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80386
SEGMENT
REGISTER
.--------~
BASE
REGISTER
SS
GS
FS
ES
DS
+
!+-
____
-I
DISPLACEMENT
(IN
INSTRUCTION)
EFFECTIVE
ADDR
ESS
LINEAR
/
SEGMENT
LIMIT
DESCRIPTOR
REGISTERS
ADDRESS
ACCESS
RIGHTS
CS
LIMIT
•
TARGET
ADDRESS
SELECTED
SEGMENT
BASE
ADDRESS
------~
SEGMENT
BASE
ADDRESS
231630-51
Figure 2-9.
Addressing
Mode
Calculations
2.5.4
Differences
Between
16
and 32
Bit
Addresses
In
order to provide software compatibility with the
80286 and the 8086, the 80386 can execute 16-bit
instructions
in
Real and Protected Modes. The proc-
essor determines the size of the instructions it
is
ex-
ecuting by examining the D bit
in
the
CS
segment
Descriptor.
If the D bit
is
° then
all
operand lengths
and effective addresses
are
assumed to
be
16
bits
long.
If the D bit
is
1 then the default length for oper-
ands and addresses
is
32
bits.
In
Real Mode the
default size for operands and addresses
is
16-bits.
Regardless of the default precision of the operands
or addresses, the
80386
is
able to execute either 16
or 32-bit instructions. This
is
specified via the use of
override prefixes. Two prefixes, the Operand
Size
Prefix and the
Address
Length Prefix, override the
value of the D bit on
an
individual instruction basis.
These prefixes are
automatically added
by
Intel as-
semblers.
18
Example: The processor
is
executing
in
Real
Mode
and the programmer needs to access the
EAX
regis-
ters. The assembler code for this might
be
MOV
EAX,
32bitMEMORYOP, ASM 386 automatically de-
termines that
an
Operand Size Prefix
is
needed
and
generates
it.
Example: The D bit
is
0,
and the programmer wishes
to use
Scaled Index addressing mode to access
an
array. The Address Length Prefix allows the use of
MOV
DX,
TABLE[ESI*2]. The assembler uses
an
Address Length Prefix since, with D =
0,
the default
addressing mode
is
16-bits.
Example: The D bit
is
1,
and the program wants to
store a 16-bit quantity. The
Operand Length Prefix
is
used to specify only a 16-bit value; MOV MEM16,
DX.