Commodore 1581 - Using Random Files

When the user selects a particular sub-directory area, it then
becomes the default working area. Accesses to the disk for directories,
loading files, saving files, etc., will all be done within this area. Files
outside of the selected area are effectively invisible.
File and local BAM information for sub-directories are stored
within the sub-directory areas themselves. The information is stored
on the first allocated track of the partition area, and has the same
format as track 40. When creating partitions and sub-directories within
sub-directories it is the responsibility of the user to make sure that he
doesn't overwrite this information! The DOS only checks to make sure
that you don't attempt to overwrite this information for the ROOT
directory (track 40). It is up to the user to make sure that this
information isn't corrupted in the sub-directories.
Partitioned areas can be freed up simply by SCRATCHING the
partition file entry in the appropriate directory. If the partition was
being used as a sub-directory, all of the files in that sub-directory will
be lost.
USING RANDOM FILES
By combining the commands in this chapter, it is possible to
develop a file-handling program that uses random files. What you
need to know now is how to keep track of which blocks on the disk
such a file has used. (Even though you know a sector has not been
allocated by your random file, you must also be sure it wasn't allocated
by another unrelated file on the diskette.)
The most common way of recording which sectors have been
used by a random file is in a sequential file. The sequential file stores a
list of record numbers, with the track, sector, and byte location of each
record. This means three channels are needed by a random file: one
for the command channel, one for the random data, and the last for
the sequential data.
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