Basic Disk Operating System (BDOS) Functions
4-14
SELECT
DISK
ENTRY
CL:
OEH
D
L:
Selected
Disk
RETURN
FUNCTION
14
SELECT
DISK
The Select
Disk
function designates the disk drive
named
in register
DL
as the
default disk for subsequent file operations.
DL
is
0 for Drive A, 1 for Drive
B,
and
so
on
through
15
for Drive P in a full sixteen-drive system. (Recall, however
that
the
APC
currently supports only Drives A through D for diskettes
and
Drives E
through
H for
hard
disk.) The function logs
in
the designated drive
if
the drive
is
currently in the reset state. Logging in a drive places it in "online" status. This
activates the drive's directory until the next cold start, warm start, disk system reset,
or
drive reset operation. FCBs which specify drive code zero
(dr
=
OOH)
automati-
cally reference the currently selected default drive. Drive code values from 1
to
15,
however, ignore the selected default drive
and
directly reference Drives A
through
P.
OPEN
FILE
ENTRY
CL: OFH
DX: FCB
Offset
RETURN
FUNCTION
15
AL: Return Code
OPEN
FILE
The
Open
File function activates
an
FCB specifying a file which exists in the disk
directory for the currently active user number. The BDOS scans the disk directory
of
the drive specified by byte 0
of
the FCB addressed by register
DX
for a match in
positions 1
through
12
of
the FCB.
An
ASCII question
mark
(3FH)
matches any
directory character in any
of
these positions. Normally, no question marks are
included
and
byte "ex"
of
the FCB
is
set
to
zero before the
Open
File call
is
made.
If
a directory element is matched, the relevant directory information
is
copied into
bytes
dO
through
dn
of
the
FCB,
thus allowing access
to
the file
through
subsequent
read
and
write operations.
An
existing file must
not
be accessed until a successful
open
operation
is completed.
Further,
an
FCB
not
activated by either
an
Open
File
or
Make
File function must
not
be used in BDOS read
or
write commands.
To Next Page
Loading...
