IBM 7074 General Information Manual page 8

access to any record on the track, additional character
positions must be used for each record on the track, or
a record address (RA). Additional character positions
are required for gaps (G) to separate addresses and
records. Since the tracks are circular, an index point
( I) must be used to indicate the beginning or the end
of the track. Therefore, each of the 10,000 data tracks
must have an index point, one home address, a record
address for each record stored on the track, and the
necessary gaps to separate the address and the rec-
ords (Figure 5).
INDEX POINT
The index point of the track is the reference point of
the track, that is, it indicates both the beginning and
the end of the track. The index point is used by the file
control in conjunction with its various modes of opera-
tion.
HOME ADDRESS (HA)
There is one home address for each of the 10,000
tracks. The home address follows the index point and
consists of two parts called home address one (HAl)
and home address two (HA2).
HAl: This portion of the home address is the track
number. HAl is prerecorded; it cannot be written by
the customer. The track number is a four-digit number
( 0000-9999) indicating the physical location of the
track within the module. The tracks are numbered
sequentially, beginning at the lowest track of the outer-
most cylinder, track number 0000, and continuing up
through the cylinder to track number 0039, represent-
ing the 40 data tracks of the outermost cylinder. The
numbers continue with the lowest track of the ad-
jacent cylinder, track number 0040, up the cylinder to
track number 0079. Continuing through each of the
cylinders in a like manner, the last track number, 9999,
is the top track of the innermost cylinder. With this
method of numbering, adjacent tracks on the same
disk always differ by 40. This provides a means of
determining the track number for any track within any
cylinder by the simple formula: Given the head num-
ber and the cylinder number, the track number is the
product of the cylinder number and 40, plus the head
number. For example, for cylinder number 241 and
head number 20 the track number is:
241
X
40
=
9640; 9640
+
20
=
9660.
Index
Point
~
HA 1
-{
~ GAP
Figure 5. Track Layout
HA 2
GAP
Record
Address 1
GAP
Also, with the track number known, the cylinder
number and head number can be determined by
dividing the track number by 40; the quotient is the
cylinder number; the remainder is the head number.
For example, given the track number, 0590,
0590 + 40
=
014 with a remainder of 30.
Thus, the cylinder number is 014 and the head number
is 30.
HA2: The second portion of the home address is
called the home address identifier. This portion of the
address further identifies the track to the using system.
HA2 is written by the user and consists of two or more
characters that can be numeric, alphabetic, or special
characters, depending upon the requirements of the
using system. Two characters are required for
HA2;
even though only two characters are compared during
address verification operations, additional characters
can be used as a convenience for fixed word length
systems.
HA2
can be written to serve any convenient purpose.
It can be used as a coded file protect device or it can
be used to identify or tag a category of records, such
as receipts, withdrawals, payments, inventory, etc.
HAl
and
HA2
can be treated as one six-position ad-
dress that corresponds to a six-position key of record,
part number, policy number, account number, etc.
RECORD ADDRESS
The record address consists of six or more characters
that can be numeric, alphabetic, or special characters.
It is used to identify the individual record on the track
to the using system. A record address is assigned and
written by the user to fit any convenient addressing
scheme; it need not have any relationship to the home
address of the track where it is written, since an ad-
dress is transmitted to the file control for each func-
tional order. The home address, used for seek orders,-
and the record address, used in the prepare to verify
(single record) order, need not be related in any way.
The record address normally consists of six charac-
ters and even though the first four (high-order posi-
tions) can be alphabetic or special characters with
the 7080 and 7090, only the numeric bit positions of
these characters are compared during address verifica-
tion operations. All bits of the' fifth and sixth charac-
ters are compared during address verification. The
Record
t
GAP
Record
Address 2 Record 2
GAP
Introduction 7