1. Processing String Data and Binary Data (continue)

2. Code for last practice exercise
DATASEG SEGMENT PARA 'Data'
; Please insert your data declaration here
DES_STR DB 50 DUP ('$')
ParaList label byte
maxlen DB 50
actlen DB ?
SRC_STR DB 50 DUP ('$')
DATASEG ENDS

3. Code for last practice exercise
; Code for reading input from keyboard
MOV AH, 0AH
LEA DX, ParaList
INT 21H
; Code for reversing the string
LEA SI, SRC_STR
LEA DI, DES_STR
MOV CH,0
MOV CL, ACTLEN
DEC CX
ADD DI, CX
INC CX
L10:
LODSB
MOV [DI], AL
DEC DI
LOOP L10

4. CMPSB, CMPSW Compare String Instruction
Compare bytes: ES:[DI] from DS:[SI]. DS:[SI] - ES:[DI]
set flags according to result: OF, SF, ZF, AF, PF, CF
if DF = 0 then
SI = SI + 1
DI = DI + 1
else
SI = SI - 1
DI = DI - 1

5. Example STRING1 DB ‘COMPUTER’ STRING2 DB ‘COMPUTER’ MOV CX, 8
LEA DI, STRING2
LEA SI, STRING1
REPE CMPSB
JNE exit
exit: …

6. REPE (repeat if equal)
Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 1 (result is Equal), maximum CX times. check_cx: if CX <> 0 then
do following chain instruction
CX = CX - 1
if ZF = 1 then:
go back to check_cx
else
exit from REPE cycle

7. SCASB: SCAN STRING INSTRUCTION
Scan a string for a specified value. Continue to Compare bytes: AL from ES:[DI] while the comparison is not equal or until CX is 0
(Similar to indexOf in java or strstr in C) ES:[DI] - AL
set flags according to result: OF, SF, ZF, AF, PF, CF
if DF = 0 then
DI = DI + 1
else
DI = DI - 1

8. Example: STRING1 DB ‘COMPUTER’ … MOV AL, ‘r’ MOV CX, 8 LEA DI, STRING1
REPNE SCASB

9. REPNE
Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 0 (result is Not Equal), maximum CX times. check_cx: if CX <> 0 then
do following chain instruction
CX = CX - 1
if ZF = 0 then:
go back to check_cx
else
exit from REPNE cycle

10. Replicating a pattern PATTERN DB ‘|----|’ RESULT DB 42 DUP (‘ ‘)
MOV CX, 21
LEA DI, RESULT
LEA SI, PATTERN
REP MOVSW

11. Lab/Practice 1. Open your browser and open this page:
C:\emu8086\documentation\8086_instruction_set.html
And
C:\emu8086\documentation\8086_and_dos_interrupts.html
2. Open your emu8086 software
3. Cut and paste (or type) the following code (as shown in the next page) and save move.asm

12. page 60,132
TITLE MovePractice Move ;
STACK SEGMENT PARA STACK 'Stack'
DW DUP(0)
STACK ENDS ;
DATASEG SEGMENT PARA 'Data'
; Please insert your data declaration here
DATASEG ENDS
CODESEG SEGMENT PARA 'Code'
MAIN PROC FAR
MOV AX, DATASEG
MOV DS, AX
MOV ES, AX
; Please insert your code here
MOV AX,4C00H ;exit procedure
INT 21H
MAIN ENDP
CODESEG ENDS
END MAIN ;End of program

13. Practice 4. Modify your code so that:
- Define: BUS_TITLE DB ‘COMPUTER WIZARDS’
- WORK_SPACE DB 16 DUP(20H)
- Move BUS_TITLE to WORK_SPACE from left to right (CLD)
- MOV BUS_TITLE to WORK_SPACE from right to left (STD)
- COMPARE BUS_TITLE with WORK_SPACE, print “equal” if equal, print “not equal” if not
5. Compile and run

14. Code ; Data segment declaration EQUALPROMPT DB 'Equal', '$'
NOTEQUALPROMPT DB 'Not Equal', '$'
BUS_TITLE DB 'COMPUTER WIZARDS','$'
WORK_SPACE DB 17 DUP(20H)

15. Code LEA SI, BUS_TITLE LEA DI, WORK_SPACE MOV CX, 17 REP MOVSB
; Moving from left to right
CLD
LEA SI, BUS_TITLE
LEA DI, WORK_SPACE
MOV CX, 17
REP MOVSB
; Moving from right to left
STD
LEA SI, BUS_TITLE+16
LEA DI, WORK_SPACE+16

16. Code ; Print equal if equal, print not equal if not MOV CX, 17
LEA DI, BUS_TITLE
LEA SI, WORK_SPACE
REPE CMPSB
JE EQUALPRINT
MOV AH, 09H
LEA DX, NOTEQUALPROMPT
INT 21H
JMP exit
EQUALPRINT:
LEA DX, EQUALPROMPT

17. Binary Data
ADC
CBW
NEG
SBB

18. ADC
Add with Carry. Algorithm: operand1 = operand1 + operand2 + CF

19. Example
STC ; set CF =1
MOV AL, 5 ; AL = 5
ADC AL, 1 ;
AL =?

20. CBW Convert byte into word. Algorithm: if high bit of AL = 1 then:
AH = 255 (0FFh)
else
AH = 0

21. Example MOV AX, 0 ; AH = 0, AL = 0 MOV AL, -5 ; AX = 000FBh (251)
CBW ; AX = 0FFFBh (-5)

22. NEG
Negate. Makes operand negative (two's complement). Algorithm: Invert all bits of the operand
Add 1 to inverted operand

23. Example MOV AL, 5 ; AL = 05h NEG AL ; AL = 0FBh (-5)
NEG AL ; AL = 05h (5)

24. SBB
Subtract with Borrow. Algorithm: operand1 = operand1 - operand2 - CF

25. Example
STC ; Set CF=1
MOV AL, 5
SBB AL, 3
AL =?

26. Distinguish between a carry and an overflow
Arithmetic carry (CF)
An arithmetic operation transfers the resulting sign bit (0 or 1) to CF.
When the carry occurs with unsigned data, the result is invalid. When carry occurs with signed data, the result is valid

27. Distinguish between a carry and an overflow
Arithmetic overflow (OF)
Example: unsigned signed decimal
(1) (invalid) (invalid)
CF = 1
OF = 1

28. Note about DIV/ IDIV
If divisor is a byte, a value must be greater than the left byte of the dividend (AH)
If divisor is a word, its value must be greater than left word of the dividend (DX)

29. Define doubleword using words
Assume, we have H, we can define this as:
DDVAR DW 0147H
DW 1329H

30. Practice 1. Open your browser and open this page:
C:\emu8086\documentation\8086_instruction_set.html
And
C:\emu8086\documentation\8086_and_dos_interrupts.html
2. Open your emu8086 software
3. Cut and paste (or type) the following code (as shown in the next page) and save add.asm

31. page 60,132
TITLE AddPractice Add ;
STACK SEGMENT PARA STACK 'Stack'
DW DUP(0)
STACK ENDS ;
DATASEG SEGMENT PARA 'Data'
; Please insert your data declaration here
DATASEG ENDS
CODESEG SEGMENT PARA 'Code'
MAIN PROC FAR
MOV AX, DATASEG
MOV DS, AX
MOV ES, AX
; Please insert your code here
MOV AX,4C00H ;exit procedure
INT 21H
MAIN ENDP
CODESEG ENDS
END MAIN ;End of program

32. Practice 4. Define data: BIN1 DW 0147H DW 1329H BIN2 DW 02B3H DW 0241H
SUM DW DW
Write a program to add the doublewords beginning at BIN1 and BIN2 and store the result to doubleword beginning at SUM
SUM= 156A03FAH