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Calling
the
BIOS
All
BIOS
calls
are
made
via
software
interrupts.
This presents
a
clean, easily
documented
interface to
the
BIOS.
If
the user
documentation
describes only
this
interface
to
the
BIOS
and
a note discouraging the
use
of
undocumented
ROM
routines,
it
will
be
possible
for
the
ROM
to
be
easily
revised without introducing
incompatibilities.
Except as otherwise
noted,
registers that
are not
used
to return
values
must be
preserved by
the
BIOS.
For
any
BIOS
calls
where
no
provisions are
made
for
the
BIOS
to
return error
codes
on
"out-
of-range"
parameters,
it
may
be
assumed
that
those
BIOS
functions
will
never be
called with
an
illegal
parameter.
Except
for
MNI
interrupt
entry
and
exit,
the
BIOS
functions
should switch
to
a
BIOS
stack
on
entry
and
restore the user's stack
on
exit.
The BIOS may
not
assume
that
there
is
sufficient
space on
the
calling
program's
stack
for
it
to function.
127
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