The workings of a normal disc drive

Press, 22 July 1986, Page 19

The workings of a normal disc drive

Disc drives for saving information are a big improvement on what went before. This was magnetic tape. Accessing information on tape took so long you built your social life around loading and unloading the computer. The technology is still not totally out of date, but for speed, for quantity of information stored, and for all-round convenience, discs wins hands down. Disc drives so improved the speed and ability of the computer that they led to the introduction of virtual memory, where the computer decides what information is not needed in the main memory, saves it on to the disc and accesses it only when it is needed.

Disc drives and highspeed computers are a marriage made in heaven. Floppies have partially the same technology as tape recorders in that the read/write head actually comes in contact with the surface of the disc instead of floating just above it.

There are three main components that allow the normal disc drive its wonders to perform. The con-

Mr Chris Angove (above) has been appointed the manager of Honeywell’s information systems division in New Zealand. He joined Honeywell in 1984 and has been a branch manager In Papua New Guinea and Sydney. Before joining Honeywell Mr Angove spent 10 years with Computer Consultants, Ltd.

troller, the drive itself, and the disc. The controller is a computer, keeping track of the part of the disc which is being read, where the information it needs is hiding and whether any other information is required apart from the information on the disc. It also works in reverse; that is, it decides where information should be written to a disc, how much can be written to one disc and the way the disc will be formated. There are two motors in a normal disc drive. One is not unlike the oldfashioned motor on the wind-up gramophone, but is driven by electricity, and nips along at a considerably faster rate than 78 rpm — 300 or 600 rpm depending on the machine. Part of the drive is the head, which collects information from the briskly rotating disc, and is not dissimilar to a read/write head on a tape recorder. The second motor in the drive pushes the recording/read head back and forwards so that it is positioned on precisely the right spot of the disc — having been told where to go by the controller — for the appropriate information to be read or written. The head writes information to the disc by rearranging the ways the iron oxide molecules are lying — each piece of information represented by a single rearrangement called a transition — and reads them back in the same way.

The third essential part of this trio is the disc. I have a tendency to refer to discs flippantly as pieces of plastic covered with rust. Very well refined, extremely pure rust; plastic manufactured to very fine tolerances. But still rust and plastic nevertheless.

One way to think of a disc is a biscuit cut out of a large piece of recording tape, and carefully polished so there is the minimum amount of wear on the head. This disc is contained in either a semi-stiff cardboard con-

tainer with a lubricant liner or, more generally these days, in a stiff plastic container.

The aim is to protect the disc of iron-oxide-covered plastic from the elements, your morning cup off coffee, magnetised paper clips and other undesirable materials. Treat your discs in exactly the same way you handle high-quality long-playing records. A 5.25-inch disc, with only 160 K of memory, is a fairly sturdy beast and can go through shot and shell losing nary a scrap ’ of data. The same size disc with 1.2 megabytes recorded thereon can be a skittish beast and deserves and needs tender loving care. If you were at a convention of hackers and you asked all who had overwritten priceless data by accident to raise their hands, you would be looking at a forest of waving arms. We have all done so, and will do. it again. In theory, there is a write-protect notch on the disc, which, if covered with opaque material, blocks out a small light and a sensor and gstops any information being written to the disc. Hackers are a careless lot, however, and most have no idea whether the current disc in their drive is protected or not. The reason so many programmers wear beards is that it gives them something handy to tear when they overwrite a vital disc.