whirlwind

 

revolving mass of air resulting from local atmospheric instability, such as that caused by intense heating of the ground by the sun on a hot summer day. Examples of whirlwinds are waterspouts, tornadoes, small whirls of dust or leaves, and the sand whirls of the desert, called dust devils or dust whirls.

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.

Techno-historical background

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.

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 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.

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. When the Air Force decided to construct the SAGE air defense system, IBM, the prime contractor for the AN/FSQ-7 computer based the machine's design more on the stillborn Whirlwind II design than on the original Whirlwind. Thus the AN/FSQ-7 is sometimes incorrectly referred to as "Whirlwind II", even though they were not the same machine or design.

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 was essentially sixteen times as fast as other machines. Today almost all machines work in this fashion, albeit with larger 32- or 64-bit words.

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 2000 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. 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 already lost interest. The USAF picked up the work under Project Claude.

The core of the machine

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 (or, more accurately, Williams-Kilburn tubes) used 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. Speed was roughly doubled (40 KIPS) as a result of using core when the new version was completed in 1953. The addition time was 49 microseconds and the multiplication time was 61 microseconds (before the main memory was converted to magnetic core).

After the magnetic core memory was installed, the Whirlwind became the fastest computer of its time. With the change it had an addition time of 8 microseconds, a multiplication time of 25.5 microseconds, and a division time of 57 microseconds (excluding memory access time). The access time had been about 16 microseconds for the CRT memory which was reduced to only 8 microseconds with the magnetic core.

The Cape Cod System and SAGE

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 first tests of the Cape Cod System, beginning in September 1953, used only simulated data, but later tests used U.S. Air Force B-47 Stratojet bombers as stand-ins for Soviet bombers, and real interceptors scrambled from four Air Force bases.

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. RCA was a front-runner, but IBM was eventually selected instead. They started production in 1957, along with a massive construction project to build the buildings, power and communications network needed to feed the SAGE systems with data.

What came of the Whirlwind?

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.

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.

References

• Redmond, Kent C.; Thomas M. Smith (1980). Project Whirlwind: The History of a Pioneer Computer. Bedford, MA: Digital Press. ISBN 0-932376-09-6. 

• Everett, R.R., and Swain, F.E. (1947). "Whirlwind I Computer Block Diagrams". Report R-127. MIT Servomechanisms Laboratory. Retrieved on 2006-06-21.

• John F. Jacobs, The SAGE Air Defense System: A Personal History (MITRE Corporation, 1986) also contains much material on the Whirlwind

See also

• History of computing hardware
• Laning and Zierler system

External links

• Computer Structures: Readings & Examples — The Whirlwind I computer
• Whirlwind documentation on Bitsavers.org
• Overview of the Lincoln Laboratory Ballistic Missile Defense Program (PDF)

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