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ARCHITECTURE
AND
INSTRUCTIONS
FIRST
OPERAND
CHOICE
DEPENDS ON
ADDRESSING
MODE:'
FIRST
OPERAND
IN
MEMORY
INDIRECT
ADDRj:SSING
00';
DISP = 0
MOD=01
; OISP = DISP-LO SIGN
EXTENDED
10
; DISP = DISP-HI; DISP-LO
OPERAND
RIM:
EFFECTIVE
ADDRESS
000
(BX) + (SI) + DISP
001
(BX) + (DI) + DISP
010
(BP) + (SI) + DISP
011
(BP) + (DI) +
DISP
100
(SI) + DISP
101
(DI) + DISP
i
110
(BP)
+ DISP
,
111
(BX) + DISP
,
Where
( )
means
"contents
of"
'Exception-direct
addressing
mode
DIRECT ADDRESSING
MOD
=
00
AND
R/M
=
110
OPERAND EFFECTIVE
"ADDRESS
=
DISP-HI, DISP-LO
Figure 2-5. Determining First Operand
,
DATA
DATA
MEMORY
STRUCTURE
,WITHOUT
BASE
WITH BASE
SIMPLE
DIRECT
BX
+:
OFFSET
VARIABLE
ARRAYS
SI
BX +
SI
DI
BX +
DI
ARRAYS
SI
+ OFFSET BX +
SI
+ OFFSET
OF RECORDS
DI
+ OFFSET
BX +
DI
+:
9FFSET
FIRST
OPERAND
IN REGISTER
MOD
=
11
REGISTER
RIM:
' 8-BIT
16-BIT
(W
=
0)
(W =
1)'
000
AL
AX
001
CL
CX
010
DL
DX
011
BL
BX
100
AH
SP
101
CH
BP
110
DH SI
111
BH '
DI
STACK
BP + OFFSET
BP
-+-
SI
BP +
DI,
BP +
SI+
OFFSET
BP +
DI
+ OFFSET
Figure
2~6.
Effective Addresses Used
with
Different Data
Structures'
DEFAULT
ALTERNATE
TYPE
OF
MEMORY
REFERENCE
SEGMENT
SEGMENT
LOGICAL
ADDRESS
BASE BASE
Instruction
Fetch
CS
NONE.
IP
Stack
Operation
SS
NONE
SP
String,
Source
DS CS,ES,SS
SI
String
Destination
ES
NONE
DI
BP
Used
As
Base
Register
SS
CS,DS,ES
Effective
Address
General Data
Read/Write
DS CS,ES,SS
Effective
Address
Figure 2-7. 8088 Address Components
2-7