Information Technology Reference
In-Depth Information
heads to specific track locations. If a read/write head were to over
shoot slightly, it might read or write from the space between
tracks. If it were to overshoot a bit more, it might have moved to
the location of the next track. Thus, the read/write heads must
consistently move to exactly the correct location—within reason
ably tight tolerances.
As a disk functions over time, it is not unknown for this
read/write head movement to come out of alignment slightly. In this
case, a write operation would still place data on the disk, and a read
operation would still retrieve it appropriately. However, misalign
ment means that the location of the data would not be quite right—
perhaps the data would be stored on the space between where the
tracks should be rather than on the desired track itself. If the same
read/write head were used for reading and writing, any such mis
alignment would be the same for both reading and writing, and the
operations would appear to work fine. However, if the disk were
moved to a new machine and the data were not exactly where they
should be, the reading operation would fail. Of course, it could be
that the home machine's disk drive works fine, and the one in the
lab is misaligned. This could be checked by trying to use the disk on
other lab machines. When a disk from home fails in several lab ma
chines, then it may be time for the servicing or replacement of a
home disk drive.
Operating systems handle common tasks and administrative
functions that allow users to run computers with relative ease. Such
tasks include identifying one user from another, keeping track of
distinct requests (e.g., associating typing in a window with the cor
rect processing), scheduling and coordinating processing, allocating
resources (e.g., files, printers, memory), handling files, and commu
nicating with other machines. When these tasks are performed cen
trally, individual users need not worry about doing these tasks
themselves.
Multitasking allows computers to split their processing time
and resources among several processes or users. Processing re
quires coordination when computers work on several tasks at the
same time.
