We have mentioned mass storage in passing previously, in the history of the computer series. The term mass storage is applied to a group of media, including drums magnetic tape and disks. It has the advantage of high capacity, but slow access times, in comparison to RAM. 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. |
