When you want to extend a data set written on a tape cartridge by an IBM magnetic tape subsystem using OPEN for OUTPUT or OUTIN with DISP=MOD, or OPEN for OUTINX or EXTEND, you can use the high-speed search function. This function quickly positions the volume to the end of the requested data set. After the tape is positioned, the open module processes the trailer labels of the data set to be extended.

To invoke the high-speed search function when extending a data set, use the high-speed search interface as follows:

1. Obtain the JFCB for the requested data set by using the read JFCB-RDJFCB macro.
2. Set the fast positioning indicator in the JFCB (JFCPOSID in JFCBFLG3).
3. Set the block identifier in the JFCB (JFCRBIDO=blk-id).
4. Execute OPEN TYPE=J with the modified JFCB.

The block identifier for high speed positioning is for the tape mark immediately following the last block of user data. You can obtain the block identifier when the data set is created by issuing the NOTE macro with the ABS parameter before issuing CLOSE. This can be done only with BSAM or EXCP, not with QSAM.