Aviation Electronics Technician 1 - Historic Naval Ships Association

to represent the same data that was stored in thatlocation.Transfer of the data from constantly rotatingmagnetic drums, on the other hand, is complicated.Timing pulses are not used to synchronize the drumspeed, which may vary slightly from time to time.Thus, some method must be used to ensure that dataread into the drum memory in a given bit position willbe read from the memory with the same timereference. The probability y of an incompatible timerelationship between the drum speed andsynchronizing pulses makes it necessary to establishsome means of compensating for variations in drumspeeds.In practice, the drum contains a control point anda number of sectors in a specific format. The controlpoint is a magnetic mark that specifies a startinglocation on the drum. All data stored on the drum isreferenced to this indexing point or reference pulse, asshown in figure 8-3.Magnetic TapesMagnetic tape is widely used as a storage mediumfor large amounts of data, or it may be used as a mainstorage backup. However, it is normally not used asan internal (main) storage medium because of its longaccess time. This is readily realized if you considerthat needed information is widely, and sometimesrandomly, distributed along the tape. Magnetic tapehas two main advantages—its large storage capacityand its low cost.Magnetic DisksThe magnetic disk is a convenient medium forsemipermanent storage of mass volumes ofproduction programs. For many applications, disksFigure 8-4.-Circular data track.8-8