The Whirlwind computer was developed at the Massachusetts Institute of Technology. It is the first computer that operated in real time, used video displays for output, and the first that was not simply an electronic replacement of older mechanical systems. Its development led directly to the United States Air Force's Semi Automatic Ground Environment (SAGE) system, and indirectly to almost all business computers and minicomputers in the 1960s.
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During World War II, the U.S. Navy 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 like 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.
A short study by the MIT Servomechanisms Laboratory concluded that such a system was certainly possible. The Navy decided to fund development under Project Whirlwind, 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.
In 1945 Perry Crawford, another member of the MIT team, saw a demonstration of ENIAC and suggested that a digital computer was the solution. Such a machine would allow the accuracy of the simulation 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.
Up until this point all computers constructed were dedicated to single tasks, 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.
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 was essentially sixteen times as fast as other machines. Today almost all CPUs do 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 5 bits — and so it was 16-bits.[1] Nevertheless the small word size led John von Neumann to conclude the machine would be worthless.
The Whirlwind design incorporated a control store driven by a master clock. Each step of the clock selected a signal line 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. The design inspired Maurice Wilkes to develop the concept of microprogramming.
Construction of the machine started the next year, an effort that employed 175 people including 70 engineers and technicians. Whirlwind took 3 years to build and first went online on April 20, 1951. The project's budget was $1 million a year, and 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.
The speed of the original design (20 KIPS) turned out to be too slow to be very useful, and most of the problem was attributed to the fairly slow speed of the Williams tubes for main memory of 256 words. Forrester started looking at replacements, first using magnetic tape formed into spirals, even at one time considering using a 3-D array of neon lamps, and eventually creating core memory.
Before its conversion to core memory, the machine could add two numbers in 49 μs and multiply them in 61 μs. After conversion, addition time was 8 μs, multiplication 25.5 μs, and a division 57 μs. Speed was roughly doubled (40 KIPS) when the new version was completed in 1953.
The Cape Cod System was designed to demonstrate a computerized air defence system, covering southern New England. Signals from three long range (AN/FPS-3) radars, eleven gap-filler radars, and three height-finding radars were converted from analog to digital format and transmitted over telephone lines to the Whirlwind I computer in Cambridge, Massachusetts. The Cape Cod System verified that the new core-based machine was fast enough for use in SAGE, and an industrial effort was started in order to mass-produce the AN/FSQ-7 computers for this role.
After Whirlwind was completed and running, a design for a larger and faster machine to be called Whirlwind II was begun. But the design soon became too much for MIT's resources. It was decided to shelve the Whirlwind II design without building it and concentrate MIT's resources on programming and applications for the original machine, now called Whirlwind I. IBM based their production designs, the AN/FSQ-7, on the stillborn Whirlwind II. Thus the AN/FSQ-7 is sometimes incorrectly referred to as "Whirlwind II", even though they were not the same machine or design.
Whirlwind I ran in a support role for SAGE until June 30, 1959. A member of the project team, Bill Wolf, then rented the machine for a dollar a year until 1973. Ken Olsen and Robert Everett then saved the machine from the scrap heap and it became the basis for the Digital Computer Museum, which would later become The Computer Museum on Boston's Museum Wharf. Today it is in the collection of the Computer History Museum in Mountain View, California, and a portion of the machine is currently on display. As of February 2009, one of Whirlwind's core memory units is on display at the Charles River Museum of Industry in Waltham, Massachusetts.
The Whirlwind used approximately 5000 vacuum tubes. An effort was also started to convert the Whirlwind design to a transistorized form, led by Ken Olsen and known as the TX-0. TX-0 was very successful and plans were made to make an even larger version known as TX-1. However this project was far too ambitious and had to be scaled back to a smaller version known as TX-2. Even this version proved troublesome, and Olsen left in mid-project to start Digital Equipment Corporation (DEC). DEC's PDP-1 was essentially a collection of TX-0 and TX-2 concepts in a smaller package.
MIT's Barta Building (now building N42) which housed Whirlwind during the project's lifetime, is now home to MIT's campus-wide IT department, Information Services & Technology. In 1997-8, the building was restored to its original exterior design, and can be found at 211 Massachusetts Ave, Cambridge.[2]