ISO/ANSI standard data set label 2 (HDR2/EOV2/EOF2)

The ISO/ANSI standard data set label 2 (HDR2/EOV2/TRL2) is 80 characters in length and is used to identify additional information about the data set. The system records the data set label in ASCII.

Table 1. Format of the IBM^® Standard Data Set Label 2 (HDR2/EOV2/EOF2)
Offset	Data type	Length	Contents
0	Character	3	Label identifier
3	Character	1	Label number
4	Character	1	Record Format
5	Character	5	Block Length
10	Character	5	Record Length
15	Character	35	Reserved for Operating System
50	Character	2	Buffer offset
52	Character	28	Reserved

The manual describes the contents and function of each of the following fields.

Label identifier
The characters identify the type of data set label.
HDR

Header Label (the beginning of a data set)

EOF

Trailer Label (the end of a data set)

EOV

Trailer Label (the end of a data set that is continued on another volume)
Label number
The relative position of this label within the set of labels of the same type; it is always 2 for the data set label 2.
Record Format
An alphabetic character that indicates the format of the records in the data set. The record format indicates to the operating system how to interpret the blocks of data that the program reads from the tape volume.
F

Fixed length records

V

Variable length records

U

Undefined length records
Block Length
A number indicating the block length, in bytes, of the data blocks on the tape. The number in this field can range from 18 to 2048. This block length should include the buffer offset and padding.

Note that the 18-byte to 2048 byte limit on block length is an ISCII/ASCII standard. You can specify larger blocks (up to 9999 bytes) with the agreement of the interchange parties. However, for tapes with Version 3 labels, exceeding the 2048 byte limit might create incompatible tapes.

Interpretation of the number depends on the associated record format field as follows:
Record format F

Maximum block length.

Record format D

Maximum block length that includes the 4 byte length indicator in the records and the optional block prefix.

Record format S

Maximum block length that includes the optional block prefix, plus one or more pairs of 5 byte segment control words and segments.
Record Length
A number indicating the record length, in bytes, of the logical records on the tape volume. The interpretation of the number depends on the Record Format field.
- Record format F
  Actual record length
- Record format D
  Maximum record length that includes the 4 byte length indicator in the records
- Record format S
  Maximum record length that excludes all the 5 byte segment control words that describe the record. If the record length is larger than 99999, this field is 0.
Reserved for Operating System
The content of this field is optional for each operating system. i5/OS™ has chosen similar meaning to the same bytes in the IBM Standard Data Set Label 2.
- Tape Density (1 byte)
  3
  
  1600 bpi
  
  4
  
  3200 bpi
  
  5
  
  6250 bpi
  
  blank
  
  All other densities/formats
- Data Set Position (1 byte)
  A code indicating a volume switch is as follows:
  0
  
  No volume switch has occurred
  
  1
  
  A volume switch previously occurred
- Job/Job Step Identification (17 bytes)
  This field identifies the job and job step that created or extended the data set. i5/OS does not use this field.
- Tape Recording Technique (2 bytes)
  This field indicates the tape recording technique used in creating the data set.
  blank
  
  No Improved Data Recording Capability (IDRC) used.
  
  'P '
  
  Improved Data Recording Capability (IDRC) used.
- Control Character (1 byte)
  A printer control code indicating whether the program uses a control character set to create the data set and the type of control characters the program uses:
  A
  
  Contains ISO/ANSI control letters
  
  blank
  
  Contains no control characters
- Buffer Alignment Block (1 byte)
  i5/OS does not use this field.
- Block Attribute
  A code indicating the block attribute used to create the data set:
  B
  
  Blocked records
  
  blank
  
  Records not blocked
- Reserved (11 bytes)
  i5/OS does not use this field.
Buffer Offset
The length of the block prefix (from 0 to 99). Used to determine the length of an optional prefix that might be a part of a physical block on tape. The version of the prefix for variable and spanned record formats is known as a block descriptor word (BDW). A BDW is always 4 bytes long and contains the block length of the physical record it describes, including the BDW.