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Check Disk Track
CTN or 36 -
Q
11
of 7
This instruction causes a full track of addresses and
data (20 sectors) to be read from disk storage and
compared with addresses and data in core storage.
The operation is the same as for Check Disk Track/
WLRC,
except that group marks do not affect checking.
Group marks in disk storage and core storage are
compared as data.
Execution Time. Average T
==
62 ms
Branch No Group Mark
BNG or 55
See Branch instructions.
Summary of the Effects of Group Marks in
Disk Storage Operations
A group mark is used to establish record length
when the Wrong-Length Record Check feature is
used. The group mark must be placed in
core storage
following the last position of the record to be written
in to disk storage, or following the last position of
the area in core storage reserved for reading data
from disk storage. Figure 49 is a block diagram
illustrating the effects of group marks in disk storage
NO
operations. The effect of group marks is the same for
both track mode and sector mode operations.
The following two points are programming situa-
tions that may arise, and the programmer should be
aware of how they affect a disk storage operation.
YES
1. If the group mark terminates a record that
does not consist of an even increment of 100
positions (i.e., 85, 145, 470, etc.) the group
mark is
written in disk storage and the Wrong-
Length Record Check indicator turns on. When
the same record is read back into core storage,
the group mark in
disk storage stops the opera-
tion and turns on the Wrong-Length Record
Check indicator.
2. If 300-position records (for example) are to
be used in a program, and the sector count in
the disk control field has been set to 3, but
the group mark has been incorrectly placed in
position 201 of the record instead of in position
301; then, during a
write operation, the group
mark in core storage stops the operation and the
Wrong-Length Record Check indicator is turned
on, but the group mark is
not written in disk
storage. In this example, although the group
mark had
not been placed in the correct posi-
tion, it did establish a record in an increment
Group marks have no effect on the operation.
Group marks are transmitted as data.
A group mark must be placed in
£Q!!t
storage following last pcnition of the record.
Operation is terminated by a group mark
in core storage following last position of
record, or is terminated by a group mark
anywhere in the record being transmitted
from disk storage. Group marks in core
storage, other than the terminal group
mark are replaced by data.
READ
Operation is terminated by first group
mark encountered in core storage or
disk storage.
Figure 49.
Effects of Group Marks on Disk Storage Operations
50
WRITE
Operation is terminated by a group mark
in core storage following the last position
of record, or by a group mark anywhere
in the record being transmitted from core
storage. Group marks in the record in
disk storage are replaced by data.
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