IBM 7074 General Information Manual page 7

The magnetic disk data surface can be used repeti-
tively. Each time new information is stored on a
track, the old is erased as the new information is re-
corded. The recorded data may be read as often as
desired; data remain recorded in the tracks of a disk
until written over.
Disk storage, like trays of cards or magnetic tape,
provides external storage capacity to supplement in-
ternal core storage of a computer. Disk storage has a
major advantage, however, in that all records con-
tained in the 1301 are program-addressable, providing
random access to any record or group of records (that
is, faster access to any record).
Although the total number of character positions of
the track is fixed, the arrangement as to the number of
records and the number of characters per record can
be arranged to suit the needs of the using system. Thus,
data can be stored on the track in any convenient
arrangement within the limitations of track require-
ments. Addresses must be provided to identify the
track and the individual records to the computer;
also, space must be provided in the form of gaps to
separate address and record areas, as well as the areas
for the records.
A format track is used to provide a means of de-
fining and monitoring the address, record, and gap
areas. for the data tracks. In the 1301, one format track
monitors 40 associated data tracks. The format track
can be written and rewritten to describe the desired
data track format as often as required to suit the needs
of the user.
A disk storage module is comprised of the stack of
25 magnetic disks and its associated access mechanism.
Of the 25 disks, 20 disks (40 surfaces) are used to
store data; of the other ten surfaces, six are used as
alternate surfaces, one is a format surface, one is a
clock surface (Figure 3). The other two are not used.
The data storage surfaces and the format surface each
contain 250 concentric tracks that are accessible for
reading and writing.
Cylinder Concept
Since the heads and disk tracks are mechanically
aligned one above the other, the vertical alignment of
the tracks can be considered as a cylinder of tracks
(Figure 4). Thus, with the access mechanism placed
in anyone of the 250 cylinders, 40 tracks of data are
available without any further access motion; only
electronic head switching is necessary. For example,
for one access mechanism setting, as many as 112,000
characters (7080) are available.
The tracks are numbered sequentially, from the
bottom to the top of the cylinder (corresponding to the
6
Figure 3. Disk Surfaces
40 heads, 00 to 39), starting at the outermost cylinder
(000) to the innermost cylinder (249). Thus with
large storage areas for reference tables, the data can
be conveniently stored in a cylinder of tracks or in a
number of adjacent cylinders. This technique will re-
duce access time to a minimum.
The cylinder arrangement of tracks also permits the
optional feature, the cylinder mode of operation, to
read or write a cylinder, (or part of a cylinder) of
tracks in one operation.
Data Tracks
The basic fixed recording area of the 1301 is the data
track. The physical make-up of the track limits the
track to a specific overall recording capacity. All data
tracks are equal in overall storage capacity. The entire
recording area cannot be used to store data, however;
a number of character positions must be set aside to
provide a means of identifying the track to the using
system, or a home address (
HA).
To provide random
One of
250
Cylinders
Figure 4. Cylinder Concept
--1- -
20 Data
Disks
(40 Surfaces)
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