Figure 8-4.-Circular data track

to represent the same data that was stored in that location. Transfer of the data from constantly rotating magnetic drums, on the other hand, is complicated. Timing pulses are not used to synchronize the drum speed, which may vary slightly from time to time. Thus, some method must be used to ensure that data read into the drum memory in a given bit position will be read from the memory with the same time reference. The probability y of an incompatible time relationship between the drum speed and synchronizing pulses makes it necessary to establish some means of compensating for variations in drum speeds. In practice, the drum contains a control point and a number of sectors in a specific format. The control point is a magnetic mark that specifies a starting location on the drum. All data stored on the drum is referenced to this indexing point or reference pulse, as shown in figure 8-3. Magnetic Tapes Magnetic tape is widely used as a storage medium for large amounts of data, or it may be used as a main storage backup. However, it is normally not used as an internal (main) storage medium because of its long access time. This is readily realized if you consider that needed information is widely, and sometimes randomly, distributed along the tape. Magnetic tape has two main advantages—its large storage capacity and its low cost. Magnetic Disks The magnetic disk is a convenient medium for semipermanent storage of mass volumes of production programs. For many applications, disks Figure 8-4.-Circular data track. 8-8