| We have mentioned mass storage in passing | | | | coating used finer particles. This allowed the heads to |
| previously, in the history of the computer series. The | | | | be moved closer together, and more data to be held |
| term mass storage is applied to a group of media, | | | | around the drum, and on more tracks. Speed could |
| including drums magnetic tape and disks. It has the | | | | also be increased, improving access times.Another |
| advantage of high capacity, but slow access times, in | | | | application of the drum, the Fastrand from the Sperry |
| comparison to RAM.For this reason, it is used as a long | | | | Univac company, had two horizontally mounted drums |
| term (computer time), or back-up medium. The current | | | | of approximately 2ft diameter and about 8ft long. A |
| data and programs the processor is working on are | | | | beam mounted between the drums and carrying read |
| kept immediately accessible in main memory, RAM, | | | | write heads was able to move horizontally, so |
| and they are replenished as required from mass | | | | increasing the number of tracks each head could |
| storage. At the same time, programs and data which | | | | address. This system was used extensively on Univac |
| are no longer required to be instantly available are | | | | computers of various sizes starting in the 1960s. The |
| written out to mass storage.MAGNETIC DRUM | | | | capacity was equivalent to around 100MBThe drums |
| STORAGEEarly computers used drums for both main | | | | rotated in opposite directions, at 880 RPM, and the |
| memory and mass storage, but with the development | | | | beam was driven by a 'voice coil'. The voice coil was |
| of core memory in the 1950s, drums were used as | | | | named thus as it resembled the voice coil of a |
| mass storage, backing up the core memory. This was | | | | loudspeaker. This works by passing an analog |
| necessary as core was expensive, being hand made, | | | | proportional current, representing music, or speech, |
| a typical capacity was 28Kbits.A typical drum storage | | | | from an amplifier. The loudspeaker cone, attached to |
| unit used a magnetic recording medium coated on a | | | | the voice coil, and in the presence of a permanent |
| vertical cylindrical drum approximately 10in. in diameter, | | | | magnet, is driven by the electromagnetic action, and |
| and about 20in. high. The drum rotated at a speed of | | | | converts the current into sound waves.In the case of |
| around 430, later 880 and 1760 rpm. Data was written | | | | the fastrand, the voice coil was fixed, and a section of |
| to and read from this drum by a series of read/write | | | | the beam passed through it. A servo system drove |
| heads which almost touched the surface of the drum, | | | | the current in the coil to move the beam to the desired |
| held away by air pressure in a similar manner to an | | | | address.Also mounted between the drums, not on the |
| aircraft wing. These were known as 'flying heads'.The | | | | moving beam were a number of 'fixed heads'. These |
| data was written to the drum by passing a current | | | | provided fast access to data, as they didn't need to |
| through a coil on a C-shaped electromagnet, the gap | | | | wait for the beam to move to access the data. They |
| between the poles of the magnet being aligned close | | | | were typically used for the 'boot block' to give a fast |
| to the drum. Thus the circular track around the drum | | | | start.Problems with all drums occurred when the head |
| carried sequentially written data (serial).The data was | | | | contacted the magnetic surface. A 'hit' might cause the |
| located by referring to a once per revolution pulse, | | | | data to be lost at that point but was a warning of an |
| generated by a mark on the the drum, and by | | | | impending 'crash' - a catastrophic event. This would |
| addressing a particular head. Reading was carried out | | | | entail recovering bits of read/write head, repairing the |
| by addressing the required head, or track, and the | | | | drum surface, then replacing and re-aligning the head. |
| distance around the drum. A current was generated in | | | | Cabinets were pressurised to keep out dust, to |
| the coil on the read/write head, and data passed back | | | | minimise this happening.In part 2 we look at magnetic |
| to the system.As technology improved, the heads | | | | disks.Tony is an experienced computer engineer. |
| were made with smaller gaps, and the magnetic | | | | |