5. FILES › 5.2 Intermediate Files › 5.2.2 Compressed Format

5.2.2 Compressed Format

The compressed format is produced by the module VMTCMP when
the parameter for compressed data is selected.  This is a
variable blocked file.  The maximum record length is governed
by the VMTXFR BUFFSIZE parameter.

Record header format:

    RDW      DS    H         RDW
             DS    H         RESERVED
    COUNT    DS    F         COUNT

Header record extension:

    ORG   COUNT
    DC    F'0'      COUNT OF ZERO
    DC    C'MAI'    IDENTIFIER
    DC    C'1'      VERSION
    DC    X'C0FFEE' UNIQUE ID
    DS    C         RECFM: 'F' OR 'V'
    DS    C         RESERVED
    DS    H         LRECL
    DS    H         BUFFER SIZE

Trailer record extension:

    ORG   COUNT+4
    DC    C'MAI'    IDENTIFIER
    DC    C'1'      VERSION
    DC    X'DECAFE' UNIQUE ID
    DS    C         RECFM: 'F' OR 'V'
    DS    C         RESERVED
    DS    H         LRECL
    DS    H         BUFFER SIZE

Data segment:

Data segments follow a record header in two types, for
repeating and non-repeating data.  Each segment begins with a
one-byte header.  The header for a non-repeating data segment
has the form

    DC      B'cnnnnnnn'

    where

    c  is 0, indicating the following data is not compressed.

    nnnnnn
       is one less than the count of bytes following the
       header that constitute the data in the segment.  This
       count does not include the header.  The number of
       bytes following the header is nnnnnn+1.

The header for a repeating data segment has the form

    DC      B'cnnnnnnn'

    where

    c  is  1,   indicating   that  the  following  data    is
       compressed, and

    nnnnnn
       is the number of times the byte following the header
       must be repeated to recreate the original repeating
       data.  The total number of bytes in the original data
       is nnnnnn+2.

For example, the following original data

    DC      C'******ABCDEF'

produces

    DC      X'84',C'*'         six bytes repeating, and
    DC      X'05',C'ABCDEF'    six bytes non-repeating.

Note:  This structure places a limit of 128 bytes of
non-repeating data per segment, and 129 bytes of repeating
data per segment.