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
