68
as
Tape LabelsFor ASCII interchange tapes, this set of labels (HDR2, EOV2, and EOF2) is optional.
This practice differs from IBM labeling conventions, which require header 2 and trailer 2 labels. The header 2 and trailer 2 labels have the same format. They are used to show information about the structure of records on the file. Because these labels are optional, and because their formats may vary depending on the system that produces them (non-IBM systems may use different formats), the processing of header 2 and trailer 2 labels will vary. If the labels are produced by the operating system, they are treated like IBM header 2 and trailer 2 labels. If the labels are produced by another system, only the fields that conform to ANSI interchange standards will be processed, as explained later in this section.
Figure 16 shows the format of HDR2, EOV2, and EOF2. The shaded areas represent fields that the operating system writes in the label, but that are not used or verified during processing. The contents and processing of each field of the label are described, as are differences between ANSI labels and IBM labels.
1 - Label Identifier (3 bytes)
• Contents: Three characters that identify the label are as follows:
HDR Header label (at the beginning of a data set)
EOV Trailer label (at the end of a tape volume, when the data set continues on another volume)
EOF Trailer label (at the end of a data set)
• Processing: Data management checks this field to verify that the record is an ANSI standard data set label.
For input data sets, data management checks the label identifier to determine whether data set processing is to be continued. When data management finds an EOV label, it performs volume switching. When data management finds an EOF label, it passes control to the user's end-of-data routine.
If the DD statement specifies OPTCD=B for an input data set, data management accepts either EOV or EOF as the trailer label identifier, and the identifier is not used to determine whether a volume switch is necessary. If more volumes are available, data management performs the switching. available, data management passes control to the user's end-of-data routine.
When creating trailer labels, the EOV routine writes EOV in this field, and the Close routine writes EOF.
2 - Label Number (1 byte)
• Contents: The relative position of this label within a set of labels of the same type;
it is always 2 for data set label 2.
• Processing: Verified and written in conjunction with Field 1 to identify this label as HDR2, EOV2 or EOF2.
3 - Record Format (1 byte)
• Contents: An alphabetic character that indicates the format of the records in the associated data set:
F Fixed length
D Variable length with a zoned decimal length indicator U Undefined length
)
)
)
~ (~
(3) 3
4 1
5 (1)
6
(5) 10
11
(5) 15
16
(35)*
Field Number and Name
o lobel 'd.,,,n,,}
I
HDR~OV'IEOF2Label Number Record Format
• Block length
• Record length
• Reserved for Operating System
• Buffer Offset
o
Reserved for Operating SystemD -
Functional Field . - Field with No ProcessingFunction
* -ANSI Field Differs From Corresponding IBM Field
Figure 16. Format of ANSI Header 2 and Trailer 2 Labels
• Processing: For input, the record format is obtained from this label and recorded in the JFCB (if the JFCB field is 0). Then the record format in the JFCB is recorded in the DCB (if the DCB field is 0). Note that this is a merging process in which existing specifications in the JFCB and DCB cannot be overridden.
When creating labels, a reverse merge follows the forward merge described above.
The record format in the DCB overrides the record format in the JFCB, and the updated JFCB provides the information for the label.
This merging process is explained and illustrated in the introduction to this manual.
American National Standard Labels 69
70
as
Tape Labels4 - Block Length (5 bytes)
• Contents: A number from 18 to 2048 that indicates the block length (including buffer offset and padding) in bytes. Note: The 18-byte to 2048-byte limit on block length is an ASCII standard. Larger blocks may be specified with the agreement of the interchange parties. Interpretation of the number depends on the associated record format in Field 3, as follows:
Format F Block length
Format D Maximum block length (including the 4-byte length indicator in the records and the optional block prefix)
Format U Maximum·blo~k length
• Processing: The number in the label is converted to binary and merged with appropriate fields in the JFCB and DCB. The merging process is the same as that for the record format code in Field 3 of this label.
5 - Record Length (5 bytes)
• Contents: A number that indicates the record length in bytes. Interpretation of the number depends on the associated record format in Field 3, as follows:
Format F Logical record length
Format D Maximum logical record length (including the 4-byte length indicator in the records and the optional block prefix)
Format U Zeros
• Processing: The number in the label is converted to binary and merged with the appropriate fields in the JFCB and DCB. The merging process is the same as for the record format code in Field 3 of this label.
6 - Reserved (35 bytes)
• Contents: If the label is produced by the operating system, the content and format of this field are identic.al to the content and format of the seven fields following the record)ength field in an mM standard label. If the label is produced by another system, the content and format of this field are optional, and the field is not processed by the operating system. The fields produced by the operating system are:
A. Tape Density (1 byte)
• Contents: A DEN value of 2 indicates a 9-track tape recorded in 800 CPI (characters per inch), NRZI (non-return-to-zero-inverted). If the tape is recorded in ASCII code but is not an interchange tape, other values (as described in the section on IBM standard labels) may be placed in this field.
• Processing: Not used or verified. When data management creates labels, the information for this field is obtained from the JFCB.
B. Data Set Position (1 byte)
• Contents: A code indicating a volume switch is as follows:
o
No volume switch has occurred.1 A volume switch previously occurred.
• Processing: Not used or verified. When creating labels, the Open routine writes
o
in this field, and the EOV routine writes a 1. The Close routine determines which code to write by comparing the volume serial number in the JFCB to the)
)
)
number in the UCB. It writes 0 if the numbers are equal, and 1 if they are not equal.
C. Job/Job Step Identification (17 bytes)
• Contents: Identification of the job and job step that created the data set. The first 8 bytes contain the name of the job; the ninth byte is a slash (/), and the last 8 bytes contain the name of the job step.
• Processing: Not used or verified. When data management creates labels, the names of the job and job step are obtained from the TIOT.
D. Tape Recording Technique (2 bytes)
• Contents: Recorded as blanks for 9-track tape; 9-track tape can only be recorded in odd parity with no translation.
• Processing: Not used or verified.
E. Control Characters (1 byte)
• Contents: A code indicating whether a control character set was used to create the data set, and the type of control characters used:
A Contains ASCII control characters b Contains no control characters
• Processing: The specification in the label is converted to a bit code and merged with the appropriate fields of the JFCB and DCB. The merging process is the same as that for the record format code in Field 3 of this label.
F. Buffer Alignment Block (1 byte)
• Contents: Reserved for future use-recorded as blanks.
• Processing: Not used or verified. When creating labels, data management writes blanks in this field.
G. Block Attribute (1 byte).
• Contents: A code indicating the block attribute used to create the data set:
B Blocked records b Records not blocked
• Processing: The specification in the label is converted to a bit code and merged with the appropriate fields of the JFCB and DCB. The merging process is the same as for the record format code in Field 3 of this label.
7 - Buffer Offset (2 bytes)
• Contents: The length of the block prefix.
• Processing: Used to determine buffer-offset length.
• Difference from IBM Field: This field is not present in mM standard labels.
8 - Reserved (28 bytes)
• Contents: Reserved for future use-recorded as blanks. (Blanks are translated to ASCII space characters on output.)
• Processing: Not used or verified. When creating labels, data management writes blanks in this field.
American National Standard Labels 71