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80386
In
Virtual 8086 Mode, a slightly different set of
in-
structions are made 10PL-sensitive. The following
in-
structions are 10PL-sensitive
in
Virtual 8086 Mode:
INT
n;
STI;
PUSHF;
eLI;
POPF;
IRET
The
PUSHF,
POPF,
and IRET instructions
are
10PL-
sensitive
in
Virtual 8086 Mode only. This provision
allows the
IF
flag (interrupt enable flag) to
be
virtual-
ized to the Virtual 8086 Mode program. The INT n
software interrupt instruction is also 10PL-sensitive
in
Virtual 8086 Mode. Note, however, that the INT 3
(opcode
OCCH),
INTO,
and
BOUND instructions are
not
10PL-sensitive
in
Virtual 8086 mode (they aren't
10PL
sensitive
in
Protected Mode either).
Note that the
I/O
instructions
(IN,
OUT,
INS,
OUTS,
REP
INS,
and
REP
OUTS)
are
not
10PL-sensitive
in
Virtual 8086 mode. Rather, the
I/O
instructions be-
come automatically sensitive to the
I/O
Permission
Bitmap contained
in
the 386 Task State Segment.
The
I/O
Permission Bitmap, automatically used by
the 80386
in
Virtual 8086 Mode,
is
illustrated
by
Fig-
ures 4.15a and 4-15b.
The
I/O
Permission Bitmap can be viewed
as
a
0-
64 Kbit bit string, which begins
in
memory at offset
BiLMap_Offset
in
the current
TSS.
the 16-bit
pointer
BiLMap_Offset
(15:0) is found
in
the word
beginning at offset 66H (102 decimal) from the TSS
base,
as
shown
in
Figure 4-15a.
Each bit
in
the
I/O
Permission Bitmap corresponds
to a single byte-wide
I/O
port,
as
illustrated
in
Figure
4-15a. If a bit
is
0,
I/O
to the corresponding byte-
wide port can occur without generating
an
excep-
tion. Otherwise the
I/O
instruction causes
an
excep-
tion 13 fault. Since every byte-wide
I/O
port must be
protectable,
all
bits corresponding to a word-wide or
dword-wide port must
be
a for the word-wide or
dword-wide
I/O
to.be permitted. If all the referenced
bits
are
0,
the
I/O
will
be
allowed. If any referenced
bits are
1,
the attempted
I/O
will cause
an
exception
13 fault.
Due
to the
use
of a pointer to the base of the
I/O
Permission Bitmap, the bitmap
may
be located any-
where within the
TSS,
or
may
be ignored completely
by pointing the
BiLMap_Offset
(15:0) beyond the
limit of the TSS segment.
In
the same manner, only
a small portion of the 64K
I/O
space need have
an
associated
map
bit, by adjusting the TSS limit to
truncate the bitmap. This eliminates the commitment
of 8K of memory when a complete bitmap
is
not
required, while allowing the fully general case if de-
sired.
57
EXAMPLE
OF
BITMAP
FOR
I/O
PORTS
0-255:
Setting the TSS limit to
{biLMap_Offset
+
31
+
1"l
[0'
see note below] will allow a 32-byte bit-
map
for the
I/O
ports
#0-255,
plus a terminator
byte of
all
1 's
["
see note below]. This allows the
I/O
bitmap to control
I/O
Permission to
I/O
port
0-
255 while causing
an
exception
13
fault
on
attempt-
ed
I/O
to any
I/O
port 256 through 65,565.
"IMPORTANT
IMPLEMENTATION NOTE: Beyond
the
last byte of
I/O
mapping information
in
the
I/O
Permission Bitmap
must
be
a
byte
containing all 1
'so
The byte of all 1 's must be within the limit of the 386
TSS segment (see Figure 4-15a).
4.6.5
Interrupt
Handling
In
order to fully support the emulation of
an
8086
machine, interrupts
in
Virtual 8086 Mode
are
han-
dled
in
a unique fashion.
When
running
in
Virtual
Mode
all
interrupts
and
exceptions involve a privi-
lege
change back to the host 80386 operating sys-
tem. The 80386 operating system determines if the
interrupt comes from a Protected Mode
application
or from a Virtual Mode program
by
examining the
VM
bit
in
the EFLAGS image stored
on
the stack.
When a Virtual Mode program
is
interrupted and ex-
ecution passes to the interrupt routine at level
0,
the
VM
bit
is
cleared. However, the
VM
bit is still set
in
the EFLAG image
on
the stack.
The 80386 operating system
in
turn handles the
ex-
ception or interrupt and then returns control to the
8086 program. The 80386 operating system may
choose to
let the 8086 operating system handle the
interrupt or it may
emulate the function of the inter-
rupt handler. For example,
many
8086 operating
system
calls
are
accessed
by
PUSHing
parameters
on
the stack,
and
then executing
an
INT n instruc-
tion. If the
10PL
is set to a then
all
INT n instructions
will be intercepted
by
the 80386 operating system.
The 80386 operating system could emulate the
8086 operating system's call. Figure 4-25 shows
how the 80386 operating system could intercept
an
8086 operating system's call to "Open a File".
An
80386 operating system
can
provide a Virtual
8086
Environment which
is
totally transparent to the
application software via intercepting and then emu-
lating 8086 operating system's calls, and intercept-
ing
IN
and
OUT
instructions.
4.6.6 Entering and Leaving Virtual
8086 Mode
Virtual 8086 mode
is
entered
by
executing
an
IRET
instruction (at CPL=O), or Task Switch (at any
CPL)
to a 386 task whose 386 TSS has a FLAGS image
containing a 1
in
the
VM
bit position while the proc-