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Communications/Modem
Support
The RS-232
receiver
is
interrupt driven.
The BIOS
handles
the receiver
interrupts
and checks
for
error conditions.
If
an
error
is
detected
within
the
interrupt
service
routine,
the
error
code
must be saved so
that
it
can be
returned
as
part of
the
exit
conditions
for
the next
call
to
INT
59h, functions
8
-
C. Just
prior \o
placing the
data
into
the
queue,
the
BIOS
must
execute
an
INT 4Ch. This
allows
code
to
be
hooked
into
the receiver
interrupt routine
for
purposes
of
data
filtering
and
event
trapping.
INT
4Ah
-
Ring Detect
Hook
Entry:
No
registers specified
Exit:
No
registers specified
This
interrupt
should
be
invoked
when
an incoming
call is
detected
on
the
telephone
line.
INT
4Ch
-
Serial
Receiver
Queue Hook
Entry:
AL
= Character Received
AH
=
Serial
device the character
came
from
FLAGS.C
=
Exit:
FLAGS.C
=
-
the character
(in
AL) should be
put
into
the
queue
FLAGS. C =
1
-
the character
should
be
discarded
(i.e.,
do
not put the
character
into
the
queue)
AL
=
Character
to
put
in
queue
There must be
at
least
one
serial I/O port,
although there
may
be
more
if
desired.
Function
OFh
is
used
to
select
which
serial port
is
to
become
the active
port.
After a
call
to
function
OFh,
the selected
port
will
become
the
port
acted
upon
by
function
calls
07h
-
OEh.
The
following interrupts
support
the
serial I/O ports,
modem,
and
telephone
dialing:
174.
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