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THE BLOCK AVAILABILITY MAP (BAM)
The BAM is a disk memory representation of available and allocated space on
a disk. When the system stores information on a disk, the BAM will be auto
matically referenced by the DOS to determine what space is available and how
many blocks can be allocated. Iff sufficient space is available to store a given file,
it will be stored on the disk and the BAM updated to account for the space
allocated. However, if the DOS detects that a file will occupy more space than
available, an error message will be generated.
Formatting a disk creates the BAM which is then loaded into DOS memory upon
initialization. The BAM is stored on diskette in varying locations depending
upon the drive used:
BAM Location and Memory Required
Track 18, Sector 0
128 bytes
As changes occur to the BAM in DOS memory, the BAM on disk will be updated
to reflect these changes. Updates to the BAM occur when a program is saved or
a CLOSE is performed on a new SEQuential data file.
THE DISK OPERATING SYSTEM (DOS)
The DOS is responsible for managing information exchange between the disk
controller and the computer.
The DOS performs many functions which are transparent to the user but which
are vital to the operation of the system. For example, the DOS monitors the
input/output (I/O) of the disk so that channels are properly assigned and that
no lengthy waits for an open channel occur. In addition to monitoring of disk
I/O, the DOS also uses the channel structure to search the directory and to
delete and copy files.
DISK MAINTENANCE COMMANDS
The following disk commands permit the user to perform file manipulation and
disk maintenance.
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