20 Macintosh
User's
Handbook
DISKS
AND
DISK DRIVES
The disk drive
is
one
of
the most important parts
of
a
computer. Disk drives allow the storage
of
large amounts
of
data. This data
is
actually stored on a disk (or diskette) which
is
read from
or
written to within the drive. The
3~
inch disk
currently used with the Mac can store
about
400,000 (or 400K)
characters of data on one
of
its sides. Eventually, the disks used
with the Mac will be expanded giving a total capacity
of800K.
This
is
about
6~
times as much data as can be stored on 128K
of
RAM.
Unlike RAM storage, when information
is
stored on a
disk, the information
is
not lost when the computer
is
turned
off. In other words, disks offer a permanent means
of
storing
data. Disk storage
is
not as permanent as ROM
data
storage,
however, since a computer can alter the
data
on
a disk whereas
a
ROM
cannot be altered without destroying it.*
A disk stores
data
in a magnetic form much like
data
is
stored on magnetic tape. The main difference between storage
on a magnetic tape and storage on a disk
is
that
the disk allows
data
to be accessed more quickly.
The disk drive contains a device known as a
read/write
head,
which
is
used to read and write information. The compu-
ter
can move the head
to
any
position desired
on
the disk
surface. This
is
in contrast
to
magnetic tape, where
data
is
read
from
or
written onto the tape in consecutive order.
Consider the difference between
an
audio cassette tape
and
a phonograph record. In order
to
locate a song on the
cassette, you must wind through the tape to the appropriate
place. With magnetic tape, the computer must wind through the
tape until it finds the information it needs. This
is
known as
sequential access. On the other hand, with the phonograph
record, you simply
jump
ahead to the desired position. Sim-
ilarly, with a magnetic disk, the computer just advances
* With the exception
of
PROM's (programmable ROM).
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