8086 Programming
Data Transfer Instructions
Push and Pop Instructions
 Last in first out.
 The PUSH instruction transfers two bytes to the top of the
stack.
037FF
AX
6AB3
6A
037FE
B3
037FD
-2
+
SP
SS
07FF
stack
segment
0300
*16
03000
+
The effect of PUSH AX instruction
PUSH
CX
[SS:SP-1]
CH
[SS:SP-2]
CL
SP
SP-2
1
Symbolic
PUSH reg16
PUSH seg
PUSH mem
Example
PUSH BX
PUSH DS
PUSH [DI+2]
PUSHA
PUSHA
PUSHF
PUSHF
Table The PUSH instructions
Note
16-bit register
Segment register
memory
Save all 16-bit
registers
Save flags
The PUSHA instruction pushes all the internal registers onto the
stack in the following order: AX, CX, DX, BX, SP, BP, SI, DI.
The value of the SP is that before the PUSHA instruction.
The PUSHF (push flag) instruction copies the contents of the flag register
to the stack.
2
 The POP instruction performs the inverse operation of a PUSH
instruction.
037FF
BX
6A
6A
037FE
B3
B3
037FD
2
+
SP
SS
07FD
stack
segment
0300
*16
03000
+
The effect of POP BX instruction
POP BX
BL
BH
SP
[SS:SP]
[SS:SP+1]
SP+2
3
-d300:7f0l10
0300:07F0 FA 8B F4 2E 89 36 D4 04-8C D6 2E 89 36 D2 04 B8 .....6......6...
-u100l7
1582:0100 B8B36A
1582:0103 50
1582:0104 5B
1582:0105 CD20
MOV
PUSH
POP
INT
AX,6AB3
AX
BX
20
-r
AX=0000 BX=0000 CX=0000 DX=0000 SP=07FF BP=0000 SI=0000
DI=0000
DS=1582 ES=1582 SS=0300 CS=1582 IP=0100 NV UP EI PL NZ NA PO NC
1582:0100 B8B36A
MOV AX,6AB3
-t
AX=6AB3 BX=0000 CX=0000 DX=0000 SP=07FF BP=0000 SI=0000
DI=0000
DS=1582 ES=1582 SS=0300 CS=1582 IP=0103 NV UP EI PL NZ NA PO NC
1582:0103 50
PUSH AX
-t
AX=6AB3 BX=0000 CX=0000 DX=0000 SP=07FD BP=0000 SI=0000
DI=0000
DS=1582 ES=1582 SS=0300 CS=1582 IP=0104 NV UP EI PL NZ NA PO NC
1582:0104 5B
POP BX
-t
AX=6AB3 BX=6AB3 CX=0000 DX=0000 SP=07FF BP=0000 SI=0000
DI=0000
DS=1582 ES=1582 SS=0300 CS=1582 IP=0105 NV UP EI PL NZ NA PO NC
1582:0105 CD20
INT 20
-
4
The XCHG instruction
Performs a register-to-register or register-to-memory
swap
XCHG
AX,BX
AX
XCHG
AL,BH
AL
XCHG
[SI],DX
[DS:SI]
BX
BH
DX
The LAHF and SAHF instructions
LAHF
SAHF
AH
Flagslow
Flagslow
AH
5
The LEA instruction
Loads the effective address of the source to the destination
LEA
BX,NUMB
;NUMB is assumed to point at
the byte at location 10FD in the
data segment.
BL
FD
BH
10
LEA BX,NUMB = MOV BX,OFFSET NUMB
LEA
BX,[DI]
; BX
DI
MOV
BX,[DI]
; BX
[DI]
Example:
6
offset directive can be used with simple operands such as LIST.
But cannot be used for operands such as [di], LIST[di], ..
Using offset directive the assembler calculates the offset address, while
in case of LEA the microprocessor calculates. Hence, MOV …offset is
faster.
LEA cx,[bx+si]
; CX
(BX+SI) mod 64K
If bx=2f00 H; si=10d0H
cx
3fd0H
if bx=ff00H, si=1000H
cx
0f00H
The LDS AND LES instructions
LDS
BX,DWORD PTR[SI]
BL
[SI]; BH
[SI+1]
DS
[SI+3:SI+2]; in the data segment
LES
BL
ES
BX,DWORD PTR[SI]
[SI]; BH
[SI+1]
[SI+3:SI+2];
in the extra segment
7
7F
057FF
00
057FF
6A
6A
057FE
B3
B3
057FD
BX
DI
DS
07FD
data
segment
0500
*16
05000
+
The effect of LDS BX,[DI] Instruction
8
String Instructions
Instructions for moving large blocks of data or strings.
For all these instructions the memory source is DS:SI and the
destination is ES:DI.
The offset memory pointers, SI and DI are automatically
incremented or decremented depending on the state of DF.
STOS
Stores AL or AX at the extra segment memory location addressed
by the DI register; if DF = 0, increment DI, else decrement DI
STOSB
STOSW
ES:[DI]
AL
If DF = 0, DI
DI+1
If DF = 1, DI
DI-1
ES:[DI]
ES:[DI+1]
AL
AH
If DF = 0, DI
DI+2
If DF = 1, DI
DI-2
LODS
Loads AL or AX with the data stored at the data segment memory
location addressed by the SI register; if DF = 0, increment SI, else
decrement SI
9
LODSB
LODSW
AL
DS:[SI]
If DF = 0, SI
SI+1
If DF = 1, SI
SI-1
AL
DS:[SI]
AH
DS:[SI+1]
If DF = 0, SI
SI+2
If DF = 1, SI
SI-2
MOVS
Transfers data from a memory location to another.
This is the only memory-to- memory transfer allowed.
The MOVS instruction transfer a byte or a word from the data
segment location addressed by the SI to the extra segment
location addressed by the DI. The pointers then increment or
decrement according to DF.
MOVSB
ES:[DI]
If DF = 0,
If DF = 1,
DS:[SI]
SI
SI+1
DI
DI+1
SI
SI-1
DI
DI-1
10
MOVSW
ES:[DI]
DS:[SI]
ES:[DI+1]
DS:[SI+1]
If DF = 0, SI
SI+2
DI
DI+2
If DF = 1, SI
SI-2
DI
DI-2
Setting the Direction Flag
Two instructions used:
STD set direction flag so that the pointers are auto decremented.
CLD clear direction flag so that the pointers are auto incremented.
11
Repeat Prefix
Preceding the string instructions STOS or MOVS with REP
causes these instructions to be repeated a number of times equal
to the contents of CX register.
REP
STOSB
;STOSB ; CX
CX-1
;repeat until CX = 0
d1c66:20l201C66:0020 FE FE FE FE FE FE FE FE-FE FE FE FE FE FE
FE FE
................
1C66:0030 FE FE FE FE FE FE FE FE-FE FE FE FE FE FE
FE FE
................
u100l81C66:0100 B0DD
MOV
AL ,DD
1C66:0102 B92000
MOV
CX,0020
1C66:0105 F3
REPZ
1C66:0106 AA
STOSB
1C66:0107 CC
INT
3
rAX=00FE BX=0000 CX=0000
BP=0000 SI=0000 DI=0020
DX=0000
SP=FFEE
DS=1C66 ES=1C66 SS=1C66 CS=1C66 IP=0100
NV UP
12
EI PL NZ NA PO NC
DD,1C66:0100 B0DD
MOV
AL
g-
AX=00DD BX=0000 CX=0000
BP=0000 SI=0000 DI=0040
DX=0000
SP=FFEE
DS=1C66 ES=1C66 SS=1C66 CS=1C66 IP=0107
EI PL NZ NA PO NC
1C66:0107 CC
INT
NV UP
3
d1c66:20l201C66:0020 DD DD DD DD DD DD DD DD-DD DD DD DD
DD DD DD DD
................
1C66:0030 DD DD DD DD DD DD DD DD-DD DD DD DD
DD DD DD DD
................
-
13
XLAT
Translate instruction
14
The IN and OUT instructions
An IN instruction transfers data from an external I/O device to
AL or AX.
An OUT instruction transfers data from AL or AX to an external
I/O device.
The I/O device address is called port address.
In direct addressing mode the address is a single byte.
In indirect addressing mode the address is two bytes in DX.
IN
AL,2EH
;AL
port 2EH
IN
AX,2EH
;AL
port 26H
;AH
port 27H
IN
AL,DX
;AL
port DX
IN
AX,DX
;AL
port DX
;AH
port DX+1
OUT
2EH,AL
; port 2EH
AL
OUT 2EH,AX
; port 26H
AL
; port 27H
AH
; port DX
AL
OUT DX,AL
15
OUT DX,AX
; port DX
; port DX+1
AL
AH
16
17