When a program finishes reading the portion of a data set that is on a tape reel or cartridge, the system checks that the expected number of blocks was read. The system does this if the volume contains IBM standard or ISO/ANSI labels and the program uses BSAM, QSAM, or EXCP. With EXCP, the program maintains the block count in the DCB and the programmer codes REPOS=Y on the DCB macro. For more information, see Verifying block count in label when reading, and Verifying block count in label when reading.

When a program finishes reading or writing data on a cartridge (not a reel), the system does additional block count checking unless the program is using EXCP and the programmer did not code REPOS=Y on the DCB macro. The system can check for this on all cartridges, including unlabeled tapes. This type of block count checking is independent of the checking that is done only on labeled tapes.

The additional checking is to compare the count accumulated in the data control block with a count that the system calculates based on information maintained by the tape subsystem (hardware). If the two counts differ, the system does not call the block-count-unequal exit. Instead, the system calls the optional DCB ABEND exit for abnormal completion code 117 or 237. If the exit is not defined, or if it does not ignore or recover, then the system issues an ABEND macro. For more information on DCB ABEND exit, see z/OS DFSMS Using Data Sets. For more information on the installation DCB ABEND exit, see z/OS DFSMS Installation Exits.

For more information on maintaining and verifying block count when using EXCP, refer to z/OS DFSMSdfp Advanced Services.