Recording Data Set Positioning

The checkpoint routine records positioning information for user data sets as follows:

Data sets are repositioned at restart only if they were open when the checkpoint was taken. The OPEN routine positions all data sets opened after the checkpoint was taken.
Unit record data sets (printer, punch, or card reader) are not repositioned at restart.
If you use EXCP to process a reel tape data set open at a checkpoint, ensure that the block count in the data set's data control block is correct. You can do that by coding REPOS=Y in the DCB macro. If the block count is incorrect, the data set may be incorrectly positioned by restart.

If input/output operations were requested, but not begun (for example, if a READ macro instruction was executed, but the related channel program was not started), the checkpoint routine then stops any processing associated with the I/O request, records the positioning information, and reestablishes I/O operations.

If I/O operations have already begun, the checkpoint routine waits until they are complete before recording positioning information.