When a job step is restarted from a checkpoint, the restart routine mounts and positions any tape volumes containing data sets that were open when the checkpoint was taken. Specifically, the restart routine performs the following functions:

• Restores applicable control blocks to the conditions that existed when the checkpoint was taken.
• Ensures that the first existing record on the tape is not a standard volume label (VOL1 in EBCDIC or ASCII).
• If the volume is a reel:
  • Uses the data set sequence number shown in the JFCB to position the tape to the required data set. The method of positioning is the same as previously explained for opening an input data set.
  • Uses the block count shown in the DCB to reposition the tape to the proper record within the data set. For forward read operations, this positioning is performed in a forward direction. If the block count is 0 or negative, the tape remains positioned at the interrecord gap preceding the first record. For backward read operations, this positioning is performed in a backward direction. If the block count is 0 or a positive number, the tape is positioned at the interrecord gap following the last record of the data set.
  • If the volume is a cartridge, then the block identifier saved in the checkpoint is used to position the tape to the proper record.