Chapter 6 ÷ The Whirlwind I computer 143
Fig. 9. View of test control.
arithmetic element before electrostatic storage was available but also for testing electrostatic storage itself. When not in use for test purposes test storage earns its keep as part of the terminal equipment system. The toggle-switches hold a standard read-in program; the flip-flop registers are used as in-out registers for special purposes.
Checking
Logical checking facilities built into WWI are rather inconsistent. A complete bus transfer checking system has been provided, duplicate checking of some terminal equipment is permitted, but little else is thoroughly checked. We felt that it was worthwhile to thoroughly check some substantial portion of the machine. This portion would then serve as a prototype for studying the tube circuitry used throughout the machine. We did not feel it was worthwhile to check all the machine, a procedure that requires a great deal of added equipment and logical complexity plus a substantial loss in computing speed.
Operating experience has shown us that it is not worthwhile to provide detailed logical checking of a machine. In a new machine we would leave out the transfer checking. The amount of information and security given by the detailed checking system is not enough to warrant the expense of building and maintaining it.
This decision is based on the expectation that a computing machine should operate 95 per cent of total time or better and that the average time between random failures should be of the order of 5 to 10 hours or approximately 109 operations.
In our opinion the way to achieve the extremely high reliability needed in some real-time control problems is to provide three or more identical but distinct machines, thus obtaining error correction as well as detection, plus such features as standby, safety, and damage control. Even so the failure probability of each machine must be kept low by proper design, marginal checking, and preventive maintenance.
Extremely high reliability means a reliability far beyond that achieved in existing machines and not conveniently represented as a per cent. Consider a system consisting of three machines, each operable 98 per cent of the time and each averaging 10 hours between random errors.
One machine will be out of operation 1/2 hour per day.
Two machines will be out of operation 1/4 hour per month.
All three machines will be out of operation 4 minutes per year. Furthermore undetected random errors might occur on the average of once a year. Such reliability is needed in some systems.
Our decision to omit detailed checking does not extend to checking devices intended to detect programming errors. Devices to check for overflow from the arithmetic element or for nonexistent order configurations are necessary. Programmers make many mistakes. Techniques for dealing with programming errors are very important and need future development.
Terminal equipment
At the present time, Whirlwind is using the following terminal equipment:
1 A photoelectric paper tape reader
2 Mechanical paper tape readers and punches
3 Mechanical typewriters
4 Oscilloscope displays 5 to 16 inches in diameter with phosphors of various persistencies including a computer-controlled scope camera
5 Inputs from various analogue equipments needed for control studies
6 Outputs to analogue equipment
To be added during the next year:
1 Magnetic Tape (units by Raytheon). One such unit is now being integrated with machine.
2 Magnetic drums (units by Engineering Research Associates, Inc.).
3 Many more analogue inputs and outputs.