Because the parse service routine checks the command operands only for syntax errors, you must write validity checking routines when it is also necessary to check positional operands for logical errors. Each positional operand can have a unique validity checking routine.

To indicate that a validity checking routine is to receive control, code the entry point address of the routine on the parse macro instruction that describes the operand. The validity checking routine you provide for a positional operand receives control after the parse service routine determines that the operand is specified and is syntactically valid.

When parse passes control to a validity checking routine, it passes a validity check parameter list, which contains the address of the PDE parse built to describe the positional operand. Your validity checking routine can use the information in the PDE to perform additional checking on the operand.

When processing is complete, the validity checking routine must pass a return code in general register 15 to the parse service routine. The return code informs parse of the results of the validity check and determines the action that parse takes.