Index Track Data Set Labels - IBM 3741 Data Station Reference Manual

Index Track Data Set Labels
Sectors 8 through 26 of the index track are used primarily
for data set labels. A data set label defines required infor-
mation about a data set on disk such as beginning of ex-
tent (BOE), end of data (EOD), end of extent (EO E), and
the record length of records for the data set.
Data Set Label Field Description
Figure 8 shows the field layout of a data set label. The
following is a description of each of the fields in the data
set label.
Header 1 Field (Positions 1-4): The header 1 field, a re-
quired field, is a data set label identifier for system
application. It must be HDR1 in order to be a valid data
set label identifier. When a disk is initialized, only sector
8 of the index track has HDR1. Sectors 9-26 contain
DDR1 in positions 1 through 4 and are deleted. The
deleted label identifier can be modified to a valid data
set label identifier (HDR 1) by using the modify index
mode. Modifying the label in this way allows you to de-
fine more data sets on one disk.
Data Set Name Field (Positions 6-13): The data set name
field, an optional field, is provided to allow you to give
the data set a descriptive name. When a disk is initialized,
the data set name field of the data set label in sector 08
contains DATA as a name. The data set name field of the
data set labels in sectors 09 through 26 contains DATA
09 through DATA 26, respectively. The data set name can
be changed to a descriptive data set name by using the
modify index mode.
Record Length Field (Positions 23-27): The record length
field, a required field, must contain the logical record length
for the records to be placed in the data set. Disk initializa-
tion places the record length into this field for you. For
the 3741, the record length field can be 00001 through
00128. Disk initialization can be used to set the record
length to
t6t6
128. The modify index mode can be used to
change the 128 to any value from 1 through 128.
Beginning of Extent Field (Positions
29-33): The
beginning of extent (BOE) field, a required field, identi-
fies the address of the first sector of the data set (where
the first record of the data set will be). The BOE field
has th is format: TTOSS where IT is the track number
and SS is the sector number. The track number must
be in positions 29 and 30, a 0 must always be in posi-
tion 31, and the sector number must be in positions 32
and 33. The BOE address must be 01001 or greater.
8
End of Extent Field (Positions 35-39): The end of extent
(EOE) field, a required field, identifies the address of the
last sector of the data set (where the last record of the
data set will be). The EOE field has the same format as
the BOE field. The track number (TT) must be in posi-
tions 35 and 36, a 0 must always be in position 37, and
the sector number (SS) must always be in positions
38 and 39. The EOE address cannot be larger than 74026
or an incorrect data set label (B) error will occur. How-
ever, track 74 is reserved and should not be used for data
entry.
Bypass Data Set Field (Position
41): The bypass data set
field is a field for use by the 3747 or the 3741 with the
communications feature, data recorder attachment feature,
and I/O adapter. This field must contain a B or be blank.
If the field contains a B, the 3747 bypasses the data set (the
data in the data set is not converted to tape). If the field is
blank, the data in the data set is converted to tape. This
field allows you to store programs and data on the same
disk. A B would be put into the bypass data set field of the
data set label describing the data set containing the programs
The B would keep the programs from being converted to
tape.
For the communications feature, the station transmitting
will bypass a data set if the bypass data set field contains
a B. For a receiving station, if the bypass data set field
contains a B, the data set defined by the data set label
will not be bypassed (data will be received and written
into the data set).
For the 3540 Diskette Input/Output Unit, a B in position
41 causes the data set to be bypassed during read opera-
tions.
Accessibility Field (Position 42):
The accessibility field
is used to control access to a data set. If this field is blank,
the data set is processed. If any character is in this field,
the 3741 does not allow the data set to be processed and
a disk accessibility (A) error is displayed. For the 3741,
this field cannot be modified.
Write Protect Field (Position 43):
The write protect field
controls writing of records on the disk. If this field is
blank, records in a data set are read from the disk and
records are written on the disk. If this field contains a
P, the data set is write protected (data cannot be written
on the disk),