Radio Shack TRS-80 model III Owner's Manual page 82

TRSDOS
DCB
while
$OPEN
Address Length Explanation
DCB
+
3 Reserved
+
3 2 Physical Buffer
address
(lsb/msb)
+
5 Offset to delimiter at
end
of
current record
+
6
File
drive
number
residence
+
7
Reserved
+
8
eof
offset of
last
delimiter
in
last
physical record
+
9 lrl
(logical
record length)
+
10 2
nrn
(next
record
#
—
$open
sets
=
x'ooocr
—
lsb/msb)
+
12
2
ern
(ending record
#
—
(last in file)
lsb/msb)
+
14
50 Reserved
NRN
Next Record
Number
defines
which
record
is
to
be read
or written
by
the
next
system
call
for
$read
or $write.
It is
automatically
incremented
by one
after
each system
call.
In
order
to
process
random
files,
use the
$posn
call to
direct
trsdos
to
the record
you
wish
to transfer next.
ERN
Ending Record
Number
is
the
last
record
number
currently in
the
file. It
is
put into the directory
at
$close
time, so
if
it is
expected
to
be
correct, the
user
must
close his
files after
adding records
to
a
file.
This value
may
also
be used
to
position
to
end
of
file
so
that
new
records
may
be
added
to
the
end of
the
file.
To
position to the
end
of
file
use a
call to
$posn
with a record
number
of
ern
+
l
$posn
is
described
later.
Physical
and
Logical
Records
in
TRSDOS
A
physical record
is
defined as
one
sector
of
disk.
One
sector of disk contains
256
user data bytes.
The
artificial
term
'"granule"
is
defined
to
be
3 sectors
of
disk space.
There
are
6 granules
on each
of the
40
tracks
on
the disk.
A
granule
is
the
least
amount
of space allocated
by trsdos. For programming
purposes,
the
physical records
in a
file
are
numbered from
to
N.
The
largest
record
number
(N)
in
a
file
will
then be
3
times
the
number
of granules
allocated
minus
one ((3*G)
-
1).
All
trsdos
granule
allocations are
made
as
needed
at the time
of
write,
not
when
the
file
is
created.
Bytes Sectors
Granules Tracks
Disk
.
256
1
— —
768
3
1
—
4608
18
6
1
184320
720 240 40
Disk Space
Table:
For each
5 l A'
f
Disk Drive
A
logical
record
is
defined
by
the user
of trsdos.
It
may
be
anywhere from
1
to
255
bytes in length.
Once
a
file is
opened with
a specific
lrl
(Logical
Record
77