Small-Scale Experimental Machine
Encyclopedia
The Manchester Small-Scale Experimental Machine (SSEM), nicknamed Baby, was the world's first stored-program computer
. It was built at the Victoria University of Manchester
by Frederic C. Williams
, Tom Kilburn
and Geoff Tootill, and ran its first program on 21 June 1948.
The machine was not intended to be a practical computer but was instead designed as a testbed
for the Williams tube
, an early form of computer memory. Although considered "small and primitive" by the standards of its time, it was the first working machine to contain all of the elements essential to a modern electronic computer. As soon as the SSEM had demonstrated the feasibility of its design, a project was initiated at the university to develop it into a more usable computer, the Manchester Mark 1
. The Mark 1 in turn quickly became the prototype for the Ferranti Mark 1, the world's first commercially available general-purpose computer.
The SSEM had a 32-bit
word length and a memory
of 32 words. As it was designed to be the simplest possible stored-program computer, the only arithmetic operations implemented in hardware were subtraction
and negation
; other arithmetic operations were implemented in software. The first of three programs written for the machine found the highest proper divisor of 218 (262,144), a calculation that was known would take a long time to run—and so prove the computer's reliability—by testing every integer from 218 − 1 downwards, as division was implemented by repeated subtraction of the divisor. The program consisted of 17 instructions and ran for 52 minutes before reaching the correct answer of 131,072, after the SSEM had performed 3.5 million operations (for an effective CPU speed of 1.1 kIPS
).
The first design for a program-controlled computer was Charles Babbage
's Analytical Engine
in the 1830s. A century later, in 1936, mathematician Alan Turing
published his description of what became known as a Turing machine
, a theoretical concept intended to explore the limits of mechanical computation. Turing was not imagining a physical machine, but a person he called a "computer", who acted according to the instructions provided by a tape on which symbols could be read and written sequentially as the tape moved under a tape head. Turing proved that if an algorithm can be written to solve a mathematical problem, then a Turing machine can execute that algorithm.
Konrad Zuse
's Z3 was the world's first working programmable, fully automatic computer, with binary digital arithmetic logic, but it lacked the conditional branching of a Turing machine. On 12 May 1941, it was successfully presented to an audience of scientists of the Deutsche Versuchsanstalt für Luftfahrt ("German Laboratory for Aviation") in Berlin. The Z3 stored its program on an external tape, but it was electromechanical rather than electronic. The Colossus of 1943 was the first electronic computing device, but was not a general-purpose machine.
The ENIAC
(1946) was the first machine that was both electronic and general purpose. It was Turing complete, with conditional branching, and programmable to solve a wide range of problems, but its program was held in the state of switches in patchcords, not in memory, and it could take several days to reprogram. Researchers such as Turing and Konrad Zuse
investigated the idea of using the computer's memory to hold the program as well as the data it was working on, but it was mathematician John von Neumann
who became widely credited with defining that computer architecture, still used in almost all computers.
The construction of a von Neumann computer depended on the availability of a suitable memory device on which to store the program. During the Second World War researchers working on the problem of removing the clutter from radar
signals had developed a form of delay line memory
, the first practical application of which was the mercury delay line, developed by J. Presper Eckert
. Radar transmitters send out regular brief pulses of radio energy, the reflections from which are displayed on a CRT screen. As operators are usually interested only in moving targets, it was desirable to filter out any distracting reflections from stationary objects. The filtering was achieved by comparing each received pulse with the previous pulse, and rejecting both if they were identical, leaving a signal containing only the images of any moving objects. To store each received pulse for later comparison it was passed through a transmission line, delaying it by exactly the time between transmitted pulses.
Turing joined the National Physical Laboratory
(NPL) in October 1945, by which time scientists within the Ministry of Supply
had concluded that Britain needed a National Mathematical Laboratory to coordinate machine-aided computation. A Mathematics Division was set up at the NPL, and on 19 February 1946 Alan Turing presented a paper outlining his design for an electronic stored-program computer to be known as the Automatic Computing Engine (ACE). This was one of several projects set up in the years following the Second World War with the aim of constructing a stored-program computer. At about the same time, EDVAC
was under development at the University of Pennsylvania
's Moore School of Electrical Engineering
, and the University of Cambridge Mathematical Laboratory was working on EDSAC
.
The NPL did not have the expertise to build a machine like ACE, so they contacted Tommy Flowers
at the General Post Office
's (GPO) Dollis Hill Research Laboratory
. Flowers, the designer of Colossus, the world's first programmable electronic computer, was committed elsewhere and was unable to take part in the project, although his team did build some mercury delay lines for ACE. The Telecommunications Research Establishment
(TRE) was also approached for assistance, as was Maurice Wilkes at the University of Cambridge Mathematical Laboratory.
The government department responsible for the NPL decided that, of all the work being carried out by the TRE on its behalf, ACE was to be given the top priority. NPL's decision led to a visit by the superintendent of the TRE's Physics Division on 22 November 1946, accompanied by Frederic C. Williams
and A. M. Uttley, also from the TRE. Williams led a TRE development group working on CRT stores for radar applications, as an alternative to delay lines. He had already accepted a professorship at the University of Manchester
, and most of his circuit technicians were in the process of being transferred to the Department of Atomic Energy. The TRE agreed to second a small number of technicians to work under Williams' direction at the university, and to support another small group working with Uttley at the TRE.
made successful use of mercury delay line memory, the technology had several drawbacks; it was heavy, it was expensive, and it did not allow data to be accessed randomly. In addition, because data was stored as a sequence of acoustic waves propagated through a mercury
column, the device's temperature had to be very carefully controlled, as the velocity of sound through a medium varies with its temperature. Williams had seen an experiment at Bell Labs
demonstrating the effectiveness of cathode ray tube
s (CRT) as an alternative to the delay line for removing ground echoes from radar signals. While working at the TRE, shortly before he joined the University of Manchester in December 1946, he had developed a form of electronic memory known as the Williams tube
based on a standard CRT, the first random-access
digital storage device. The Manchester Small-Scale Experimental Machine (SSEM) was designed to prove that the Williams tube was a practical storage device, by testing that data held within it could be continuously updated at the speed necessary for use in a computer.
For use in a binary
digital computer, the tube had to be capable of storing either one of two states at each of its memory locations, corresponding to the binary digits (bits) 0 and 1. It exploited the positive or negative electrostatic charge generated by displaying either a dash or a dot at any position on the CRT screen, a phenomenon known as secondary emission
. A dash generated a positive charge, and a dot a negative charge, either of which could be picked up by a detector plate in front of the screen; a negative charge represented 0, and a positive charge 1. The charge dissipated in about 0.2 seconds, but it could be automatically refreshed from the data picked up by the detector.
The Williams tube was initially based on the CV1131, a commercially available 12 inches (30.5 cm) diameter CRT, but a smaller 6 inches (15.2 cm) tube, the CV1097, was used in the SSEM.
Following his appointment to the Chair of Electrical Engineering at Manchester University, Williams recruited his TRE colleague Tom Kilburn
on secondment. By the autumn of 1947 the pair had increased the storage capacity of the Williams tube from one bit to 2,048, arranged in a 64 by 32-bit array, and demonstrated that it was able to store those bits for four hours. Engineer Geoff Tootill joined the team on loan from TRE in September 1947, and remained on secondment until April 1949.
Max Newman
had been appointed to the Chair of Pure Mathematics at Manchester University in 1945. During the Second World War he had worked as a cryptanalyst at Bletchley Park
, and had led the team which in 1943 produced the first of the Colossus code-breaking computers. Although Newman played no active role in the development of the SSEM, or any of the subsequent Manchester computers
, he was generally supportive and enthusiastic about the project, and arranged for the acquisition of war-surplus supplies for its construction, including GPO
metal racks from Bletchley.
By June 1948 the SSEM had been built and was working. It was 17 feet (5.2 m) in length, 7 in 4 in (2.24 m) tall, and weighed almost 1 LT. The machine contained 550 valves
—300 diode
s and 250 pentode
s—and had a power consumption of 3500 watt
s. The arithmetic unit was built using EF50 pentode valves, which had been widely used during wartime. The SSEM used one Williams tube to provide 32 by 32-bit words of random-access memory
(RAM), a second to hold a 32-bit accumulator
in which the intermediate results of a calculation could be stored temporarily, and a third to hold the current program instruction along with its address
in memory. A fourth CRT, without the storage electronics of the other three, was used as the output device, able to display the bit pattern of any selected storage tube.
Each 32-bit word of RAM could contain either a program instruction or data. In a program instruction, bits 0–12 represented the memory address of the operand
to be used, and bits 13–15 specified the operation
to be executed, such as storing a number in memory; the remaining 16 bits were unused. The SSEM's single operand architecture meant that the second operand of any operation was implicit: the accumulator or the program counter (instruction address); program instructions specified only the address of the data in memory.
A word in the computer's memory could be read, written, or refreshed, in 360 microseconds. An instruction took four times as long to execute as accessing a word from memory, giving an instruction execution rate of about 700 per second. The main store was refreshed continuously, a process which took 20 milliseconds to complete, as each of the SSEM's 32 words had to be read and then refreshed in sequence.
The SSEM represented negative numbers using two's complement
, as most computers still do. In that representation, the value of the most significant bit
denotes the sign of a number; positive numbers have a zero in that position and negative numbers a one. Thus the range of numbers that could be held in each 32-bit word was −231 to +231 − 1 (decimal: −2,147,483,648 to +2,147,483,647).
allowed a maximum of eight (23) different instructions. In contrast to the modern convention, the machine's storage was arranged with the least significant digits
to the left; thus a one was represented in three bits as "100", rather than the more conventional "001".
The awkward negative operations were a consequence of the SSEM's lack of hardware to perform any arithmetic operations except subtraction
and negation
. It was considered unnecessary to build an adder
before testing could begin as addition can easily be implemented by subtraction, i.e., x+y can be computed as −(−x−y). Therefore adding two numbers together, X and Y, required four instructions:
Programs were entered in binary form by stepping through each word of memory in turn, and using a set of 32 switches known as the input device to set the value of each bit of each word to either 0 or 1. The SSEM had no paper-tape reader or punch
.
Three programs were written for the computer. The first, consisting of 17 instructions, was written by Kilburn, and so far as can be ascertained first ran on 21 June 1948. It was designed to find the highest proper factor
of 218 (262,144) by trying every integer from 218 − 1 downwards. The divisions were implemented by repeated subtractions of the divisor. The SSEM took 3.5 million operations and 52 minutes to produce the answer (131,072). The program used eight words of working storage in addition to its 17 words of instructions, giving a program size of 25 words.
Geoff Tootill wrote an amended version of the program the following month, and in mid-July Alan Turing—who had been appointed as a reader
in the mathematics department at Manchester University in September 1948—submitted the third program, to carry out long division. Turing had by then been appointed to the nominal post of Deputy Director of the Computing Machine Laboratory at the University, although the laboratory did not become a physical reality until 1951.
, published in September 1948. The machine's successful demonstration quickly led to the construction of a more practical computer, the Manchester Mark 1
, work on which began in August 1948. The first version was operational by April 1949, and it in turn led directly to the development of the Ferranti Mark 1, the world's first commercially available general-purpose computer.
In 1998, a working replica of the SSEM, now on display at the Museum of Science and Industry in Manchester
, was built to celebrate the 50th anniversary of the running of its first program. Demonstrations of the machine in operation are held regularly at the museum. In 2008 an original panoramic photograph of the entire machine was discovered at the University of Manchester. The photograph, taken on 15 December 1948 by a research student, Alec Robinson, had been reproduced in The Illustrated London News in June 1949.
Stored-program computer
A stored-program computer is one which stores program instructions in electronic memory. Often the definition is extended with the requirement that the treatment of programs and data in memory be interchangeable or uniform....
. It was built at the Victoria University of Manchester
Victoria University of Manchester
The Victoria University of Manchester was a university in Manchester, England. On 1 October 2004 it merged with the University of Manchester Institute of Science and Technology to form a new entity, "The University of Manchester".-1851 - 1951:The University was founded in 1851 as Owens College,...
by Frederic C. Williams
Frederic Calland Williams
Sir Frederic Calland Williams CBE, FRS , known as 'Freddie Williams', was an English engineer....
, Tom Kilburn
Tom Kilburn
Tom Kilburn CBE, FRS was an English engineer. With Freddie Williams he worked on the Williams Tube and the world's first stored-program computer, the Small-Scale Experimental Machine , while working at the University of Manchester.-Computer engineering:Kilburn was born in Dewsbury, Yorkshire and...
and Geoff Tootill, and ran its first program on 21 June 1948.
The machine was not intended to be a practical computer but was instead designed as a testbed
Testbed
A testbed is a platform for experimentation of large development projects. Testbeds allow for rigorous, transparent, and replicable testing of scientific theories, computational tools, and new technologies.The term is used across many disciplines to describe a development environment that is...
for the Williams tube
Williams tube
The Williams tube or the Williams-Kilburn tube , developed in about 1946 or 1947, was a cathode ray tube used to electronically store binary data....
, an early form of computer memory. Although considered "small and primitive" by the standards of its time, it was the first working machine to contain all of the elements essential to a modern electronic computer. As soon as the SSEM had demonstrated the feasibility of its design, a project was initiated at the university to develop it into a more usable computer, the Manchester Mark 1
Manchester Mark 1
The Manchester Mark 1 was one of the earliest stored-program computers, developed at the Victoria University of Manchester from the Small-Scale Experimental Machine or "Baby" . It was also called the Manchester Automatic Digital Machine, or MADM...
. The Mark 1 in turn quickly became the prototype for the Ferranti Mark 1, the world's first commercially available general-purpose computer.
The SSEM had a 32-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...
word length and a memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
of 32 words. As it was designed to be the simplest possible stored-program computer, the only arithmetic operations implemented in hardware were subtraction
Subtraction
In arithmetic, subtraction is one of the four basic binary operations; it is the inverse of addition, meaning that if we start with any number and add any number and then subtract the same number we added, we return to the number we started with...
and negation
Negation (algebra)
Negation is the mathematical operation that reverses the sign of a number. Thus the negation of a positive number is negative, and the negation of a negative number is positive. The negation of zero is zero...
; other arithmetic operations were implemented in software. The first of three programs written for the machine found the highest proper divisor of 218 (262,144), a calculation that was known would take a long time to run—and so prove the computer's reliability—by testing every integer from 218 − 1 downwards, as division was implemented by repeated subtraction of the divisor. The program consisted of 17 instructions and ran for 52 minutes before reaching the correct answer of 131,072, after the SSEM had performed 3.5 million operations (for an effective CPU speed of 1.1 kIPS
Instructions per second
Instructions per second is a measure of a computer's processor speed. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches, whereas realistic workloads typically lead to significantly lower IPS values...
).
Background
Charles Babbage
Charles Babbage, FRS was an English mathematician, philosopher, inventor and mechanical engineer who originated the concept of a programmable computer...
's Analytical Engine
Analytical engine
The Analytical Engine was a proposed mechanical general-purpose computer designed by English mathematician Charles Babbage. It was first described in 1837 as the successor to Babbage's difference engine, a design for a mechanical calculator...
in the 1830s. A century later, in 1936, mathematician Alan Turing
Alan Turing
Alan 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...
published his description of what became known as a Turing machine
Turing machine
A Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...
, a theoretical concept intended to explore the limits of mechanical computation. Turing was not imagining a physical machine, but a person he called a "computer", who acted according to the instructions provided by a tape on which symbols could be read and written sequentially as the tape moved under a tape head. Turing proved that if an algorithm can be written to solve a mathematical problem, then a Turing machine can execute that algorithm.
Konrad Zuse
Konrad Zuse
Konrad Zuse was a German civil engineer and computer pioneer. His greatest achievement was the world's first functional program-controlled Turing-complete computer, the Z3, which became operational in May 1941....
's Z3 was the world's first working programmable, fully automatic computer, with binary digital arithmetic logic, but it lacked the conditional branching of a Turing machine. On 12 May 1941, it was successfully presented to an audience of scientists of the Deutsche Versuchsanstalt für Luftfahrt ("German Laboratory for Aviation") in Berlin. The Z3 stored its program on an external tape, but it was electromechanical rather than electronic. The Colossus of 1943 was the first electronic computing device, but was not a general-purpose machine.
The ENIAC
ENIAC
ENIAC was the first general-purpose electronic computer. It was a Turing-complete digital computer capable of being reprogrammed to solve a full range of computing problems....
(1946) was the first machine that was both electronic and general purpose. It was Turing complete, with conditional branching, and programmable to solve a wide range of problems, but its program was held in the state of switches in patchcords, not in memory, and it could take several days to reprogram. Researchers such as Turing and Konrad Zuse
Konrad Zuse
Konrad Zuse was a German civil engineer and computer pioneer. His greatest achievement was the world's first functional program-controlled Turing-complete computer, the Z3, which became operational in May 1941....
investigated the idea of using the computer's memory to hold the program as well as the data it was working on, but it was mathematician John von Neumann
John von Neumann
John von Neumann was a Hungarian-American mathematician and polymath who made major contributions to a vast number of fields, including set theory, functional analysis, quantum mechanics, ergodic theory, geometry, fluid dynamics, economics and game theory, computer science, numerical analysis,...
who became widely credited with defining that computer architecture, still used in almost all computers.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
signals had developed a form of delay line memory
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...
, the first practical application of which was the mercury delay line, developed by J. Presper Eckert
J. Presper Eckert
John Adam Presper "Pres" Eckert Jr. was an American electrical engineer and computer pioneer. With John Mauchly he invented the first general-purpose electronic digital computer , presented the first course in computing topics , founded the first commercial computer company , and...
. Radar transmitters send out regular brief pulses of radio energy, the reflections from which are displayed on a CRT screen. As operators are usually interested only in moving targets, it was desirable to filter out any distracting reflections from stationary objects. The filtering was achieved by comparing each received pulse with the previous pulse, and rejecting both if they were identical, leaving a signal containing only the images of any moving objects. To store each received pulse for later comparison it was passed through a transmission line, delaying it by exactly the time between transmitted pulses.
Turing joined the National Physical Laboratory
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...
(NPL) in October 1945, by which time scientists within the Ministry of Supply
Ministry of Supply
The Ministry of Supply was a department of the UK Government formed in 1939 to co-ordinate the supply of equipment to all three British armed forces, headed by the Minister of Supply. There was, however, a separate ministry responsible for aircraft production and the Admiralty retained...
had concluded that Britain needed a National Mathematical Laboratory to coordinate machine-aided computation. A Mathematics Division was set up at the NPL, and on 19 February 1946 Alan Turing presented a paper outlining his design for an electronic stored-program computer to be known as the Automatic Computing Engine (ACE). This was one of several projects set up in the years following the Second World War with the aim of constructing a stored-program computer. At about the same time, EDVAC
EDVAC
EDVAC was one of the earliest electronic computers. Unlike its predecessor the ENIAC, it was binary rather than decimal, and was a stored program computer....
was under development at the University of Pennsylvania
University of Pennsylvania
The University of Pennsylvania is a private, Ivy League university located in Philadelphia, Pennsylvania, United States. Penn is the fourth-oldest institution of higher education in the United States,Penn is the fourth-oldest using the founding dates claimed by each institution...
's Moore School of Electrical Engineering
Moore School of Electrical Engineering
The Moore School of Electrical Engineering at the University of Pennsylvania came into existence as a result of an endowment from Alfred Fitler Moore on June 4, 1923. It was granted to Penn's School of Electrical Engineering, located in the Towne Building...
, and the University of Cambridge Mathematical Laboratory was working on EDSAC
EDSAC
Electronic Delay Storage Automatic Calculator was an early British computer. The machine, having been inspired by John von Neumann's seminal First Draft of a Report on the EDVAC, was constructed by Maurice Wilkes and his team at the University of Cambridge Mathematical Laboratory in England...
.
The NPL did not have the expertise to build a machine like ACE, so they contacted Tommy Flowers
Tommy Flowers
Thomas "Tommy" Harold Flowers, MBE was an English engineer. During World War II, Flowers designed Colossus, the world's first programmable electronic computer, to help solve encrypted German messages.-Early life:...
at the General Post Office
General Post Office
General Post Office is the name of the British postal system from 1660 until 1969.General Post Office may also refer to:* General Post Office, Perth* General Post Office, Sydney* General Post Office, Melbourne* General Post Office, Brisbane...
's (GPO) Dollis Hill Research Laboratory
Post Office Research Station
The Post Office Research Station at Dollis Hill, London, was first established in 1921 and opened by Prime Minister Ramsay MacDonald in 1933.In 1943 the world's first programmable electronic computer, Colossus Mark 1 was built by Tommy Flowers and his team, followed in 1944 and 1945 by nine...
. Flowers, the designer of Colossus, the world's first programmable electronic computer, was committed elsewhere and was unable to take part in the project, although his team did build some mercury delay lines for ACE. The Telecommunications Research Establishment
Telecommunications Research Establishment
The Telecommunications Research Establishment was the main United Kingdom research and development organization for radio navigation, radar, infra-red detection for heat seeking missiles, and related work for the Royal Air Force during World War II and the years that followed. The name was...
(TRE) was also approached for assistance, as was Maurice Wilkes at the University of Cambridge Mathematical Laboratory.
The government department responsible for the NPL decided that, of all the work being carried out by the TRE on its behalf, ACE was to be given the top priority. NPL's decision led to a visit by the superintendent of the TRE's Physics Division on 22 November 1946, accompanied by Frederic C. Williams
Frederic Calland Williams
Sir Frederic Calland Williams CBE, FRS , known as 'Freddie Williams', was an English engineer....
and A. M. Uttley, also from the TRE. Williams led a TRE development group working on CRT stores for radar applications, as an alternative to delay lines. He had already accepted a professorship at the University of Manchester
Victoria University of Manchester
The Victoria University of Manchester was a university in Manchester, England. On 1 October 2004 it merged with the University of Manchester Institute of Science and Technology to form a new entity, "The University of Manchester".-1851 - 1951:The University was founded in 1851 as Owens College,...
, and most of his circuit technicians were in the process of being transferred to the Department of Atomic Energy. The TRE agreed to second a small number of technicians to work under Williams' direction at the university, and to support another small group working with Uttley at the TRE.
Williams tube
Although early computers such as CSIRACCSIRAC
CSIRAC , originally known as CSIR Mk 1, was Australia's first digital computer, and the fourth stored program computer in the world. It was first to play digital music and is one of only a few surviving first-generation computers .The CSIRAC was constructed by a team led by Trevor Pearcey and...
made successful use of mercury delay line memory, the technology had several drawbacks; it was heavy, it was expensive, and it did not allow data to be accessed randomly. In addition, because data was stored as a sequence of acoustic waves propagated through a mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
column, the device's temperature had to be very carefully controlled, as the velocity of sound through a medium varies with its temperature. Williams had seen an experiment at Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
demonstrating the effectiveness of cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s (CRT) as an alternative to the delay line for removing ground echoes from radar signals. While working at the TRE, shortly before he joined the University of Manchester in December 1946, he had developed a form of electronic memory known as the Williams tube
Williams tube
The Williams tube or the Williams-Kilburn tube , developed in about 1946 or 1947, was a cathode ray tube used to electronically store binary data....
based on a standard CRT, the first random-access
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
digital storage device. The Manchester Small-Scale Experimental Machine (SSEM) was designed to prove that the Williams tube was a practical storage device, by testing that data held within it could be continuously updated at the speed necessary for use in a computer.
For use in a binary
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
digital computer, the tube had to be capable of storing either one of two states at each of its memory locations, corresponding to the binary digits (bits) 0 and 1. It exploited the positive or negative electrostatic charge generated by displaying either a dash or a dot at any position on the CRT screen, a phenomenon known as secondary emission
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
. A dash generated a positive charge, and a dot a negative charge, either of which could be picked up by a detector plate in front of the screen; a negative charge represented 0, and a positive charge 1. The charge dissipated in about 0.2 seconds, but it could be automatically refreshed from the data picked up by the detector.
The Williams tube was initially based on the CV1131, a commercially available 12 inches (30.5 cm) diameter CRT, but a smaller 6 inches (15.2 cm) tube, the CV1097, was used in the SSEM.
Development and design
Tom Kilburn
Tom Kilburn CBE, FRS was an English engineer. With Freddie Williams he worked on the Williams Tube and the world's first stored-program computer, the Small-Scale Experimental Machine , while working at the University of Manchester.-Computer engineering:Kilburn was born in Dewsbury, Yorkshire and...
on secondment. By the autumn of 1947 the pair had increased the storage capacity of the Williams tube from one bit to 2,048, arranged in a 64 by 32-bit array, and demonstrated that it was able to store those bits for four hours. Engineer Geoff Tootill joined the team on loan from TRE in September 1947, and remained on secondment until April 1949.
Max Newman
Max Newman
Maxwell Herman Alexander "Max" Newman, FRS was a British mathematician and codebreaker.-Pre–World War II:Max Newman was born Maxwell Neumann in Chelsea, London, England, on 7 February 1897...
had been appointed to the Chair of Pure Mathematics at Manchester University in 1945. During the Second World War he had worked as a cryptanalyst at Bletchley Park
Bletchley Park
Bletchley Park is an estate located in the town of Bletchley, in Buckinghamshire, England, which currently houses the National Museum of Computing...
, and had led the team which in 1943 produced the first of the Colossus code-breaking computers. Although Newman played no active role in the development of the SSEM, or any of the subsequent Manchester computers
Manchester computers
The Manchester computers were an innovative series of stored-program electronic computers developed during the 30-year period between 1947 and 1977 by a small team at the University of Manchester, under the leadership of Tom Kilburn...
, he was generally supportive and enthusiastic about the project, and arranged for the acquisition of war-surplus supplies for its construction, including GPO
General Post Office
General Post Office is the name of the British postal system from 1660 until 1969.General Post Office may also refer to:* General Post Office, Perth* General Post Office, Sydney* General Post Office, Melbourne* General Post Office, Brisbane...
metal racks from Bletchley.
By June 1948 the SSEM had been built and was working. It was 17 feet (5.2 m) in length, 7 in 4 in (2.24 m) tall, and weighed almost 1 LT. The machine contained 550 valves
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...
—300 diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
s and 250 pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
s—and had a power consumption of 3500 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s. The arithmetic unit was built using EF50 pentode valves, which had been widely used during wartime. The SSEM used one Williams tube to provide 32 by 32-bit words of random-access memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
(RAM), a second to hold a 32-bit 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...
in which the intermediate results of a calculation could be stored temporarily, and a third to hold the current program instruction along with its address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
in memory. A fourth CRT, without the storage electronics of the other three, was used as the output device, able to display the bit pattern of any selected storage tube.
Each 32-bit word of RAM could contain either a program instruction or data. In a program instruction, bits 0–12 represented the memory address of the operand
Operand
In mathematics, an operand is the object of a mathematical operation, a quantity on which an operation is performed.-Example :The following arithmetic expression shows an example of operators and operands:3 + 6 = 9\;...
to be used, and bits 13–15 specified the operation
Opcode
In computer science engineering, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question...
to be executed, such as storing a number in memory; the remaining 16 bits were unused. The SSEM's single operand architecture meant that the second operand of any operation was implicit: the accumulator or the program counter (instruction address); program instructions specified only the address of the data in memory.
A word in the computer's memory could be read, written, or refreshed, in 360 microseconds. An instruction took four times as long to execute as accessing a word from memory, giving an instruction execution rate of about 700 per second. The main store was refreshed continuously, a process which took 20 milliseconds to complete, as each of the SSEM's 32 words had to be read and then refreshed in sequence.
The SSEM represented negative numbers using two's complement
Two's complement
The two's complement of a binary number is defined as the value obtained by subtracting the number from a large power of two...
, as most computers still do. In that representation, the value of the most significant bit
Most significant bit
In computing, the most significant bit is the bit position in a binary number having the greatest value...
denotes the sign of a number; positive numbers have a zero in that position and negative numbers a one. Thus the range of numbers that could be held in each 32-bit word was −231 to +231 − 1 (decimal: −2,147,483,648 to +2,147,483,647).
Programming
The SSEM's three bit instruction setInstruction set
An 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...
allowed a maximum of eight (23) different instructions. In contrast to the modern convention, the machine's storage was arranged with the least significant digits
Endianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
to the left; thus a one was represented in three bits as "100", rather than the more conventional "001".
| Binary code | Original notation | Modern mnemonic | Operation |
|---|---|---|---|
| 000 | S, Cl | JMP S | Jump to the instruction at the address obtained from the specified memory address S (absolute unconditional jump) |
| 100 | Add S, Cl | JRP S | Jump to the instruction at the program counter plus (+) the relative value obtained from the specified memory address S (relative unconditional jump) |
| 010 | -S, C | LDN S | Take the number from the specified memory address S, negate it, and load it into the accumulator |
| 110 | c, S | STO S | Store the number in the accumulator to the specified memory address S |
| 001 or 101 |
SUB S | SUB S | Subtract the number at the specified memory address S from the value in accumulator, and store the result in the accumulator |
| 011 | Test | CMP | Skip next instruction if the accumulator contains a negative value |
| 111 | Stop | STP | Stop |
The awkward negative operations were a consequence of the SSEM's lack of hardware to perform any arithmetic operations except subtraction
Subtraction
In arithmetic, subtraction is one of the four basic binary operations; it is the inverse of addition, meaning that if we start with any number and add any number and then subtract the same number we added, we return to the number we started with...
and negation
Additive inverse
In mathematics, the additive inverse, or opposite, of a number a is the number that, when added to a, yields zero.The additive inverse of a is denoted −a....
. It was considered unnecessary to build an adder
Adder (electronics)
In electronics, an adder or summer is a digital circuit that performs addition of numbers.In many computers and other kinds of processors, adders are used not only in the arithmetic logic unit, but also in other parts of the processor, where they are used to calculate addresses, table indices, and...
before testing could begin as addition can easily be implemented by subtraction, i.e., x+y can be computed as −(−x−y). Therefore adding two numbers together, X and Y, required four instructions:
Programs were entered in binary form by stepping through each word of memory in turn, and using a set of 32 switches known as the input device to set the value of each bit of each word to either 0 or 1. The SSEM had no paper-tape reader or punch
Punched tape
Punched tape or paper tape is an obsolete form of data storage, consisting of a long strip of paper in which holes are punched to store data...
.
First programs
Divisor
In mathematics, a divisor of an integer n, also called a factor of n, is an integer which divides n without leaving a remainder.-Explanation:...
of 218 (262,144) by trying every integer from 218 − 1 downwards. The divisions were implemented by repeated subtractions of the divisor. The SSEM took 3.5 million operations and 52 minutes to produce the answer (131,072). The program used eight words of working storage in addition to its 17 words of instructions, giving a program size of 25 words.
Geoff Tootill wrote an amended version of the program the following month, and in mid-July Alan Turing—who had been appointed as a reader
Reader (academic rank)
The title of Reader in the United Kingdom and some universities in the Commonwealth nations like Australia and New Zealand denotes an appointment for a senior academic with a distinguished international reputation in research or scholarship...
in the mathematics department at Manchester University in September 1948—submitted the third program, to carry out long division. Turing had by then been appointed to the nominal post of Deputy Director of the Computing Machine Laboratory at the University, although the laboratory did not become a physical reality until 1951.
Later developments
Williams and Kilburn reported on the SSEM in a letter to the Journal NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
, published in September 1948. The machine's successful demonstration quickly led to the construction of a more practical computer, the Manchester Mark 1
Manchester Mark 1
The Manchester Mark 1 was one of the earliest stored-program computers, developed at the Victoria University of Manchester from the Small-Scale Experimental Machine or "Baby" . It was also called the Manchester Automatic Digital Machine, or MADM...
, work on which began in August 1948. The first version was operational by April 1949, and it in turn led directly to the development of the Ferranti Mark 1, the world's first commercially available general-purpose computer.
In 1998, a working replica of the SSEM, now on display at the Museum of Science and Industry in Manchester
Museum of Science and Industry in Manchester
The Museum of Science and Industry in Manchester, England, is a large museum devoted to the development of science, technology, and industry with emphasis on the city's achievements in these fields...
, was built to celebrate the 50th anniversary of the running of its first program. Demonstrations of the machine in operation are held regularly at the museum. In 2008 an original panoramic photograph of the entire machine was discovered at the University of Manchester. The photograph, taken on 15 December 1948 by a research student, Alec Robinson, had been reproduced in The Illustrated London News in June 1949.
External links
- Computer 50 – A website celebrating the 50th anniversary of the SSEM in 1998.
- Digital60 – A website celebrating the 60th anniversary of the SSEM in 2008.
- A brief history of the Small-Scale Experimental Machine
- Manchester Baby Simulator software
- BabyRace – Run original program on a mobile phone and compare the performance with the Small-Scale Experimental Machine
- Updated, Photo Realistic Manchester Baby Simulator software
- BBC article on Baby
- Listen to an oral history interview with Geoff Tootill, one member of the team that designed and built the Manchester Small-Scale Experimental Machine, recorded for An Oral History of British Science at the British Library.