Basic Disk Operating System (BDOS) Functions
The
Open
File function returns a code, called a directory code, with a value
of
0
through 3
if
the open was successful,
or
OFFH (255 decimal) if the file could
not
be
found.
If
question marks occur in the FCB, the first matching FCB
is
activated. The
current record
(" cr") must be zeroed by the
program
if
the file
is
to
be accessed
sequentially from the first record.
CLOSE
FILE
ENTRY
CL:
10H
DX: FCB
Offset
RETURN
FUNCTION
16
AL: Return Code
CLOSE
FILE
The Close File function
is
the inverse
of
the
Open
File function in its operation.
Given
that
the
FCB
addressed by
DX
has been previously activated
through
an
Open File
or
Make File function (function
15
or
22), the Close File function
permanently records the new FCB in the referenced disk directory. The FCB
matching process for the close
is
identical
to
the open function. The directory code
returned for a successful Close File function
is
0,
1,
2,
or
3,
while OFFH (255
decimal)
is
returned
if
the file name could
not
be found in the directory. A file need
not
be closed
if
only read operations have
taken
place.
If
write operations have
occurred, however, the Close File function
is
necessary
to
permanently record the
new directory information.
SEARCH
FOR
FIRST
ENTRY
CL:
IlH
DX: FCB
Offset
FUNCTION
17
SEARCH
FOR
FIRST
RETURN
AL: Return
Code
The Search for First function scans
the
directory for a match with the file given by
the FCB addressed by
DX.
The value OFFH (255 decimal)
is
returned
if
the file
is
not
found; otherwise 0,
1,
2,
or
3
is
returned indicating the file
is
present.
If
the file
is
found, the buffer
at
the current
DMA
address
is
filled with the record containing the
directory entry,
and
its relative starting position
is
calculated as
AL
* 32 (i.e., rotate
the
AL
register left 5 bits). Although it
is
not
normally required for application
programs, the directory information can be extracted from the buffer
at
this
position.
4-15
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