Whirlwind I is a vacuum-tube computer developed by the MIT Servomechanisms Laboratory for the U.S. Navy. Operational in 1951, it was among the first digital electronic computers that operated in real-time for output, and the first that was not simply an electronic replacement of older mechanical systems. It was one of the first computers to calculate in bit-parallel (rather than bit-serial), and was the second to use magnetic-core memory after it had been proven to work for some months of use on the Memory Test Computer (MTC). (The MTC received a new core stack later so that some of the work for Whirlwind could instead be done on the MTC. The MTC was a complete, functional computer similar in capability to Whirlwind.) Its development led directly to the Whirlwind II design used as the basis for the United States Air Force SAGE air defense system, and indirectly to almost all business computers and minicomputers in the 1960s, particularly because of the mantra "short word length, speed, people."
Background During World War II, the U.S. Navy's Naval Research Lab approached MIT about the possibility of creating a computer to drive a flight simulator for training bomber crews. They envisioned a fairly simple system in which the computer would continually update a simulated instrument panel based on control inputs from the pilots. Unlike older systems such as the Link Trainer, the system they envisioned would have a considerably more realistic aerodynamics model that could be adapted to any type of plane. This was an important consideration at the time, when many new designs were being introduced into service. The Servomechanisms Lab in MIT building 32 conducted a short survey that concluded such a system was possible. The Navy's Office of Naval Research decided to fund development under Project Whirlwind (and its sister projects, Project Typhoon and Project Cyclone, with other institutions), and the lab placed Jay Forrester in charge of the project. They soon built a large analog computer for the task, but found that it was inaccurate and inflexible. Solving these problems in a general way would require a much larger system, perhaps one so large as to be impossible to construct. Judy Clapp was an early senior technical member of this team. Perry Crawford, another member of the MIT team, saw a demonstration of ENIAC in 1945. He then suggested that a digital computer would be the best solution. Such a machine would allow the accuracy of simulations to be improved with the addition of more code in the computer program, as opposed to adding parts to the machine. As long as the machine was fast enough, there was no theoretical limit to the complexity of the simulation. Until this point, all computers constructed were dedicated to single tasks, and run in batch mode. A series of inputs were set up in advance and fed into the computer, which would work out the answers and print them. This was not appropriate for the Whirlwind system, which needed to operate continually on an ever-changing series of inputs. Speed became a major issue: whereas with other systems it simply meant waiting longer for the printout, with Whirlwind it meant seriously limiting the amount of complexity the simulation could include.
Technical description
Design and construction By 1947, Forrester and collaborator Robert Everett completed the design of a high-speed stored-program computer for this task. Most computers of the era operated in bit-serial mode, using single-bit arithmetic and feeding in large words, often 48 or 60 bits in size, one bit at a time. This was simply not fast enough for their purposes, so Whirlwind included sixteen such math units, operating on a complete 16-bit word every cycle in bit-parallel mode. Ignoring memory speed, Whirlwind ("20,000 single-address operations per second" in 1951) was essentially sixteen times as fast as other machines. Today, almost all CPUs perform arithmetic in "bit-parallel" mode. The word size was selected after some deliberation. The machine worked by passing in a single address with almost every instruction, thereby reducing the number of memory accesses. For operations with two operands, adding for instance, the "other" operand was assumed to be the last one loaded. Whirlwind operated much like a reverse Polish notation calculator in this respect; except there was no operand stack, only an accumulator. The designers felt that 2048 words of memory would be the minimum usable amount, requiring 11 bits to represent an address, and that 16 to 32 instructions would be the minimum for another five bits — and so it was 16 bits. The Whirlwind design incorporated a control store driven by a master clock. Each step of the clock selected one or more signal lines in a diode matrix that enabled gates and other circuits on the machine. A special switch directed signals to different parts of the matrix to implement different instructions. In the early 1950s, Whirlwind I "would crash every 20 minutes on average." Whirlwind construction started in 1948, an effort that employed 175 people, including 70 engineers and technicians. The use of carry save multiplication appears to have been first introduced in the Whirlwind computer in the late 1940s. In the third quarter of 1949, the computer was advanced enough to solve an equation and display its solution on an oscilloscope,. (By 1953 an animated, interactive bouncing-ball physics demonstration had been developed for the display and then further evolved into a simple game.) Finally Whirlwind "successfully accomplished digital computation of interception courses" on April 20, 1951. The project's budget was approximately $1 million a year, which was vastly higher than the development costs of most other computers of the era. After three years, the Navy had lost interest. However, during this time the Air Force had become interested in using computers to help the task of ground controlled interception, and the Whirlwind was the only machine suitable to the task. They took up development under Project Claude. Whirlwind weighed 20,000 pounds (10 short tons; 9.1 t) and occupied over 2,000 square feet (190 m2).
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