OSAM and VSAM ESDS block format

OSAM and VSAM ESDS blocks share the following format.

Block format

The following figure shows the OSAM and VSAM ESDS block format.

Graphic shows a 9 column, 1 row table. Column 1 references note 2; 2 references note 3; 3 references note 4; 4 references note 5; 5 references note 6; 6 references note 7; 7 references note 8. Column 8 is labeled "FREE SPACE." Column 9 is labeled "VSAM INFO." and references note 9. Columns 1 and 2 have connecting arrows across the top categorizing them as "FSEAP" with a reference to note 1 below. Column 3 has a connecting arrow across the top categorizing it as "RAP." Column 4 has a connecting arrow across the top categorizing it as "SEGMENTS." Columns 5 through 7 have a connecting arrow across the top categorizing them as "FSE." Column 8 has a connecting arrow across the top without a label. Column 9 has a connecting arrow across the top without a label.

Notes:

1.: Free space element anchor point.
2.: 2-byte offset to first free space element; contains zeros in a bit map block.
3.: 2-byte length (see 7); value is zero.
4.: 4-byte root anchor point (RAP). The number per block is specified in DBDGEN, except if HIDAM with TF (and not TB) is pointing at root level, one anchor point per block is provided and it heads a LIFO chain of roots inserted in that block. If HIDAM or PHIDAM with TB or NT is pointing at the root level, there are no anchor points provided.
5.: User database segments (prefix and data). In a bit map block, the bit map starts here and extends to the end of the block or to the VSAM control information.
6.: 2-byte offset to next free space element (FSE) from start of block.
7.: 2-byte length of free space, including 8-byte FSE.
8.: 4-byte identification of the task that freed this space.
9.: 7 bytes of VSAM control data; omitted for OSAM.

This format applies at the conclusion of initial load. The subsequent deletion of segments can result in free space elements that alternate with user database segments.