The processing of nonstandard labels is requested by means of:
the operand LABEL=NSTD of the FILE macro
the operand LABEL-TYPE=*NON-STD of the ADD-FILE-LINK command
DMS controls EOV processing by activating the appropriate routines in the user program, which then handle label processing.
When DMS detects a tape mark, it activates the user routine in order to evaluate the EOV labels. After the labels have been evaluated, DMS – if necessary executes the tape swap with messages to the operator, etc. Once the tape has been swapped, the header labels on the new tape are checked before the user program can continue reading the file.
If no tape swap is necessary, DMS activates the LABEND routine in order to evaluate the end-of-file labels. After this has been done, DMS positions the tape backwards by one tape mark and then initiates end-of-file processing (using the EOFADDR exit of the EXLST macro).