Program Buffer - IBM 3741 Data Station Reference Manual

Index Track Layout
The first track on a disk is called the index track and is
numbered 00. The index track's 26 sectors are for system
or data set label information. The 26 sectors on each of the
remaining tracks, numbered 01 through 73, are used for
data. Each sector can contain one record. Track 74 is
reserved and should not be used for data entry.
Each record on the index track has a record length of 80
characters. The index track layout is shown in Figure 7.
Sector Contents
Sector 1 80 blanks
Sector 2 80 blanks
Sector 3 Position 1: machine test character for a write test
Sector 4 80 blanks
Sector 5 Positions 1-5: ERMAP; positions 6-80: blanks
The first seven sectors of the index track may contain
system information or information about the diskette,
such as the location of tracks that have developed errors
in use. These seven sectors are not usually used or modi-
fied by the operator. The remaining sectors (08-26) may
contain data set labels, which are used to define the data
sets resident on the disk.
If the disk has 1 or 2 bad tracks, positions 7 and 8 indicate the first bad track and position 9 contains
a 0; positions 11 and 12 indicate the second bad track and position 13 contains a O.
Sector 6 80 blanks
Sector 7 Volume label: positions
Sectors
8-26
1 2 3 4
VOL 1
Data set labels
5 6 7 8 9 10 11
12 ______________ 76
kS-------_____ 16
~
)
~
Volume ID*
Accessibility -
Any non-blank
character means
disk is not
accessible
12 through 76
contains blanks
77 78 79 80
~ ~
W
Sector
sequence
information
*The volume ID field contains IBMIRD when the diskette is received from IBM. When you initialize the diskette, the volume
10 field contains either the volume 10 entered by the operator or is blank if the operator did not enter a volume 10.
Figure 7. Index Track Layout
Operation Modes and Considerations 7