Index block header

The format of an index block header in the index block is as follows:

Bytes: Description
Byte 0:: X'8A' Index block identifier
Bytes 1-2:: X'1000' Length of the index block
Byte 3:: X'4E' Index block identifier
Byte 4:: X'00' Format identifier for regular index block; X'01' Format identifier for alias index block
Byte 5:: Index-level number (01-0A)
Bytes 6-7:: Offset to the last entry in the index block. In a level-1 index block, the last entry is a sequence set chain pointer entry. In an upper-level index block, the last entry is the last index entry of the block.
Bytes 8-9:: Offset to free space in the index block
Bytes 10-11:: Offset to a table of index-entry offsets
Bytes 12-13:: Count of index entries in the block. This is also equivalent to the count of entries in the table of index-entry offsets (the bottom part of the block).

Figure 1 shows an example of the index block header set up for the example of the level-1 index block in Figure 1.

Figure 1. Example of an Index Block Header