Bitmaps
Bitmaps contain a string of bits. Each bit describes whether enough space is available in a particular control interval (CI) or block to hold an occurrence of the longest segment defined in the data set group.
The first bit says whether the CI or block that the bitmap is in has free space. Each consecutive bit says whether the next consecutive CI or block has free space. When the bit value is one, it means the CI or block has enough space to store an occurrence of the longest segment type you have defined in the data set group. When the bit value is zero, not enough space is available.
The first bitmap in an OSAM data set is in the first block of the first extent of the data set. In VSAM data sets, the second CI is used for the bitmap and the first CI is reserved. The first bitmap in a data set contains n bits that describe space availability in the next n-1 consecutive CIs or blocks in the data set. After the first bitmap, another bitmap is stored at every nth CI or block to describe whether space is available in the next group of CIs or blocks in the data set.
For a HALDB partition, the first bitmap block stores the partition ID (2 bytes) and the reorganization number (2 bytes). These are stored before the FSEAP at the beginning of the block.
