Read and Write call discussion

In this section:

The I/O exit control block field "open mode" refers to the data set open operation, therefore "input" means open for input, and "output" means open for output.
References in I/O exit control block fields to "current input record" refer to the data in the input buffers provided by File Manager to the I/O exit. Similarly, references to "current output record" refer to the data in the output buffers.

Processing flow for read and write function codes:

During a read operation, File Manager reads a record from a data set and places it in the I/O exit current input record. File Manager then calls the exit with a read function code. The exit should then copy the current input record to the current output record buffer, performing any required processing. On return from the exit, File Manager uses the processed current output record from the exit.
During a write operation, File Manager takes an in-memory record and places it into the I/O exit current input record. The exit should then copy the current input record to the current output record buffer, performing any required processing. On return from the exit, File Manager writes the processed current output record from the exit to DASD.