User`s guide
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 64
170
PRINT#l5
•.
"B-P";5;2
180 PRINT#5,CHR$(A);
190
PRINT#l5,
"U2
" ;5;0;
18;1
200 CLOSE 5
210 CLOSE
15
220END
Open command channel
Open direct access channel
Read Track
18
, Sector 1
Point
to
Byte 2
of
the buffer
Read it into memory
Tum on bit 6
to
lock
Point
to
Byte 2 again
Overwrite it
in
buffer
Rewrite buffer
to
diskette
Tidy up after
After the above program is run, the first
file
on that diskette can
no
longer be erased.
If
you later need to erase that file, re-run the same program, but substitute the revised l
ine
160 below
to
unlock the file again:
160
A=ASC(A$)
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 k
eep
other files from being written there. Blocks thus " allocated" will be safe until the disk
ette
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:
•lf you try to allocate a block that isn't available,
the
DOS
will set the error
me
ssage
to
number
65
,
NO
BLOCK, and set the track and block numbers
in
the error message
to
70
the
next available track and block number. Therefore, before selecting a block
to
write,
ll'Y
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
di
skette
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
PRfNT#5, " I THrNK
THEREFORE I AM "
130T=l
:
S=I
140
PRINT#l5
,"
B-A
";
O;T;S
150
INPUT#l5,EN
,EM$,ET,ES
160
IF
EN=O
THEN 210
170
IF
EN<~65
THEN PRINT
EN
,EM$,ET,
ES
:STOP
180
IF
ET=O
THEN PRINT
"DISK
FULL
":
STOP
190
IF
ET=
18
THEN
ET=
19
:
ES=O
200T=ET
:
S=ES
:GOTO
140
210
PRINT#l5
,
"U2
";
5;0;T;S
220
PRrNT " STORED AT
:",
T,S
230
CLOSE 5:CLOSE
15
240END
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#l5,
" BLOCK-FREE";drive #;track # ;
sector#
abbreviated as:
PRINT#l5
,"
B-F
";
drive # ;track # ;
sector#
w~~re
"drive#_"
is
the drive number (always 0 on
the
1541), and
"track#
" and "sector
I 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
#
71