IBM OS/390 manual Track and Record Addressing, Track Addressing, Record Addressing by ID

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.

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.

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.

Record Addressing

Within a track, records may be addressed either by their record number (ID) or by key.

Record Addressing by ID

Provide the record number in the R byte of the track reference field.

VSE	READ	filename,ID
MVS	READ	ecbname,DI,...,blockaddress

Blockaddress points to a field containing the complete identification of the record.

Chapter 13. A s s e m b l e r 315