Commodore 1541-II - Allocating Blocks

pointer to the first file type byte (see Appendix C for details of directory organization),
locking it by setting bit 6 and rewriting it.
110 OPEN 15,8,15 Open command channel
120 OPEN 5,8,5,"#" Open direct access channel
130 PRINT#15,"Ul";5;0;18;l Read Track 18, Sector 1
140 PRINT#15,"B-P";5;2 Point to Byte 2 of the buffer
150 GET#5,A$:IF A$=" " THEN A$ = CHR$(0) Read it into memory
160 A = ASC(A$) OR 64 Turn on bit 6 to lock
170 PRINT#15,"B-P";5;2 Point to Byte 2 again
180 PRINT#5,CHR$(A); Overwrite it in buffer
190 PRINT#15,"U2";5;0;18;l Rewrite buffer to diskette
200 CLOSE 5 Tidy up after
210 CLOSE 15
220 END
After the above program is run, the first file on that diskette can no longer be erased. If
you later need to erase that file, re-run the same program, but substitute the revised line
160 below to unlock the file again:
160 A = ASC(A$) 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,<tBLOCK-ALLOCATE";drive #; track #;sector #
usually abbreviated as: PRINT#15,"B-A";drive #; track #;sector #
where "drive #" is the drive number, always 0 on the 1541, 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
70