Writing the AVR Nonstandard Label Processing Routine

Your routine must:

Determine if the label under consideration is a valid, nonstandard label as defined by your installation.
Set general register 15 to zero if a valid label is detected, or to nonzero if the label is not recognizable. (A nonzero return causes AVR to unload the tape volume and issue an error message.)
Place the location of the volume serial number field within the label in an area provided by AVR when a valid label is detected. (The label, or the first part of it, is read into an 80-byte work area by AVR before your routine receives control; the location is defined within this work area. Also, before your routine receives control, AVR positions the tape at the inter-record gap after the nonstandard label.)
Return control to AVR. Register 14 contains the return address. (The SAVE and RETURN macro instructions can be used in your routine.)

Your label processing routine receives control when the AVR routine cannot identify the first record on a magnetic tape volume as a standard label. The various error conditions that can occur during verification of the first record are explained under Volume Label Verification and Volume Label Editor Routines.

The format of your nonstandard labels must provide for a 6-byte volume serial number field within the first 80 bytes of the label. Otherwise, the volume serial number is read into the 80-byte internal work area. This does not restrict the overall nonstandard label format from being more or less than 80 bytes in length.

Your routine is called in 24-bit mode and must return in that mode. The name of your routine must be IEFXVNSL.