Allocating Blocks; Freeing Blocks - Commodore 1541 User Manual

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
120 OPEN 5,8,5 , " #"
130 PRINT#l5, "Ul " ;5;0;18 ;1
140 PRINT#l5,"B-P" ;5;2
150 GET#5 ,A$:IF A$= "" THEN A$=CHR$(0)
160 A= ASC(A$) OR
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170 PRINT#l5 \" B-P" ;5;2
180 PRINT#5,CHR$(A);
190 PRINT#l5 ,"U2" ;5;0;18;1
200CLOSE 5
210 CLOSE 15
220 END
Open command channel
Open direct access channel
Read Track 18, Sector I
Point to Byte 2 of the buffer
Read it into memory
Tum on bit 6 to lock
Point to Byte 2 again
Overwrite it in buffer
Rewrite buffer to diskette
Tidy up after
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$) AND 191 Tum off bit 6 to unlock
ALLOCATING BLOCKS
Once you have written something in a particular sector on a diskette with the help of
direct access commands, you may wish to mark that sector as "already used, " to keep
other files from being written there. Blocks thus ''allocated'' will be safe until the diskette
is validated.
FORMAT FOR BLOCK-ALLOCATE COMMAND:
PRINT#l5 ," BLOCK-ALLOCATE";drive # ; track #;sector#
usually abbreviated as: PRINT#l5,"B-A";drive # ; track #;sector#
where " drive# " is the drive number, always 0 on the 1541, and " track#" and "sector
#" are the track and sector containing the block of data to be read into the file buffer.
ALTERNATE FORMAT:
PRINT#l5,"B-A:";drive #;track # ;sector#
EXAMPLE:
If you try to allocate a block that isn ' t available, the DOS will set the error message
to number 65, NO BLOCK , and set the track and block numbers in the error message
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the next available track and block number. Therefore , before selecting a block to write,
lf'f
to allocate that block . If the block isn ' t available; read the next available block from
the
error channel and allocate it instead. However, do not allocate data blocks in the
directory track . If the track number returned is 0, the diskette is full.
Here is a program that allocates a place to store a message on a diskette .
100 OPEN15 ,8, 15
110 OPEN5 ,8,5 , "#"
120 PRINT#5 , "I THINK
THEREFORE I AM "
1JOT= l :S= I
140 PRINT#l5 ,"B-A";O;T;S
150 INPUT#l5 ,EN,EM$ ,ET,ES
160 IF EN =0 THEN 210
170 lF EN <>65 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
200T=ET:S =ES:GOTO 140
210 PRINT#l5 ," 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
panicular sector is not in use , rather than actually erasing any data .
FORMAT FOR BLOCK-FREE COMMAND:
PRINT#l5 ,"BLOCK-FREE";drive # ;track # ;sector#
abbreviated as: PRINT#l5,"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#l5, "B-F:";drive #;track # ;sectoi" #
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