Radio Shack TRS-80 Trsdos & Disk Basic Reference Manual page 78

TRSDOS
Technical Information
DCB
before
OPEN
and
after
CLOSE:
The
DCB
is
defined
as
32 contiguous bytes of
RAM
designated
by
the
user.
Before
OPEN
and
after
CLOSE,
it is
a
left justified,
compressed
(no
spaces)
ASCII
string, as in a
standard
TRSDOS
filespec:
CONTENTS OF
32
-
BYTE DCB
8
16
24
32
1
F
1
1 1 1 1 LENA
1 1 1 I
1 1
I 1
II
M
E
/
E
X T
1 1 ! 1 i 1
I I
I i
i l i i i
PASSWORD
! I
I I I I I I I
I I I i I I I I I I
I I I I
Notes:
/EXT,
.PASSWORD,
:D are optional.
$ stands for a carriage return (X'OD'!
£ stands for a blank (X'20'J
Explanation
of
DCB
while
OPEN;
Isb/msb
is
least significant
byte followed
by most
significant
byte
in
Z80
RAM
format
(i.e.
addr=7CC8
in
RAM
is
C8
7C).
Addr. Len. Explanation
—
Reserved
—
Physical Buffer address (lsb/msb)
—
Offset to delimiter
at
end of current record
—
File drive
number
residence
—
Reserved
— EOF
offset of
last
delimiter
in last
physical record
—
LRL
(logical
record
length)
— NRN
(next record no.
—
open
sets=X'0000'
—
lsb/msb)
—
ERN
(ending record
no.
—
last in
file
—
lsb/msb)
—
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+1
.
POSN
is
described
later.
DCB+0 -
3
+3
-
2
+5
-
1
+6
-
1
+7
-
1
+8
-
1
+9
-
1
+10
-
2
+12
-
2
+
14
-
18
6-6