Commodore 1541-II - Freeing Blocks

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 returned is 0, the diskette is full.
Here is a program that allocates a place to store a message on a diskette.
100OPEN15,8,15
110OPEN5,8,5,"#"
120 PRINT#5,"I THINK
THEREFORE I AM"
130T=l:S = l
140 PRINT#15,"B-A";0;T;S
150 INPUT#15,EN,EM$,ET,ES
160 IF EN = 0 THEN 210
170 IF ENO65 THEN PRINT
EN,EM$,ET,ES:STOP
180 IF ET = 0 THEN PRINT
"DISK FULL":STOP
190 IF ET= 18 THEN ET= 19:ES = 0
200 T = ET:S = ES:GOTO 140
210 PRINT#15,"U2";5;0;T;S
220 PRINT "STORED AT:",T,S
230 CLOSE 5:CLOSE 15
240 END
FREEING BLOCKS
Open command channel
** direct access "
Write a message to buffer
Start at first track & sector
Try allocating it
See if it worked
If so, we're almost done
"NO BLOCK" means already allocated
If next track is 0, we're out of room
Don't allocate the directory!
Try suggested track & sector next
Write buffer to allocated sector
Say where message went
and tidy up
The Block-Free command is the opposite of Block-Allocate. It frees a block that you
don't need any more, for re-use by the DOS. Block-Free updates the BAM to show a
particular sector is not in use, rather than actually erasing any data.
FORMAT FOR BLOCK-FREE COMMAND:
PRINT#15,"BLOCK-FREE'';drive #;track #;sector #
abbreviated as: PRINT#15,"B-F";drive #;track #;sector #
where "drive #" is the drive number (always 0 on the 1541), and "track #" and "sector
#" are respectively the track and sector numbers containing the desired block of data to
be read into the file buffer.
ALTERNATE FORMAT:
PRINT#15,"B-F:";drive #;track #;sector #
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