Format of IBM standard data set label 2 (HDR2/EOV2/EOF2)

IBM standard data set label 2 always follows data set label 1 and contains additional information about the associated data set. The format is used for header labels (HDR2), end-of-volume trailer labels (EOV2), and end-of-data-set trailer labels (EOF2). The label is 80 characters in length. It is recorded in EBCDIC on 9-track tape units and on cartridges, or in BCDIC on 7-track tape units. Tapes created on other operating systems such as VSE might not contain header label 2.

Figure 1 shows the format of data set label 2. The shaded areas represent fields that the operating system writes in the label, but that are not used or verified during processing. The processing descriptions refer to the following system control blocks:

Data control block (DCB)
Job file control block (JFCB)
Unit control block (UCB)

Figure 1. Format of the IBM standard data set label 2

Figure 2. Format of the IBM standard data set label 2 continue

Format of the IBM standard data set label 2 continued

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: The system checks this field to verify that the record is an IBM standard data set label.
For input data sets, the system checks the label identifier to determine whether data set processing is to be continued. When the system finds an EOV label, it performs volume switching. When the system finds an EOF label, it passes control to the user's end-of-data routine or continues processing with a concatenated data set.

If the DD statement specifies OPTCD=B for an input data set, the system 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, the system performs the switching. If no volumes are available, the system passes control to the user's end-of-data routine or continues processing with a concatenated data set.

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 a 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
  
  V
  
  Variable length
  
  U
  
  Undefined length
Processing: For input, the OPEN routine obtains the record format from this label, converts it to a bit code, and records it in the JFCB (if the JFCB field is zero). Then the record format in the JFCB is recorded in the DCB (if the DCB field is zero).
Note: This is a merging process in which existing specifications in the JFCB and DCB cannot be overridden.

When creating labels, the system's OPEN and EOV routines do a reverse merge. 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 Figure 1.

4—Block Length (5 bytes)

Contents: A number up to 32 760 that indicates the block length, in bytes. Interpretation of the number depends on the associated record format in Field 3, as follows:
- Format F
  
  Maximum block length (must be a multiple of the logical record length in Field 5). If field 12 contains a blank, then this field is also the minimum block length.
  
  Format V
  
  Maximum block length (including the 4-byte length field in the blocks)
  
  Format U
  
  Maximum block length.
The system can determine the optimum block size when creating tape data sets. For more information see z/OS DFSMS Using Data Sets.

If the block length field contains zeros, then there is a large block interface block length. See field 17, Large Block Length.
Processing: The number in the label is converted to binary and merged with appropriate fields in the JFCB and DCB or DCBE. 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 V
  
  Maximum logical record length (including the 4-byte length field in the records)
  
  Format U
  
  Zeros
Processing: The number in the label is converted to binary code 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—Tape Density (1 byte)

Contents: A code indicating the recording density of the tape; the code is equivalent to the DEN parameter value on the DD statement. For the DEN parameter values, refer to Tape characteristics. For non-cartridge tapes, when density is not specified in the DCB or JCL DD statement, this field will be set to the highest density applicable to the drive (default).
Note: Specifying DEN=0 for a 7-track 3420 results in 556 bits-per-inch recording, but corresponding messages and tape labels indicate 200 bits-per-inch recording density.

For tape cartridges, this field contains a character 0.
Processing: Not used or verified. When the system creates labels, the information for this field is obtained from the JFCB.

7—Data Set Position (1 byte)

Contents: A code indicating a volume switch is as follows:
- 0
  
  No volume switch has occurred
  
  1
  
  A volume switch previously occurred.
Processing: Not used or verified. When creating labels, the open routine writes 0 in this field, and the EOV routine writes 1. The close routine determines which code to write by comparing the first 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.

8—Job/Job Step Identification (17 bytes)

Contents: Identification of the job and job step that created the data set. If your program extends the data set by using the EXTEND or OUTINX option of the OPEN routine, or by using the OUTPUT or OUTIN option of the OPEN routine with DISP=MOD, EOF2 contains the name of the job and job step that extended it. The first 8 bytes contain the name of the job, the ninth byte is a slash (/), and the final 8 bytes contain the name of the job step.
Processing: Not used or verified. When the system creates labels, it sets this field as described above.

9—Tape Recording Technique (2 bytes)

Contents: A code or blanks indicating the tape recording technique used to create the data set.
For 7-track tapes the values are:
- T␢
  
  Odd parity with translation
  
  C␢
  
  Odd parity with conversion
  
  E␢
  
  Even parity with no translation
  
  ET
  
  Even parity with translation
  
  ␢␢
  
  Odd parity with no translation or conversion.
The only technique available for 9-track tape is odd parity with no translation.
For a magnetic tape subsystem with Improved Data Recording Capability, the values are:
- P␢
  
  Record data in compacted format
  
  ␢␢
  
  Record data in standard, uncompacted format
For other tapes, this field is recorded as blanks.
Processing: For 7-track tapes and cartridges written by a magnetic tape subsystem with Improved Data Recording Capability, 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.

10—Control Character (1 byte)

Contents: A printer control code indicating whether a control character set was used to create the data set and the type of control characters used:
- A
  
  Contains ANSI control characters
  
  M
  
  Contains machine control characters
  
  (blank)
  
  Contains no control characters.
Processing: The specification in the label is converted to a bit code merged to 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.

11—Reserved (1 byte)

Contents: Reserved for possible future use (recorded as blanks).
Processing: Not used or verified. When creating labels, the system writes blanks in this field.

12—Block Attribute (1 byte)

Contents: A code indicating the block attribute used to create the data set:
- B
  
  Blocked records
  
  S
  
  Spanned records, if the record format byte = 'V'
  
  S
  
  Standard records, if the record format byte = 'F'
  
  R
  
  Blocked and spanned records, if the record format byte = 'V'
  
  R
  
  Blocked and standard records, if the record format byte = 'F'
  
  (blank)
  
  Records that are not blocked and not spanned, or records that are not blocked and not standard.
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.

13—Reserved (2 bytes)

Contents: Reserved for possible future use (recorded as blanks).
Processing: Not used or verified. When creating labels, the system writes blanks in this field.

14—Device Serial Number (6 bytes)

Contents: For tape reels used by the 3420, byte 43 contains the model number, and bytes 44 through 47 contain the last 4 digits of the serial number of the creating tape unit.
For tape cartridges written by IBM 3480 and 3490 magnetic tape subsystems, bytes 43 through 46 contain the last 4 digits of the serial number of the control unit, and byte 47 contains the device address.

For tape cartridges written by IBM 3590 and 3592

magnetic tape subsystems, bytes 42 through 47 contain the 6 digit device serial number.

Note: The serial numbers in the header and trailer labels may not be the same if the data set was extended, or if the system swapped tape units while the data set was being created.
Processing: A unique number to identify the recording unit is read off the tape during open processing, converted into binary, and inserted into the UCBCTD field in the UCB tape extension.

15—Checkpoint Data Set Identifier (1 byte)

Contents: This byte contains the character C if the data set is a secure checkpoint data set; the byte is blank if the data set is not a secure checkpoint data set.
Processing: This field is examined by open/close/EOV. If it finds the data set is a checkpoint data set, it performs the following security operations:
- Verifies (by way of messages to the operator) that the data set is a secure checkpoint data set.
- Determines whether the user is authorized. If the user is unauthorized, open/close/EOV does not allow access to the checkpoint data set directly, although it will allow the taking of checkpoints and performing restarts.

16—Reserved (22 bytes)

Contents: Reserved for possible future use (recorded as blanks).
Processing: Not used or verified. When creating labels, the system writes blanks in this field.

17—Large Block Length (10 bytes)

Contents: A number that can be greater than 32 760 that indicates the block length, in bytes. Interpretation of the number depends on the associated record format in Field 3, as follows:
- Format F
  
  Maximum block length (must be a multiple of the logical record length in Field 5). If field 12 contains a blank, then this field is also the minimum block length.
  
  Format V
  
  Maximum block length (including the 4-byte length field in the blocks)
  
  Format U
  
  Maximum block length
The system can determine the optimum block size when creating tape data sets. For more information see z/OS DFSMS Using Data Sets.
Processing: Used when field 4, the Block Length field, contains zeros. The number in the label is converted to binary and merged with appropriate fields in the JFCB, SWA, DCB, and DCBE. The merging process is the same as that for the record format code in field 3 of this label. If the maximum block length is 32 760 or less, the OPEN and EOV routines write it in field 4 and not this field. On input, the OPEN and EOV routines accept small or large values in the Large Block Length field.