Tape Label Processing Differences
There are some differences in the way tapes are processed by CMS/DOS
and the way they are processed by VSE:
- The tape error messages are CMS error messages and not VSE error messages. In some cases VSE lets the system operator reply NEWTAP to an error message. The system then waits for the operator to mount a new tape and continues processing with this new tape. Such a reply is never possible under CMS/DOS. In CMS/DOS, you usually can reply IGNORE to ignore a tape label error condition or CANCEL to cancel a job. NEWTAP is never allowed. In a few cases, CMS/DOS allows an IGNORE reply where VSE does not.
- You must specify CLOSE to process all trailer labels. No automatic CLOSE occurs at end of data or after reading a tape mark. If an input tape is positioned at an EOF1 or EOV1 record when CLOSE is run, the label is processed. If a tape file is closed before all data records are read, the trailer label is not processed. Output tapes have EOF records written only at CLOSE time. For nonstandard labeled tapes, your own routines do not receive control on input when a tape mark is read. You must enter a CLOSE macro in your EOFADDR routine to have the trailer labels processed.
- Certain fields in HDR1 and EOF1 labels default to values different from those in VSE. For example, the default volume serial number written in a HDR1 label is CMS001 and not the actual volume ID (volid) in the VOL1 label already on the tape. The default file sequence and volume sequence numbers are always one even when the file is not the first file on the tape. You should read LABELDEF Command to learn what the default values are in CMS/DOS. You also can read VSE/AF Tape Labels to find what the default values are for VSE. If you do not like the default values, you can always specify the exact values you want in label fields in a LABELDEF command.
- Expiration date specification is always done in absolute form rather than by retention period. You can use either the 5-digit form (yyddd) or the 6-digit form (cyyddd), where yy is the year (00-99) and ddd is the day (001-366). In the 6-digit form, c is the century (9=1900s, 0=2000s, 1=2100s). CMS does not handle expiration dates specified by retention periods.
- VOL1 labels written in the wrong density are not automatically rewritten by CMS/DOS as they are by VSE.
- Blank tapes should not be used for tape files specified as FILABL=STD in CMS/DOS; they will run off the reel. Use the TAPE command to write a VOL1 label or a tape mark on a blank tape before using it for a STD file.
- Not all tape movement and label checking that occurs in VSE occurs under CMS. For example, when opening an output file, a VSE system expects the tape to be positioned at a HDR1 label or a tape mark. It then backspaces the tape to read the last EOF1 label on the tape. If it does not find the label it expects, it displays an error message. This check is not performed by CMS/DOS. If the tape is not positioned at a HDR1 label or a tape mark when output open processing begins, error message 422 is issued.
- After an EOV1 label is written (see End-of-Tape Processing), the tape is always rewound and unloaded under CMS/DOS. VSE lets a DTFMT parameter control if the tape is rewound.
- User label processing routines do not receive control when an I/O error occurs under CMS/DOS.
- Control is not passed to user standard label routines in CMS/DOS when EOT has been sensed on output and an EOV1 label has been written by the system routines.
- Work tapes are not checked for an expiration date when they contain standard labels under CMS/DOS. If a tape is to be opened as a work tape, CMS/DOS tests to see if it contains a VOL1 label. If it does, a dummy HDR1 label and a tape mark are immediately written on the tape after the VOL1 label. If the tape does not contain a VOL1 label, a tape mark is written at the beginning of the tape. VSE checks expiration dates on previously labeled tapes used as work tapes and gives the operator a chance to reject the tapes if the expiration date has not expired.
For more information on VSE and CMS/DOS tape label processing, see the VSE/AF Tape Labels and VSE/AF Macro User's Guide.