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This
is
not a
problem
if
a
Warm
Start
Resume
is
performed
when
the
power
is
restored,
because
the
warm
start
resume
returns
to
whatever
was
executing
when
the
power
off
occurred.
However, as
warm
start
break
is
not
intended
to
return to
the
interrupted
process,
but
break out
if it
and
restart
the
Top
Level
program, a
potential
problem
could
occur
in
doing
a
warm
start
break.
The
following
discussion describes
what happens
during a
warm
start
break
when one
of
the
critical
regions
of
code
in
the
O/S was
interrupted:
1
.
When
a user
program
calls
an
O/S
function,
the
O/S pops
the user's
return
address
off
of
the
user's
stack
and
saves
it
rn
a
pair of local variables.
This
is
done
so
that
the
segment
portion
of
the
return
address can be
kept correct
if
the
program
is
running
from
RAM
and
the execution
of
the
system
call
causes
the
file
containing the
program code
to
move.
The O/S
then switches
to
its
own
stack
and
begins executing the
function
call.
When
the
O/S
enters
a
critical
region
of
code
that
must
complete,
it
sets
a
flag
that
will
cause
Warm
Start
Break
to
resume
that
system
call
before the
warm
start
break
occurs.
O/S
stack
O/S
return
address
O/S
is
executing
critical
region
of
code.
Power
off
hardware
interrupt
occurs
2.
The
hardware
interrupt
transfers control to
the
BIOS, which
switches
to
its
own
interrupt
handler
stack.
BIOS
stack
BIOS
Return
address
The BIOS
performs whatever
actions are
necessary,
and
then
performs an
INT 4Eh.
3.
The
INT
4Eh
transfers control
to
the
O/S Power
Off
Hook, which
saves
the
machine
state
so
that
it
can be
restored during
Warm
Start
Resume.
O/S power
off
stack
O/S
Return
address
The O/S
saves
the
address
of
the
BIOS
power
off
stack,
switches
to
its
own
power
off
stack,
and pushes
all
of
the
registers.
It
saves
the
location
of
the stack
pointer
in its
own
power
off
stack,
and
then
performs
an INT
58h,
function
1
to
tell
the
BIOS
to
turn
the
power
off.
168
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