IBM OS/390 User Manual
13.2.6.12 Track and Record Addressing
Track Addressing
In VSE and MVS, you can make track references by using either the actual or
relative addressing technique. The track reference field for actual addressing in
VSE and MVS is of the form MBBCCHHR. The contents of the M byte is different
in VSE and MVS. In VSE, M represents volume number, starting with zero for the
first volume. This must be increased by one for each subsequent volume.
relative addressing technique. The track reference field for actual addressing in
VSE and MVS is of the form MBBCCHHR. The contents of the M byte is different
in VSE and MVS. In VSE, M represents volume number, starting with zero for the
first volume. This must be increased by one for each subsequent volume.
In the MVS data set control block (label), M contains the extent sequence
number, which always starts with zero, on each volume. This must be increased
only for extents within the same volume. In the MVS data extent block, M is a
pointer to extent information. Therefore, if actual addressing is required, you
must change your calculation routine to meet the MVS requirements. However,
you should use relative addressing instead of actual addressing because of the
simplicity in calculating the proper track number.
number, which always starts with zero, on each volume. This must be increased
only for extents within the same volume. In the MVS data extent block, M is a
pointer to extent information. Therefore, if actual addressing is required, you
must change your calculation routine to meet the MVS requirements. However,
you should use relative addressing instead of actual addressing because of the
simplicity in calculating the proper track number.
In the case of relative track addressing, the correct disk addresses are
generated by the control program. If you use actual addressing, you must check
the extents on that volume to make sure nothing else is written there. Actual
addressing does not allow you to take advantage of some of the MVS facilities
and may impair the performance of the system. Relative addressing allows the
control program to place the data set where it is most convenient. It does all the
necessary checking of extents. With relative addressing, the system keeps track
of each data set, thus making programming easier and system use more
efficient.
generated by the control program. If you use actual addressing, you must check
the extents on that volume to make sure nothing else is written there. Actual
addressing does not allow you to take advantage of some of the MVS facilities
and may impair the performance of the system. Relative addressing allows the
control program to place the data set where it is most convenient. It does all the
necessary checking of extents. With relative addressing, the system keeps track
of each data set, thus making programming easier and system use more
efficient.
Record Addressing
Within a track, records may be addressed either by their record number (ID) or
by key.
by key.
Record Addressing by ID
Provide the record number in the R byte of the track reference field.
Blockaddress points to a field containing the complete identification of the
record.
record.
VSE
READ
filename,ID
MVS
READ
ecbname,DI,...,blockaddress
Chapter 13. A s s e m b l e r
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