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80386
VIRTUAL
MODE
8086
TASK
PAGE
DIRECTORY
ROOT
VIRTUAL
MODE
8086
TASK
8086
OS
EMPTY
TASK
1
PAGE
TABLE
PAGE
DIRECTORY
TASK
1
PHYSICAL
MEMORY
...........
~~~
02000000(H)
~I
TASK
1
WI!!
MEMORY
I777l
TASK
2
rtfLA
MEMORY
OOOOOOOO(H)
111
8086
OS
MEMORY
~
80386
OS
~
MEMORY
231630-69
Figure 4-24. Virtual 8086 Environment Memory Management
Figure 4·24 shows how the 80386 paging hardware
enables multiple 8086 programs to
run
under a virtu·
al
memory demand paged system.
4.6.4 Protection and
1/0
Permission
Bitmap
All Virtual 8086 Mode programs execute at privilege
level
3,
the level of least privilege. As such, Virtual
8086
Mode programs are subject to all of the protec-
tion checks defined
in
Protected Mode. (This
is
dif-
ferent from
Real Mode which implicitly
is
executing
at
privilege level
0,
the level of greatest privilege.)
Thus,
an
attempt to execute a privileged instruction
when
in
Virtual 8086 Mode will cause an exception
13
fault.
The following are privileged instructions, which may
be
executed only at Privilege Level
o.
Therefore, at-
tempting to execute these instructions
in
Virtual
8086
Mode (or anytime
CPL
>
0)
causes
an
excep-
tion
13
fault:
LIDT:
MOV
DRn,reg:
MOV
reg,DRn:
LGDT:
MOV
TRn,reg:
MOV
reg,TRn:
56
LMSW:
CLTS;
HLT:
MOV
CRn,reg:
MOV
reg,CRn.
Several instructions, particularly those applying to
the
multitasking model and protection model, are
available only
in
Protected Mode. Therefore, at-
tempting to execute the
following instructions
in
Real Mode or
in
Virtual 8086 Mode generates
an
exception 6 fault:
LTR;
LLDT;
LAR:
LSL;
ARPL.
STR;
SLDT;
VERR:
VERW;
The instructions which are IOPL-sensitive
in
Protect-
ed
Mode are:
IN;
STI:
OUT;
CLI
INS:
OUTS;
REP
INS;
REP
OUTS;