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80386
20
o
MAX
LIMIT
FIXED
AT
64K
IN
REAL
MODE
1
64K
L-_---1
- - - -
~J_----+_--'-j
~
SELECTED
SEGMENT
SEGMENT
BASE
231630-54
Figure 3-1. Real Address Mode Addressing
purpose of Real Mode
is
to set
up
the processor for
Protected Mode Operation.
The
LOCK prefix
on
the 80386, even
in
Real Mode,
is
more restrictive than on the 80286. This
is
due to
the addition of paging
on
the 80386
in
Protected
Mode and Virtual 8086 Mode. Paging makes it im-
possible to guarantee that repeated string instruc-
tions can
be
LOCKed. The 80386 can't require that
all pages holding the string
be
physically present
in
memory. Hence, a
Page
Fault (exception
14)
might
have to be taken during the repeated string instruc-
tion. Therefore the
LOCK
prefix can't
be
supported
during repeated string instructions.
These are the
only instruction forms where the
LOCK
prefix
is
legal
on
the 80386:
Opcode
Operands
(Dest, Source)
BIT Test and
Mem,
Reg/immed
SET/RESET/COMPLEMENT
XCHG
Reg,
Mem
XCHG
Mem,
Reg
ADD,
OR,
ADC,
SBB, Mem, Reg/immed
AND,
SUB,
XOR
NOT,
NEG,
INC,
DEC
Mem
An
exception 6 will
be
generated if a
LOCK
prefix
is
placed before any instruction form
or
opcode not
listed above. The
LOCK
prefix allows indivisible
read/modify/write operations
on
memory operands
using the instructions above. For example, even the
"ADD Reg/immed, Mem"
is
not LOCKable, because
the
Mem
operand is not the destination (and there-
fore
no
memory read/modify/operation
is
being per-
formed).
32
Since,
on
the 80386, repeated string instructions are
not
LOCKable, it
is
not possible to
LOCK
the
bus
for
a
long period of time. Therefore, the
LOCK
prefix
is
not IOPL-sensitive
on
the 80386. The
LOCK
prefix
can
be
used at any privilege level, but only
on
the
instruction forms listed above.
3.2 MEMORY ADDRESSING
In
Real Mode the maximum memory size
is
limited to
1 megabyte. Thus,
only address lines
A2-A
19,
BEO-BE are active. (Exception, the high address
lines A20-A31 are high during CS-relative memory
cycles until
an
intersegment jump or call
is
executed
(see section
2.10)).
Since paging
is
not allowed
in
Real Mode the linear
addresses are the same
as
physical addresses.
Physical addresses are formed
in
Real Mode
by
adding the contents of the appropriate segment reg-
ister which
is
shifted left
by
four bits to
an
effective
address. This addition results
in
a physical address
from
OOOOOOOOH
to 0010FFEFH. This
is
compatible
with
80286 Real Mode. Since segment registers
are
shifted left
by
4 bits this implies that Real Mode seg-
ments always start on 16 byte boundaries.
All segments
in
Real
Mode are exactly 64K bytes
long, and
may
be
read, written, or executed. The
80386 will generate
an
exception
13
if a data oper-
and or instruction fetch occurs past the end of a
segment.
(i.e.
if
an
operand has
an
offset greater the
FFFFH, example a word with a
low byte at FFFFH
and the high byte at
OOOOH)
Segments
may
be
overlapped
in
Real Mode. Thus, if
a particular segment does not use
all 64K bytes an-
other segment can be overlayed on top of the un-
used portion of the previous segment. This
allows
the programmer to minimize the amount of physical
memory needed for a program.