Magnetic drums are a development of wire recording technology originated by Telefunken of Germany, except that drums are used to record binary data instead of analogue audio¹. Binary information written in bands on the magnetic surface of the drum consists in the instructions and data for simulating a flight environment.

A magnetic drum is about the size of a five-gallon drum of oil. It is driven by a synchronous motor running at a speed of 3,600 revolutions per minute, when connected to a 60 cycles per second (Hertz) supply of alternating current. Fixed pick-up heads can write and read to and from the drum memory; additional heads read the clock bands to provide the master timing signals to orchestrate the electronic circuitry of the simulation.

Instructions and data are prepared on punched cards or paper-tape, loaded onto the drum via a suitable reader; cards or paper-tape form the off-line backup programme and data storage archive. Programmes for the simulation are divided into bands on the drum, each band assigned a corresponding priority. Programme bands act in similar manner to a Scheduler; for example— critical parts of the flight programme are assigned to Band-1 for execution every revolution, whereas navigation programmes might be assigned to Band-4 for execution every fourth revolution of the drum.

Each band can store 4,096 (4K) instructions and data, which flow from the drum to the computational circuitry. Partial results are stored in volatile (scratch) memory built from registers consisting of logic gates constructed from discrete transistors.

Two examples of flight simulators described here, manufactured by Link of Binghampton, New York, were equipped with magnetic drum storage.

• Boeing 727-100, on a 3-axis motion system, powered by a Link Mark-I computer.
• McDonnel-Douglas DC9, on a 3-axis motion system, powered by a Link GP4B computer.

First purpose-built digital computer for flight simulation designed by Link Corporation was the Mark-I. It was equipped with a magnetic drum memory connected to an interface and computational system made from germanium technology transistors. Germanium transistors are liable to thremal runaway and are, therefore, sensitive to extremes of heat; air conditioning is essential to smooth operation; overheating destroys the power transistors.

Being economical, the United States of America military procurement specialists insisted that their computing machinery be capable of other uses when no longer needed to run a flight simulator². Of course, the Link Mark-I was a dedicated computer, incapable of any other purpose.

To ensure award of future government contracts, Link designed a general purpose computer that became known as the GP4B (General Purpose, Mark-4B). It still relied on drum storage, but its computational components were built from Motorola emitter-coupled logic, a silicon technology more robust in operation than the germanium components of the Mark-I. As with the Mark-I, programmes for the GP4B were prepared on punched cards then loaded onto the magnetic drum via a punched card reader.

Physically, these systems were large, built into standard 19-inch racks 6 feet tall. The Mark-I computer, interface circuitry, servo-mechanical systems, power supplies, and patch panels occupied two rows of cabinetry all housed in a small computer room.