viewpoints VDOI:10.1145/2209249.2209277 Martin Campbell-Kelly Historical Reflections Alan Turing’s Other Universal Machine Reflections on the Turing ACE computer and its influence. LL COMPUTER SCIENTISTS know about the Univer- sal Turing Machine, the theoretical construct the British genius Alan Turing Adescribed in his famous 1936 paper on the Entscheidungsproblem (the halting problem). The Turing Machine is one of the foundation stones of theoreti- cal computer science. Much less well known is the practical stored program computer he proposed after the war in February 1946. The computer was called the ACE—Automatic Comput- ing Engine—a name intended to evoke the spirit of Charles Babbage, the pio- neer of computing machines in the previous century. Almost all post-war electronic com- puters were, and still are, based on the famous EDVAC Report written by John von Neumann in June 1945 on behalf The Pilot ACE, May 1950. Jim Wilkinson (center right) and Donald Davies (right). of the computer group at the Moore School of Electrical Engineering at the roles. Computers were in the air and of one, and over the next few months University of Pennsylvania. Von Neu- universities at Manchester, Cam- he evolved the design of the ACE. His mann was very familiar with Turing’s bridge, and elsewhere established report was formally presented to the 1936 Entscheidungsproblem paper. In electronic computer projects. Out- NPL’s executive committee in Febru- 1937, Turing was a research assistant side the academic sphere, in Lon- ary 1946. at the Institute for Advanced Study at don, the National Physical Laboratory Although the ACE drew heavily on Princeton University, where von Neu- (NPL—Britain’s equivalent of the Na- the EDVAC Report, it had many novel mann was a professor of mathematics. tional Bureau of Standards) started features and was a design of great Beyond these known facts, however, it a mathematics division to provide a originality. When he heard about the is not possible to say how much influ- computing service for industry. With EDSAC that was being planned at ence the Turing Machine had on the a staff of 40 and equipped with desk Cambridge University by Maurice Wil- design of the EDVAC. But there is no calculating machines, punched card kes, which was based very directly on doubt the ACE was heavily influenced equipment, and a differential analyz- the EDVAC, Turing was unimpressed. by the EDVAC Report. er, it was an impressive organization. He wrote in a private note: “The ‘code’ The war in Europe ended in May In October 1945, Turing was brought which [Wilkes] suggests is however 1945 and the institutions of Britain in to establish an electronic com- very contrary to the line of develop- began to get back to their peacetime puter project. He was a department ment here, and much more in the JULY 2012 | VOL. 55 | NO. 7 | COMMUNICATIONS OF THE ACM 31 viewpoints ACE Design The ACE was designed to fully exploit Turing’s brilliant the potential of a mercury delay-line innovation in the memory. The delay line was a spin-off from echo-cancellation devices in ra- ACE was to eliminate dar, and in 1946 it was the only proven delay-line waiting memory technology. When used in the main memory of a computer, the delay time by what was line consisted of a five-foot mercury- later called optimum filled steel tube. Digital data was con- verted to acoustic energy at one end of programming. the tube and converted back to electri- cal signals at the other end. By feeding the output back into the input, digi- tal data could be stored indefinitely. A five-foot tube had an acoustic de- lay of about one millisecond and the typical pulse duration of a computer American tradition of solving one’s was one microsecond; hence the de- problems by means of much equip- lay line could store about 1,000 bits ment rather than by thought. I should of information (usually 1,024 bits, of imagine that to put his code (which is course). This was enough to store sev- advertised as ‘reduced to the simplest eral instructions or numbers. Apart possible form’) into effect would re- from its high cost and unreliability, quire a very much more complex con- the mercury delay line memory also trol circuit than is proposed in our full- had the problem of latency. That is, ACM’s size machine.” when an instruction or number was interactions Wilkes took the view that build- needed, the machine had to wait un- ing the EDSAC to a conventional de- til it emerged from the delay line—an magazine explores sign would be enough of a challenge. average time of half a millisecond. critical relationships Moreover, the EDSAC would be several For this reason most delay-line based between experiences, people, thousand times faster than existing computers spent more time waiting techniques and users would be best for instructions and numbers than and technology, showcasing served by having a conventional ma- processing them. emerging innovations and industry chine sooner rather than a novel de- Turing’s brilliant innovation in leaders from around the world sign later. There was merit in both of the ACE was to eliminate this waiting their positions. time by what was later called optimum across important applications of design thinking and the broadening field of the interaction design. Our readers represent a growing community of practice that is of increasing and vital global importance. NIVERSITY U HESTER HESTER C UTING, MAN UTING, MP CO F ISTORY O ISTORY H OR THE F HIVE http://www.acm.org/subscribe C R A ATIONAL ATIONAL Control desk of the English Electric DEUCE, 1955. N 32 COMMUNICATIONS OF THE ACM | JULY 2012 | VOL. 55 | NO. 7 viewpoints programming. On a conventional eral months. Then in 1947 an Ameri- machine instructions were stored in can, Harry Husky, who had been a consecutive locations, so that latency member of the Moore School com- effects dramatically slowed up the puter group, began a sabbatical year machine. In the ACE design, however, at the NPL. He proposed building a each instruction specified the location small prototype of the ACE, which he of its successor. In this way instruc- called the Test Assembly. Still, little tions (and numbers) could be placed progress was made and Turing left optimally in the memory so that they the NPL in September 1947 for a sab- emerged from a delay line just as they batical year at King’s College, Cam- were needed. The optimum coding bridge University. He never returned idea showed a rather direct connec- to the NPL. At last in early 1949, work tion to the “instruction tables” of the really got going on another small ver- Turing Machine, in which each or- sion known as the Pilot ACE and the der specified the next to be obeyed. development logjam was broken by This simple idea alone would make Donald Davies, a new recruit to the the ACE about three times faster than NPL. Davies was an outstanding sci- other delay-line based computers. entist and administrator. He later Optimum coding, it should be noted, became head of computer science at A brochure for the Bendix G-15 computer, was much more difficult than conven- the NPL and was one of the inventors introduced in 1956. tional programming, but for a decade of packet switching technology. it was the way to get maximum perfor- The Pilot ACE sprang into life in tives. The most important was the mance from delay-line or drum-based May 1950. It had just 10 delay lines, but Bendix G-15 introduced in 1956. The machines. when augmented with a drum store it machine was designed for the West But there was more. In the ACE de- was a highly capable computing ma- Coast’s Bendix Aviation Corporation sign Turing avoided having a single chine. Benchmarks showed that it was by Harry Husky, who had returned to central register (known as the accu- five to 10 times faster than contem- the U.S. and joined the faculty of the mulator in EDVAC-type machines) in porary machines. Moreover, the Pilot University of California, Berkeley. Op- which all computation took place. In- ACE had just 800 tubes compared with timum coding gave the machine an stead, he had separate registers for ad- 3,000 in the Cambridge EDSAC and edge over rival machines, and the G-15 dition and subtraction, multiplication, 3,500 in the Manchester Mark I. became a workhorse for the American logical operations, shifting, and so engineering industry. About 400 were on. To a degree Turing was anticipat- ACE Derivatives eventually sold. ing what would later be called the von The English Electric Company, a Neumann bottleneck. manufacturer of airplanes and elec- Conclusion trical equipment and a contractor to As to the ACE itself, the full-scale, ACE Construction the NPL, decided make a copy of the 200-delay-line machine was finally Turing proposed that the ACE would Pilot ACE; unsurprisingly, the ma- completed in 1958. Sadly, by the time contain 200 delay lines in the main chine was named the DEUCE. This it was built the heyday of delay-line memory and estimated the con- set the ball rolling—English Electric’s storage was over, and new comput- structional cost at £11,200 (about competitors decided they would like a ers were using random access core $45,000). Turing was a better math- machine too and the company found memory. Even the NPL admitted the ematician than an engineer and his itself in the computer business.