This file contains a storage control block (SCB), which consists of summary information intended to optimize the Files-11 space allocation, and the bit map itself, which lists the availability of individual blocks.
The SCB contains summary information about the volume (cluster factor, volume size, blocking factor, and so forth). Each bit in the bitmap represents an allocatable cluster on the volume. If a bit is set, the corresponding cluster is available for use. If a bit is clear, the cluster is not available.
During normal operation, the operating system moves portions of the bitmap in and out of cache memory. The state of each bit in memory is altered as clusters are allocated and deallocated. BITMAP.SYS is updated when the portion of the bitmap in cache is swapped back to disk. Since a portion of the bitmap is always in cache, BITMAP.SYS never reflects the current state of allocated clusters on a disk (unless the disk is dismounted or write-locked).
One of the functions of ANALYZE/DISK_STRUCTURE is to build a current version of BITMAP.SYS from data extracted from INDEXF.SYS, so that BITMAP.SYS accurately reflects the status of free clusters on the disk.