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ADVANCED TOPICS
Table
11-1.
NPX
Context Switching
Step Operation
Lines
(Figure
11-3)
1. Save the working registers
28,29
2.
Set up address
for
kernel work area
30,31
3. Get current task
10
from Task Register
32
4.
Clear Task Switch flag to allow
NPX
work
34
5. Inhibit interrupts
35
6.
Compare owner with current task
10
37
If same owner:
7a. Restore working registers
48,49
7b.
and return
50
If
owner is not
current task:
8a.
Use owner
10
to save old context in Its TSS
42,43,44
8b. R,estore context
of
current task;
45
restore working registers;
46
and return 52
updated
by
a re-entrant procedure that
is
invoked by an inter-processor interrupt. The handler must
ensure that the segment registers, the LDTR and the TR, are re-loaded. This happens automatically if
the interrupt
is
serviced
by
a task switch.
11.6 SHUTDOWN
Shutdown occurs when a severe error condition prevents further processing. Shutdown
is
very similar
to
HL
T
in
that the 80286 stops executing instructions. The 80286 externally signals shutdown as a
Halt bus cycle with
Al
=0.
The
NMI
or RESET input
will
force the 80286 out of shutdown. The
INTR
input
is
ignored during shutdown.
11-7