Variable length segment structure
The following figure depicts a variable length segment (VLS) that can exist in HISAM, HDAM, and HIDAM databases; a description of the structure immediately follows the figure.
Segment code | See the definition for the appropriate database organization. |
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Delete byte | See the definition for the appropriate database organization. |
Counters and pointers | See the definition for the appropriate database organization. |
Variable length field | 2 bytes. Signed binary number that specifies the length of the data portion of the segment, including the variable length field itself. |
Data | See the definition for the appropriate database organization. If a variable length segment in a HISAM database is replaced and is longer than it was before, IMS will move the following segments to make room for the new segment. IMS does not move HDAM and HIDAM database segments once they have been inserted. Instead, it splits the segment, leaving the prefix part in the original location and inserting the data part in another location. The two parts are connected by a VLS pointer. You can make a segment variable in length in one of two ways:
Thus, use of a compression routine always makes the segment variable in length in the data, but may be presented to the user through a DL/I call as fixed length. How the user sees the segment data is determined by the SIZE= parameter in the DBD. Split variable length segment structure
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Prefix | The prefix consists of the segment code, delete byte, counters and pointers, VLS pointer, and free space. |
Segment code | See the definition for the appropriate database organization. |
Delete byte | See the definition for the appropriate database organization. BIT 4 will be on, indicating that the prefix and data are separated. Typically this will be X'08'. |
Counters and pointers | See the definition for the appropriate database organization. |
VLS pointer | 4 bytes. RBA of the data part of the variable length segment. |
Free space | Normal IMS free space (described in a later section of this section). |
Data | The data portion consists of the segment code, delete byte, a variable length field, and data. |
Segment code | See the definition for the appropriate database organization. |
Delete byte | This will always be X'FF'. |
Variable length field | 2 bytes. Signed binary number that specifies the length of the data portion of the segment, including the variable length field itself. |
Data | See the definition for the appropriate database organization. |
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