Whirlwind (computer)
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.
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 Jerry 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 Airforce decided to construct the
SAGE air defence system,
IBM, the prime contractor for the
AN/FSQ-7 computer based the machine's design more on the never built Whirlwind II design than 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.
Design and construction
By
1947, Forrester and 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 amount 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. However it was around this time that the
USSR detonated their first
atomic bomb, and 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 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.
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.
*
* John F. Jacobs,
The SAGE Air Defense System: A Personal History (MITRE Corporation, 1986) also contains much material on the Whirlwind
*
Computer Structures: Readings & Examples — The Whirlwind I computer*
Whirlwind documentation on Bitsavers.org
*
Overview of the Lincoln Laboratory Ballistic Missile Defense Program (PDF)
