After a read or write call, control is returned to the user as soon as the requested operation has been accepted. In other words, there is no need to wait for this operation to be completed.
Before a read or write operation is initiated, however, the system waits for the preceding read/write operation (if any) to terminate (i.e. implicit check-and-wait operation). Similarly, when a close operation is requested, the system waits for the last read/write operation to terminate.
Thus, after the last in a series of read/write operations, a check operation is necessary only if the file is not immediately closed again or if, in the case of chained I/O, reading is continued until the end-of-file condition is reached (bit IDMFIB or IDMFEF of sense byte =1) and the number of blocks transferred is not equal to 0.
Example
In an EAM file, 3 read operations are performed. The file is not closed after this, because it is still required for later I/O operations. However, these I/O operations are not requested until after processing of the blocks that were read has been completed:
READ : CHECK/WAIT Wait for termination of the last : I/O operation. Processing of the blocks that were read : Further I/O operations