When processing a data set, you can test for completion of any READ or WRITE request by issuing a WAIT or EVENTS macro. You can choose to do this so you can wait for multiple unrelated events. You can process whichever events have completed. The I/O operation is overlapped with processing, but the DECB is not checked for errors or exceptional conditions, nor are end-of-volume procedures initiated. Your program must perform these operations.
If you use overlapped BSAM or BPAM READ or WRITE macros, your program can run faster if you use the MULTACC parameter on the DCBE macro. If you do that and use WAIT or EVENTS for the DCB, then you must also use the TRUNC macro. See TRUNC information in Ensuring I/O Initiation with the TRUNC Macro and DASD and Tape Performance.
For BDAM, a WAIT macro must be issued for each READ or WRITE macro if MACRF=C is not coded in the associated DCB. When MACRF=C is coded, a CHECK macro must be issued for each READ or WRITE macro. Because the CHECK macro incorporates the function of the WAIT macro, a WAIT is normally unnecessary. The EVENTS macro or the ECBLIST form of the WAIT macro can be useful, though, in selecting which of several outstanding events should be checked first. Each operation must then be checked or tested separately.