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first and only data set on the tape, so the tape remains positioned immediately following the VOLl label.
To create a data set that follows another data set already stored on the tape, you specify a data set sequence number in the LABEL parameter of the DD statement.
• The sequence number can be from 1 to 9999, with 1 representing the first data set on the volume. If you specify a sequence number that is two greater than the number of data sets existing on the volume, the tape will be spaced through and removed from its reel.
• If you do not specify a sequence number, or specify zero, data management assumes that the data set is to be written as the first on the volume.
To position the tape, data management maintains a logical data set sequence number in the unit control block (UCB). The method of
positioning is the same as previously explained for opening an input data set.
Only one data set on a tape volume can be open at any given time. If you attempt to open another data set on the same volume, processing is abnormally terminated. This restriction includes system output (SYSOUT) tapes.
When the tape is positioned to receive the new data set, data
management expects to find either an existing HDRl label or a tapemark.
If neither one is present, data management assumes that other data is recorded where the HDRl label should be, and therefore processing is abnormally terminated. (If the last data set on a tape has EOV labels, another data set cannot be written to follow it.)
If a tapemark is found, i t indicates that a HDRl label does not exist at the position where the new data set is to be written. Data
management bypasses all further label verification and accepts the tape for output. The conditions under which data management finds a tapemark instead of a HDRl label are:
• When a tapemark immediately follows the VOLl label. This may occur when the tape is initialized by means other than the IEHINITT
utility program (IEHINITT writes a dummy HDRl label following the VOLl label). The tapemark is overwritten by the new HDRl label.
• When, for multi-data set organizations, the new data set is to be written following the last existing data set on the volume. In this case, data management encounters the second tapemark following the existing EOF trailer label group. The tapemark is overwritten by the new HDRl label.
If data management finds an existing HDRl label, i t checks the label to determine whether the existing data set may be overlaid.
EXpiration Date on Existing Label
The existing HDRl label is inspected for the expiration date. If the expiration date has not been reached, the operator is asked to confirm use of the tape or to mount another tape.
If other data sets exist on the same volume, data management checks only the one expiration date and assumes that all following data sets expire on the same date.
Protection and Data set Name on Existing Label
After checking the expiration date~ data management inspects the security indicator in the existing HDRl label. This indicator shows whether the existing data set is protected against unauthorized use.
If no protection is indicated, the tape is· accepted for output. Data management does not request a password, and does not check the data set name.
If protection is indicated, data management compares the data set name shown in the existing HDRl label to the name specified by the user in the DD statement. If the names are not the same, processing is abnormally terminated.
If the data set name is correct, data management requests the operator to key in the required password. The password from the operator is verified in a user-established password data set. This password data set contains the data set name, the password, and a protection mode indicator. The protection mode indicator is set to permit either read/write or read-only operations. The read/write mode is necessary for output data sets. Processing is terminated if:
• The operator, in two attempts, does not supply the correct password.
• The password record for the data set to be opened does not exist in the password data set.
• The read-only protection mode is specified.
The System Programmer's Guide publication describes the protection feature in detail, and contains the information you need to create and maintain the password data set.
Note: Verification of existing labels is considered complete after checking the HDRl label. Any labels, data, data sets, or tapemarks following the HDRl label are irrelevant, and may be overlaid by the new output.
Writing Data Set Header Labels
When the tape is accepted for output, data management creates the header labels (HDRl and HDR2) for the new data set. These labels are created from information in the updated JFCB and other system control blocks.
The source of information for each field of each label is explained in the description of label formats later in this section. The process of updating the JFCB is explained in the Introduction to the manual.
If no user header labels are to be written, data management writes a tapemark after the HDR2 label. The tape is then ready to receive the new data set.
Writing User Header Labels
When SUL is coded on the DD statement and the address of an output user header label routine is specified in the DCB exit list, data management can write as many as eight user header labels (UtiLi - UHL8).