Access time. Isn't that a new idea? It means the interval of time an electronic computer needs to discover a definite piece of information in a store and to pass it on. Imagine that you are using a magnetic tape as an intermediate storage for a long and complicated calculating operation. Three hundred intermediate results, one after the other, would be taken up by the tape in the twinkling of an eye. But the computer may need same of these intermediate results to proceed with its calculation. First of all the seventeenth. Then the two hundred and thirty eighth. Then the third. Finally the two hundred and eighty fifth ...
Even if the tape in its reading-off apparatus were to dart hither and thither like the busiest of bees working overtime, the time needed to look up the next intermediate result and forward it - the access time - would sometimes be a matter of seconds. And the arithmetic unit, which measures its work in fractions of a second, cannot be kept waiting that long.
"Magnetic drums" require a considerably shorter access time than magnetic tapes. Imagine a tall, slender South American dance-band drum. Or one of those cardboard tubes used for mailing plans and drawings: six inches in diameter, a good sixteen inches tall. On its outside, this drum is coated with the same material used on magnetic tapes. The different tracks on which information can be magnetized - in this case, circular, of course - are quite close together, as close together as the grooves in a hi-fi record. Altogether, there are between 500 and 1,000 parallel tracks, and each of them has its own magnetizing device, its own reader, and its extinguisher by means of which pulses no longer needed can be armed.
A tube like this is packed together with its thousand magnetizers, readers and extinguishers into an airtight box, a motor is added and the drum is then made to rotate at 50 revolutions per second. In this way every magnetic point must rush past its reading head (the expert calls it simply a "head") once every fiftieth of a second. So the access time amounts to a maximum of 20 milliseconds, and in the most efficient cases to no time at all - so experts speak of an "average access time" of 10 milliseconds. That is really not very slow, but still too slow for the arithmetic unit. Since the magnetic drums cannot do better then an efficient auxiliary storage, they can seldom help to make a calculation.
If it would interest you to know how much information a drum can store, the answer is - not a tremendous lot. There is room for about 500,000 bits on its surface. According to the code used, that means about 10,000 numbers, each with seven digits. (There are however, here and there big drums which can store about 5,000,000 bits.)
Until not so very long ago the magnetic drums had in addition to their storing duties another important task to perform: they marked time, set the rhythmic pace followed by everything going on in a computer. The source of this measured beat was a reading head attached to the edge of the drum. Pulses were magnetized in accordance with these regular time intervals at exactly equal spaces apart on the track which passed the reading head. The interval of time between two beats was one millionth to ten millionths of a second. Today, the drums have been relieved of these tasks, which have been banded over to a special "beat generator." In fact, this generator does just the same work as that which was formerly carried out by the magnetic track on the rim of the drum: it sends a rhythmic pulse every few millionths of a second.
Measured against these speeds, which are adapted to the speed of the arithmetic unit, the ten thousandths of a second of the average access time of which the drum is capable is at least a thousand times too slow for the calculating jobs which have to be done. Storages which have to keep up with a fast arithmetic unit need access times as short as a few millionths of a second. There are such storages. "Core storages," for instance, have an access time of about one millionth of a second. On the other hand, their construction is so complicated that with the same storage capacity as a drum they cost far more than the drum does. Moreover, they need a long-drawn-out explanation. We shall have to turn decidedly technical again.
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