A similar method is used when you want to use the high speed search function to quickly position the volume to the beginning of the data set. The difference is that the block identifier is for the first standard header label of the requested data set when positioning to the beginning of the data set.

For more information about the high speed header label search function, see 3480 Magnetic Tape Subsystem: User's Reference.

If the fast positioning indicator is set to on in a JFCB used by an open routine that is not TYPE=J, the fast positioning indicator will be reset.

When fast positioning is indicated, but a block identifier is not specified in JFCRBIDO, OPEN TYPE=J positions the tape normally and inserts a block identifier in JFCRBIDO. The block identifier is either for the first header label when opening to the beginning of a data set or for the tape mark immediately following the last block of user data when opening to extend a data set. You can use this technique with BSAM, QSAM, and EXCP.

OPEN does not update your copy of the JFCB. To retrieve the new value in the system's copy of the JFCB, issue RDJFCB after OPEN.

Issue a second RDJFCB request after the OPEN to update the JFCB JFCPOSID flag when the fast positioning is indicated.

Once you have turned on the fast positioning indicator in a JFCB for an OPEN with TYPE=J in a job step, make sure the fast positioning indicator and the block identifier in JFCRBIDO reflect your intentions before any subsequent OPEN with TYPE=J for the same data set.

After OPEN with TYPE=J uses JFCRBIDO for a high speed search, it clears JFCRBIDO in the system copy of the JFCB to prevent misinterpretation in a subsequent OPEN.