User's Manual
SYSTEM CONTROL AND INITIALIZATION
The
EM
flag indicates a processor extension function
is
to be emulated by software.
If
EM
= 1 and
MP=O,
all
ESCAPE
instructions will be trapped via the processor extension not-present exception
(#7).
MP
flag tells whether a processor extension
is
present.
If
MP=l
and
TS=I,
escape and wait instruc-
tions will cause exception
7.
If
ESC
instructions are to be used, either the
MP
or the
EM
bit must be set, but not both.
The
PE
flag indicates
that
the 80236
is
in
the protected virtual address mode. Once the PE flag
is
set,
it
can
be cleared only by a reset, which then puts the system in real address mode emulating the 8086.
Table 10-2 shows the recommended usage
of
the MSW.
Other
encodings
of
these bits are not
recommended.
10.2.3 Other Instructions
Instructions
that
verify or adjust access rights, segment limits, or privilege levels can be used to avoid
exceptions or faults
that
are correctable. Section 10.3 describes such instructions.
10.3
PRIVILEGED
AND
TRUSTED
INSTRUCTIONS
Instructions
that
execute only
at
CPL=O
are called "privileged." An
attempt
to execute the privileged
instructions
at
any other privilege level causes a general protection exception (#13) with an error code
of
zero.
The
privileged instructions manipulate descriptor tables or system registers. Incorrect use of
these instructions can produce unrecoverable conditions. Some of these instructions (LGDT,
LLDT,
and L
TR)
are discussed
in
section 10.2.
Table
10-2.
Recommended
MSW
Encodings
for
Processor
Extension
Control
Instructions
15
MP
EM
Recommended
Use
Causing
Exception 7
0 0 0 Initial encoding after
RESET.
80286 operation
is
identical to 8086,
None
8088.
Use
this encoding only if
no
ESC
instructions are to
be
executed.
0 0
1
No
processor extension
is
available. Software will emulate its
ESC
function. Wait instructions do not cause exception
7.
1
0
1
No
processor extension
is
available. Software will emulate its
ESC
function. The current processor extension context may belong to
another task.
0
1
0
A processor extension exists.
WAIT
(ifTS=1)
1 1 0
A processor extension exists. The current processor extension
ESC
or
context
may
belong to another task. The exception 7
on
WAIT WAIT
allows
software to test for
an
error pending from a previous
(if
TS=1)
processor extension operation.
10-5