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History Of The Computer Mouse

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For this reason, it is used as a long term (computer time), or back-up medium. The current data and programs the processor is working on are kept immediately accessible in main memory, RAM, and they are replenished as required from mass storage. At the same time, programs and data which are no longer required to be instantly available are written out to mass storage.

MAGNETIC DRUM STORAGE

Early computers used drums for both main memory and mass storage, but with the development of core memory in the 1950s, drums were used as mass storage, backing up the core memory. This was necessary as core was expensive, being hand made, a typical capacity was 28Kbits.

A typical drum storage unit used a magnetic recording medium coated on a vertical cylindrical drum approximately 10in. in diameter, and about 20in. high. The drum rotated at a speed of around 430, later 880 and 1760 rpm. Data was written to and read from this drum by a series of read/write heads which almost touched the surface of the drum, held away by air pressure in a similar manner to an aircraft wing. These were known as 'flying heads'.

The data was written to the drum by passing a current through a coil on a C-shaped electromagnet, the gap between the poles of the magnet being aligned close to the drum. Thus the circular track around the drum carried sequentially written data (serial).

The data was located by referring to a once per revolution pulse, generated by a mark on the the drum, and by addressing a particular head. Reading was carried out by addressing the required head, or track, and the distance around the drum. A current was generated in the coil on the read/write head, and data passed back to the system.

As technology improved, the heads were made with smaller gaps, and the magnetic coating used finer particles. This allowed the heads to be moved closer together, and more data to be held around the drum, and on more tracks. Speed could also be increased, improving access times.

Another application of the drum, the Fastrand from the Sperry Univac company, had two horizontally mounted drums of approximately 2ft diameter and about 8ft long. A beam mounted between the drums and carrying read/write heads was able to move horizontally, so increasing the number of tracks each head could address. This system was used extensively on Univac computers of various sizes starting in the 1960s. The capacity was equivalent to around 100MB

The drums rotated in opposite directions, at 880 RPM, and the beam was driven by a 'voice coil'. The voice coil was named thus as it resembled the voice coil of a loudspeaker. This works by passing an analog proportional current, representing music, or speech, from an amplifier. The loudspeaker cone, attached to the voice coil, and in the presence of a permanent magnet, is driven by the electromagnetic action, and converts the current into sound waves.

In the case of the fastrand, the voice coil was fixed, and a section of the beam passed through it. A servo system drove the current in the coil to move the beam to the desired address.

Also mounted between the drums, not on the moving beam were a number of 'fixed heads'. These provided fast access to data, as they didn't need to wait for the beam to move to access the data. They were typically used for the 'boot block' to give a fast start.

Problems with all drums occurred when the head contacted the magnetic surface. A 'hit' might cause the data to be lost at that point but was a warning of an impending 'crash' - a catastrophic event. This would entail recovering bits of read/write head, repairing the drum surface, then replacing and re-aligning the head. Cabinets were pressurised to keep out dust, to minimise this happening.

In part 2 we look at magnetic disks.

History Of The Computer Mouse

Magnetic disks used a similar recording system to the drums, but arranged on magnetic coated platters, made of aluminum, approximately 1ft in diameter. Initially fixed in a stack in a cabinet, the disks later came as a removable 'pack' of 6 platters. These packs could be 'mounted' vertically (spin horizontally) on a drive, as required, and then replaced with another to suit operating requirements.

Read write heads contained in the drive were aligned to move in towards the center of the disks,or and addressed data on both surfaces of the platters (though not the topmost or lowermost surface). A 'Track' is the circular path of one head on one platter. A 'Cylinder' refers to addressing the same track on each platter in the disk pack, forming an imaginary cylinder.

The drives spun at speeds in the hundreds of RPM range. The heads were driven by hydraulics, step motor, or voice coil, and had either a servo system or a servo track for address location. The time taken to get the heads on track is the 'seek time'. Capacities ranged from 10 to 100MB as development improved. Much time was spent aligning the heads on track, and recovering from crashes!

The HDA

The next development was the HDA, or Head and Disk Assembly. Essentially similar to the 'disk pack', but fixed in the drive cabinet, the HDA came as a sealed unit with heads ready aligned in the factory, the disks unable to be accessed by the operator (or engineer). The read/write heads were controlled by a voice call mounted in the drive.

The idea was to improve the performance by 'eliminating' the entry of dust to the HDA. Improved capacity was achieved by allowing finer tolerances to be set in the assembly plant. Capacities were improved over the range of 100 to 600MB.

Although there was a big improvement in reliability, there was still deterioration in the magnetic surface. A technique known as 'Bad-Spotting', first developed to extend the life of drums, could be used by the engineer to specify a location on the disk surface which had become unreliable. The failing address or area would have been identified by monitoring the system fault logs, showing addresses which had to be re-attempted.

Bad-Spotting made use of 'spare' areas on the same track as the faulty area. A special type of read would identify the area, and the entire track after this 'bad-spot' would be shifted, so that the bad area was skipped. Alternatively, if no spare area was available, a separate track, specifically reserved for this purpose would be used to relocate this address.

A bad spot table kept a record of all relocated addresses. Following this exercise the disk had to be re-initialised or formatted to use the new physical locations.

Eventually the disk surface would deteriorate to such an extent, or a head crash would occur, requiring the HDA to be replaced.

The advent of the PC and the requirement for small parts accelerated the development of smaller disks. When the pc disk capacities entered the 100MB and up range, they became viable alternatives to the large HDAs. These drives resembled a miniature version of the disk drive cabinet. They had a stack of several platters and heads arranged on arms. The stepper motors used a worm drive arrangement to locate the correct track.

A cabinet of PC style disk drives might contain several banks of 8 drives to a bank, giving better total capacity than the HDA, and possibilities of redundancy within the cabinet(see below). Also air conditioning requirements were relaxed, and a smaller footprint (space taken by the cabinet) was possible. As disk drives improved they were incorporated into the cabinets in ever increasing capacity.

Redundancy became possible relatively cheaply with the lower cost of drives. The principle is to write identical data to two drives for each address, so that, if one drive fails, the other can be used. A sophisticated version would switch in a spare drive to take over from the defective one, allowing the faulty drive to be replaced, while the user continues on without interruption.

The RAID system (Redundant Array of Independent Devices) is a refinement of this technique. Redundancy can be had in pairs of drives, as above, or in stripes across a series of drives, or a combination of both.

In part 3 we look at magnetic tapes.

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