The Whirlwind I computer1
R. R. Everett
Project Whirlwind is a high-speed computer activity sponsored at the Digital Computer Laboratory, formerly a part of the Servo-mechanisms Laboratory, of the Massachusetts Institute of Technology (M.I.T.) by the Office of Naval Research (O.N.R.) and the United States Air Force. The project began in 1945 with the assignment of building a high-quality real-time aircraft simulator. Historically, the project has always been primarily interested in the fields of real-time simulation and control; but since about the beginning of 1947 most of its efforts have been devoted to the design and construction of the digital computer known as Whirlwind I (WWI). This computer has been in operation for about 1 year and an increasing proportion of project effort now is going into application studies.
Applications for digital computers are found in many branches of science, engineering, and business. Although any modern general-purpose digital computer can be applied to all these fields, a machine is generally designed to be most suited to some particular area. Whirlwind I was designed for use in control and simulation work such as air traffic control, industrial process control, and aircraft simulation. This does not mean that Whirlwind will not be used on applications other than control. About one-half the available computing time for the next year will be assigned to engineering and scientific calculation including research in such uses supported by the O.N.R. through the M.I.T. Committee on Machine Methods for Computation.
These control and simulation problems result in a specialized emphasis on computer design.
Short register length
WWL has 16 binary digits and the control problems are usually very simple mathematically. Furthermore, the computer is almost always part of a feedback rather than an open-ended system. Consequently, roundoff errors are seldom troublesome and the register length can be shortened to something comparable to the sensitivity of the physical quantities involved, perhaps five decimal places or less.
WWI has a register length of 16 binary digits including sign or about four and one-half decimals. The register length was chosen as the minimum that would provide a usable single-address order, in this case five binary digits for instruction and 11 binary digits for address. In a future machine we would probably increase this register length to 20 or 24 binary digits to get additional order flexibility; the increased numerical precision is less important.
For scientific and engineering calculation, greater than 16-digit precision is often required. There is available a set of multiple-length and floating point subroutines which make the use of greater precision very easy. It is true that these subroutines are slow, bringing effective machine speed down to about that obtained by acoustic memory machines. It is much more efficient occasionally to waste computing time this way than continuously to waste a large part of the storage and computing equipment of the machine by providing an unnecessarily long register.
High operating speed
WWI performs 20,000 single-address operations per second. Control and simulation problems require very high speeds. The necessary calculations must be carried out in real time; the more complex the controlled system is, the faster the computer must be. There is no practical upper limit to the computing speed that could be used if available.
Where the problems are large enough, and these problems are, one high-speed machine is much better than two simpler machines of half the speed. Communication between machines presents many of the same problems that communication between human beings presents.
Great effort was put into WWI to obtain high speed. The target speed was 50,000 single-address operations per second, and all parts of the machine except storage meet this requirement. The actual WWI present operating speed of 20,000 single-address operations per second is on the lower edge of the desired speed range.
Large internal storage
WWL now has 1,280 registers. A large amount of high-speed internal storage is needed since it is not in general possible to use slow auxiliary storage because of the time factor. In many cases a magnetic drum can be useful since its access time is short com-
1AIEE-IRE Conf., 70-74 (1951)