Pegasus (computer)
Encyclopedia
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
computer built by Ferranti
Ferranti
Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. Known primarily for defence electronics, the Company was once a constituent of the FTSE 100 Index but ceased trading in 1993.The...
, Ltd of Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
.
The Pegasus 1 was first delivered in 1956 and the Pegasus 2 was delivered in 1959. Ferranti sold twenty-six copies of the Pegasus 1 and twelve copies of the Pegasus 2, making it Ferranti's most popular valve (vacuum tube) computer.
At least two Pegasuses survive, one in The Science Museum (London)
Science Museum (London)
The Science Museum is one of the three major museums on Exhibition Road, South Kensington, London in the Royal Borough of Kensington and Chelsea. It is part of the National Museum of Science and Industry. The museum is a major London tourist attraction....
and one in The Manchester Museum of Science and Industry. The Pegasus in The Science Museum (London)
Science Museum (London)
The Science Museum is one of the three major museums on Exhibition Road, South Kensington, London in the Royal Borough of Kensington and Chelsea. It is part of the National Museum of Science and Industry. The museum is a major London tourist attraction....
ran its first program in December 1959 and is still regularly demonstrated. It is the oldest working digital electronic computer in the world.
Christopher Strachey
Christopher Strachey
Christopher Strachey was a British computer scientist. He was one of the founders of denotational semantics, and a pioneer in programming language design...
recommended these design objectives:
- The necessity for Optimum programmingOptimum programmingIn the history of computing, optimum programming is the practice of arranging a computer program's instructions in memory so as to minimize the time the machine spends waiting for instructions. It is of historical interest mainly due to the design of many early digital computers.Most early...
(favored by Alan TuringAlan TuringAlan Mathison Turing, OBE, FRS , was an English mathematician, logician, cryptanalyst, and computer scientist. He was highly influential in the development of computer science, providing a formalisation of the concepts of "algorithm" and "computation" with the Turing machine, which played a...
) was to be minimised, "because it tended to become a time-wasting intellectual hobby of the programmers"; - The needs of the programmer were to be a governing factor in selecting the order code (instruction set)Instruction setAn instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
; and - It was to be cheap and reliable.
The first objective was only partially met: because both program and the data on which it was to operate had to be in the 56 words of primary storage, it was often necessary to resort to tricks in order to reduce the number of transfers between that store and the drum memory
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....
. To what extent the third objective was reached, depends on how one views a price of £50,000 for Pegasus 1 without tape drive
Tape drive
A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
s, line printer
Line printer
The line printer is a form of high speed impact printer in which one line of type is printed at a time. They are mostly associated with the early days of computing, but the technology is still in use...
or punched card
Punched card
A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...
input and output, which required an hour or more of preventative maintenance by a resident engineer every morning, before a programmer or operator was allowed near it.
Pegasus had eight accumulator
Accumulator (computing)
In a computer's central processing unit , an accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation to main memory, perhaps only to be read right back again for...
s, seven of which could also be used as index register
Index register
An index registerCommonly known as a B-line in early British computers. in a computer's CPU is a processor register used for modifying operand addresses during the run of a program, typically for doing vector/array operations...
s. (It was the first computer to allow this dual use.) Accumulator
Accumulator (computing)
In a computer's central processing unit , an accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation to main memory, perhaps only to be read right back again for...
s 6 and 7 were known as p and q and were involved in multiply and divide and some double length shift instructions. It had 56 words of fast memory stored in nickel delay lines
Delay line memory
Delay line memory was a form of computer memory used on some of the earliest digital computers. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory, but as opposed to modern random-access memory, delay line memory was serial-access...
, which was supplemented by a magnetic drum
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....
holding 5120 words. A word was 40 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s, of which one bit was for parity checking. Two 19-bit instructions were packed into one word and the extra bit (not counting the parity bit
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
) could be used to indicate a breakpoint
Breakpoint
In software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause....
(optional stop), to assist in debugging
Debugging
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...
. It had a relatively generous instruction set for a second generation computer, but there was almost no provision for handling either characters or floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
numbers.
The speed of arithmetic operations was about the same as the Elliott 402 computer, which could add in 204 microseconds and multiply in 3366 microseconds. The Pegasus basic instruction cycle time for add/subtract/move and logical instructions was 128 microseconds. Multiply, divide, justify and shift instructions took a variable time to complete. Transfers to and from magnetic drum
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....
were synchronous and had to be optimised where possible. The layout of blocks on the magnetic drum
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....
was interleaved to allow some processing between transfers to/from consecutive blocks.
In 1956 the first Pegasus was used to calculate the stresses and strains in the tail plane of the first vertical take off aeroplane, the Saunders-Roe SR.53
Saunders-Roe SR.53
|- See also :-References:NotesBibliography* Jones, Barry. "Saro's Mixed Power Saga". Aeroplane Monthly, November 1994, Vol 22 No 11 Issue 259. pp. 32–39. London:IPC. ISSN 0143-7240....
; the results were used to check the manufacturers figures; the programmer was Anne Robson. Because of the importance of a computer it was housed in the drawing room, complete with an Adam's ceiling
Adam style
The Adam style is an 18th century neoclassical style of interior design and architecture, as practiced by the three Adam brothers from Scotland; of whom Robert Adam and James Adam were the most widely known.The Adam brothers were the first to advocate an integrated style for architecture and...
, of Ferranti's London office in Portland Place.
In 1957, a Pegasus computer was used to calculate 7480 digits of pi
Pi
' is a mathematical constant that is the ratio of any circle's circumference to its diameter. is approximately equal to 3.14. Many formulae in mathematics, science, and engineering involve , which makes it one of the most important mathematical constants...
, a record at the time.
Hugh McGregor Ross
Hugh McGregor Ross
Hugh McGregor Ross is an early pioneer in the history of British computing. He worked for Ferranti from the mid-1960s, where he worked on the Pegasus thermionic valve computer. He was involved in the standardization of ASCII and ISO 646 and worked closely with Bob Bemer. ASCII was first known in...
was one of the people who worked on the Pegasus.
External links
- The Industrial Era, 1955–1957
- The 1962 edition of the Pegasus Programming Manual (38 MB PDF)
- The Computer Conservation Society, who help maintain the Pegasus installation in the London Science Museum