Electronic Delay Storage Automatic Calculator

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Electronic Delay Storage Automatic Calculator (EDSAC) was an early British computer.[1] Inspired by John von Neumann's seminal First Draft of a Report on the EDVAC, the machine was constructed by Maurice Wilkes and his team at the University of Cambridge Mathematical Laboratory in England. EDSAC was the second electronic digital stored-program computer to go into regular service.[2]

Later the project was supported by J. Lyons & Co. Ltd., a British firm, who were rewarded with the first commercially applied computer, LEO I, based on the EDSAC design. EDSAC ran its first programs on 6 May 1949, when it calculated a table of squares[3] and a list of prime numbers. EDSAC 1 was finally shut down on 11 July 1958, having been superseded by EDSAC 2, which remained in use until 1965.[4]

Technical overview[edit]

Physical components[edit]

As soon as EDSAC was operational, it began serving the University's research needs. It used mercury delay lines for memory, and derated vacuum tubes for logic. Input was via five-hole punched tape and output was via a teleprinter.

Initially registers were limited to an accumulator and a multiplier register. In 1953, David Wheeler, returning from a stay at the University of Illinois, designed an index register as an extension to the original EDSAC hardware.

A tape store was added in 1952 but never worked sufficiently well to be of real use.[5]

Initially, the available store was only 256 36-bit words most – but not all – of the time and no backing store.[6] The delay lines (or "tanks") were arranged in two batteries providing 512 words each. The second battery came into operation in 1952.[7]

The full 1024-word delay line store was not available until 1955 or early 1956,[8] limiting programs to about 800 words until then.

John Lindley (Diploma Student 1958–59) mentioned "the incredible difficulty we had ever to produce a single correct piece of paper tape with the crude and unreliable home-made punching, printing and verifying gear available in the late 50's.".[9]

Memory and instructions[edit]

Maurice Wilkes and Bill Renwick in front of the complete EDSAC

The EDSAC's main memory consisted of 1024 locations, though only 512 locations were initially implemented. Each contained 18 bits, but the first bit was unavailable due to timing restrictions, so only 17 bits were used. An instruction consisted of a five-bit instruction code (designed to be represented by a mnemonic letter, so that the Add instruction, for example, used the bit pattern for the letter A), one unused spare bit, ten bits for a memory address, and one bit to control whether the instruction operated on a number contained in one word or two.

Internally, the EDSAC used two's complement, binary numbers. These were either 17 bits (one word) or 35 bits (two words) long. Unusually, the multiplier was designed to treat numbers as fixed-point fractions in the range −1 ≤ x < 1, i.e. the binary point was immediately to the right of the sign. The accumulator could hold 71 bits, including the sign, allowing two long (35-bit) numbers to be multiplied without losing any precision.

The instructions available were: add, subtract, multiply, collate,[10] shift left, shift right, load multiplier register, store (and optionally clear) accumulator, conditional skip, read input tape, print character, round accumulator, no-op and stop. There was no division instruction (though a number of division subroutines were available) and no way to directly load a number into the accumulator (a “store and zero accumulator” instruction followed by an “add” instruction were necessary for this).

System software[edit]

The initial orders were hard-wired on a set of uniselector switches and loaded into the low words of memory at startup. By May 1949, the initial orders provided a primitive relocating assembler taking advantage of the mnemonic design described above, all in 31 words. This was the world's first assembler, and arguably the start of the global software industry. There is a simulation of EDSAC available and a full description of the initial orders and first programs.

The machine was used by other members of the University to solve real problems, and many early techniques were developed that are now included in operating systems. Users prepared their programs by punching them (in assembler) onto a paper tape. They soon became good at being able to hold the paper tape up to the light and read back the codes. When a program was ready it was hung on a length of line strung up near the paper tape reader. The machine operators, who were present during the day, selected the next tape from the line and loaded it into EDSAC. This is of course well known today as job queues. If it printed something then the tape and the printout were returned to the user, otherwise they were informed at which memory location it had stopped. Debuggers were some time away, but a CRT screen could be set to display the contents of a particular piece of memory. This was used to see if a number was converging, for example. After office hours certain "Authorised Users" were allowed to run the machine for themselves, which went on late into the night until a valve blew – which usually happened according to one such user.[11]

Programming technique[edit]

EDSAC control desk

The early programmers had to make use of techniques frowned upon today—especially altering the code. As there was no index register until much later the only way of accessing an array was to alter the memory location that a particular instruction referenced.

David Wheeler, who earned the world's first Computer Science PhD working on the project, is credited with inventing the concept of a subroutine. A user wrote a program that called a routine by jumping to the start of the subroutine with the address of the program counter plus one in the single register (a Wheeler jump). By convention the subroutine expected this and the first thing it did was to overwrite its final jump instruction with that address so that it returned. Multiple and nested subroutines could be called so long as the user knew the length of each one in order to calculate the location to jump to. The user then copied the code for the subroutine from a master tape onto their own tape following the end of their own program.

Application software[edit]

The subroutine concept led to the availability of a substantial subroutine library. By 1951, 87 subroutines in the following categories were available for general use: floating point arithmetic; arithmetic operations on complex numbers; checking; division; exponentiation; routines relating to functions; differential equations; special functions; power series; logarithms; miscellaneous; print and layout; quadrature; read (input); nth root; trigonometric functions; counting operations (simulating repeat until loops, while loops and for loops); vectors; and matrices.

Applications of EDSAC[edit]

  • In 1950, M. V. Wilkes and Wheeler used EDSAC to solve a differential equation relating to gene frequencies in a paper by Ronald Fisher.[12] This represents the first use of a computer for a problem in the field of biology.
  • In 1951, Miller and Wheeler used the machine to discover a 79-digit prime[13] – the largest known at the time.
  • In 1952, Sandy Douglas developed OXO, a version of noughts and crosses (tic-tac-toe) for the EDSAC, with graphical output to a VCR97 6" cathode ray tube. This may well have been the world's first video game.

Further developments[edit]

EDSAC's successor, EDSAC 2, was commissioned in 1958.

In 1961, an EDSAC 2 version of Autocode, an ALGOL-like high-level programming language for scientists and engineers, was developed by David Hartley.

In the mid-1960s, a successor to the EDSAC 2 was planned, but the move was instead made to the Titan, a prototype Atlas 2 developed from the Atlas Computer of the University of Manchester, Ferranti, and Plessey.

EDSAC Replica Project[edit]

On the 13 January 2011, the Computer Conservation Society announced that it planned to build a working replica of EDSAC, at the National Museum of Computing (TNMoC) in Bletchley Park.[14] The target is to have the replica operational by the end of 2015 and, in common with most major exhibits at TNMoC, to run it regularly in public. The first parts of the recreation were switched on in November 2014.[15]

The project is managed by Andrew Herbert, who studied under Maurice Wilkes.

See also[edit]


  1. ^ Wilkes, W. V.; Renwick, W. (1950). "The EDSAC (Electronic delay storage automatic calculator)". Math. Comp. 4: 61–65. doi:10.1090/s0025-5718-1950-0037589-7. 
  2. ^ The Manchester Small-Scale Experimental Machine, nicknamed "Baby", predated EDSAC as a stored-program computer, but was built as a test bed for the Williams tube and not as a machine for practical use. http://www.cl.cam.ac.uk/conference/EDSAC99/history.html. However, the Manchester Mark 1 of 1949 (not to be confused with the 1948 SSEM prototype) was available for general use by other university departments and Ferranti in April 1949 http://www.computer50.org/mark1/MM1.html despite being still under development.
  3. ^ "Pioneer computer to be rebuilt". Cam 62: 5. Lent 2011.  Check date values in: |date= (help) To be precise, EDSAC's first program printed a list of the squares of the integers from 0 to 99 inclusive.
  4. ^ EDSAC 99: 15–16 April 1999, University of Cambridge Computer Laboratory, 1999-05-06, pp. 68, 69, retrieved 2013-06-29 
  5. ^ http://www.cl.cam.ac.uk/events/EDSAC99/statistics.html
  6. ^ http://www.cl.cam.ac.uk/events/EDSAC99/reminiscences/#EDSAC%201%20applications
  7. ^ http://www.cl.cam.ac.uk/events/EDSAC99/statistics.html
  8. ^ http://www.cl.cam.ac.uk/events/EDSAC99/reminiscences/#EDSAC%201%20maintenance
  9. ^ http://www.cl.cam.ac.uk/events/EDSAC99/reminiscences/#EDSAC%202%20operating
  10. ^ This instruction added the bitwise AND of the specified memory word and the multiplier register to the accumulator.
  11. ^ Professor David Barron, Emeritus Professor of the University of Southampton at a Cambridge Computer Lab seminar to mark the 60th anniversary May 6th 2009.
  12. ^ Gene Frequencies in a Cline Determined by Selection and Diffusion, R. A. Fisher, Biometrics, Vol. 6, No. 4 (Dec., 1950), pp. 353–361
  13. ^ Caldwell – largest known primes by year One reference gives Miller, J. C. P. "Larger Prime Numbers" (1951) Nature 168(4280):838, but the abstract does not mention it.
  14. ^ Ward, Mark (2011-01-13). "Pioneering Edsac computer to be built at Bletchley Park". BBC News. Retrieved 2011-01-13. 
  15. ^ Museum switches on historic computer

External links[edit]