most of the signalling carried on wires. There was a second cabinet about half the size used for the power supply, which was unusually based on a large nickel-cadmium battery with charger, an early form of uninterruptible power supply. There was an operator's control console, Creed teleprinter and high-speed paper tape reader and punch for input/output, using 5-track Elliott telecode code, not Baudot. Tape could be read at 500 characters per second and punched at 100 cps.The operator's console, about 60 inches long, allowed low-level instructions to be entered manually to manipulate addresses and data and could start, stop and step the machine: there was a loudspeaker (pulsed by the top bit of the instruction register) which allowed the operator to judge the status of a computation. The system required air conditioning, drawing about 3.5 kW of power in a minimal configuration.

Optional mass storage was available on an unusual magnetic tape system based on standard 35 mm film stock coated with iron oxide (specially manufactured by Kodak, who had a factory near that of Elliott Bros). The 1000 foot reels could hold about 7 million characters.

Instruction set

Instructions and data were based on 39-bit word length with binary representation in 2's complement (invert and add 1 for negative) arithmetic.The instruction set operated on a single address and single accumulator register, with an additional auxiliary register for integer multiply and divide. An instruction was composed of a 3-bit group and 3-bit subtype with 13 bits memory address field giving an addressable range of 8192 words. These 19-bit instructions were packed two to a word with an additional 39th bit, the so-called B-bit. Setting the B-bit had the effect of adding the contents of the memory address of the first instruction to the second instruction at execution time enabling indirect addressing and other run-time instruction modifications. The bit time was 6 microseconds, jumps executed in 288 microseconds and simple arithmetic instructions in 576 microseconds. Floating point operations could take several milliseconds. IO was direct and there were no interrupts.

Compilers

Memory locations 0-4 contained a hard-wired program for loading binary code from paper tape into memory. Machine code in a simple octal notation could then be loaded using a standard loader program called "T2". There was an Autocode for simple programming tasks.

The 803B with 8192 words of memory was capable of running the Elliott ALGOL compiler [http://www.billp.org/ccs/A104/] , a major subset of the Algol60 language, capable of loading and running several ALGOL programs in succession.This was largely written by Tony Hoare, employed by Elliotts as a programmer in August 1960. Hoare recounts some of his experiences at Elliotts in his 1980 ACM Turing Award lecture.

References

* Adrian Johnstone, "The Young person's Guide to... The Elliott 803B", Resurrection (Bulletin of the Computer Conservation Society) 1 (Spring 1991) [http://www.cs.man.ac.uk/CCS/res/res03.htm#e]
* Tony Hoare, "The Emperor's Old Clothes", Communications of the ACM 24 (February 1981)

External links

* [http://www.sli-institute.ac.uk/~bob/elliott803.htm http://www.sli-institute.ac.uk/~bob/elliott803.htm]
* [http://www.ourcomputerheritage.org/wp/ Our Computer Heritage pilot study]