Commodore 1581 - Allocating Blocks

After the above program is run, the first file on that diskette can no
longer be erased. If you later need to erase that file, rerun the same
program, but substitute the revised line 160 below to unlock the file
again:
160 A=ASC(A$) AND 191 Turn off bit 6 to unlock
ALLOCATING BLOCKS
Once you have written something in a particular sector on a
diskette with the help of direct access commands, you may wish to
mark that sector as "already used", to keep other files from being
written there. Blocks thus allocated will be safe until the diskette is
validated.
FORMAT FOR BLOCK-ALLOCATE COMMAND:
PRINT#15,uBLOCK-ALLOCATE";drive #; track #;sector #
usually abbreviated as: PRINT#15,"B-A";drive #; track #;sector #
where "drive #" is the drive number, and "track #" and "sector #"
are the track and sector containing the block of data to be read into the
file buffer.
ALTERNATE FORMAT:
PRINT#15,"B-A:";drive #; track #;sector #
EXAMPLE:
If you try to allocate a block that isn't available, the DOS will set
the error message to number 65, NO BLOCK, and set the track and
block numbers in the error message to the next available track and
block number. Therefore, before selecting a «block to write, try to
allocate that block. If the block isn't available, read the next available
block from the error channel and allocate it instead. However, do not
allocate data blocks in the directory track. If the track number re
turned is 0, the diskette is full.
Here is a program that allocates a place to store a message on a
diskette.
100 OPEN15,8,15 Open command channel.
110 OPEN5,8,5,"#" " direct access "
75