To overlap I/O and processor activity, you can start several BSAM read or write operations before checking the first operation for completion. You cannot overlap read with write operations, however, because operations of one type must be checked for completion before operations of the other type are started or resumed. Note that each pending read or write operation requires a separate DECB. If a single DECB were used for successive read operations, only the last record read could be updated.

Related reading: See Figure 1 for an example of an overlap achieved by having a read or write request outstanding while each record is being processed.