Figure 8-5.-Data storage disk assembly.
are superior to magnetic tapes for rapid acquisition
and storage of mass volumes of system programs and
data.
Magnetic disks resemble phonograph records that
have been coated with iron oxide. The disks are
arranged in stacks in much the same way as a record
stack in a jukebox.
All the disks are continuously
revolving and spaced apart so that a record head
driven by an access mechanism can be positioned
between the disks.
Data is recorded at a certain address on a
specified disk. When readout of a particular bit of
data is desired, the recording head is automatically
positioned, and the data is read serially from the
surface of the selected disk.
The basic unit of information on the disk is called
a character. By design, each character contains a
given number of bits for fixed-word applications.
One or more of these characters in a group form a
record. A circular data track (fig. 8-4) consists of one
or more records, associated record addressees, gaps,
and data track identification. A number of data tracks
aligned on vertically arranged disks (fig. 8-5) form a
cylinder of information. A magnetic disk file system
may contain one or more bands (modules). Each
module contains a specified number of disks with
their associated cylinders and data tracks. The flow
chart in figure 8-6 illustrates the procedures necessary
to retrieve or store information.
INPUT/OUTPUT (I/O) SECTION
Learning Objective: Describe how a digital
computer communicates with external
peripheral devices.
The I/O section is that portion of the digital
computer through which the CPU communicates with
the external peripheral devices. In a useful computer
function, data is read into the computer, processed,
and then transferred to the output. The peripheral
units handle the data input and output display
functions. The I/O section controls the transfer of
data between the computer and the peripherals.
The I/O section is the interface between the
computer and any extenal devices. An interface is an
assembly of electronic circuits that make the
computer compatible with the peripheral units. This
Figure 8-6.-Flow chart for storage and retrieval of data from disks.
8-9
