Basic Disk Operating System (BDOS) Functions
READ
SEQUENTIAL
ENTRY
CL: 14H
DX:
FCB
Offset
RETURN
FUNCTION
20
AL: Return Code
READ
SEQUENTIAL
Given
that
the
FCB
addressed by
DX
has been activated
through
an
Open
File
or
Make
File function (function
15
or
22), the
Read
Sequential function reads the next
128-byte record from the file into memory
at
the current
DMA
address.
The
record
is
read
from
position"
cr"
of
the extent,
and
the"
cr" field
is
automatically incre-
mented to the next record position.
If
the "Cf" field overflows, the next logical extent
is automatically
opened
and
the "cr" field
is
reset
to
zero in
preparation
for the next
read operation. The "cr" field must be set
to
zero by the user following the
open
call
if
the
intent
is
to
read sequentially from the beginning
of
the file. The value
OOH
is
returned
in register
AL
if the read
operation
was successful. A value
of
01
H is
returned if
no
data
exists
at
the next record position
of
the file. The
no
data
situation
is
encountered
at
the end
of
a file. However, it can also occur if an
attempt
is
made
to
read a
data
block which has
not
been previously written,
or
an
extent which has
not
been created. These situations are usually restricted
to
files created
or
appended
by--
use
of
the BDOS Write
Random
function (function 34).
WRITE
SEQUENTIAL
ENTRY
CL: ISH
DX: FCB
Offset
RETURN
FUNCTION
21
AL: Return Code
WRITE
SEQUENTIAL
Given
that
the FCB addressed by
DX
has been activated
through
an
Open
File
or
Make
File function (function
15
or
22), the Write Sequential
function
writes the
128-byte
data
record
at
the current
DMA
address
to
the file named by the
FeB.
The
record
is
placed
at
position "cr"
of
the file,
and
the "cr" field
is
automatically
incremented
to
the next record position.
If
the"
cr" field overflows, the next logical
extent
is
automatically opened
and
the"
cr" field
is
reset to zero in
preparation
for
the next write operation. Write
operations
can
take place into
an
existing file, in
which case newly written records overlay those which already exist in the file.
The
"cr" field must be set to zero by the user following
an
open
or
make call
if
the intent
is
4-17
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