Processing String Data and Binary Data continue

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

4
CMPSB, CMPSW Compare String Instruction

• Compare bytes ESDI from DSSI. DSSI -
  ESDI
• set flags according to resultOF, 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_cxif CX ltgt 0 then
• do following chain instruction
• CX CX - 1
• if ZF 1 then
• go back to check_cx
• else
• exit from REPE cycle
• else
• exit from REPE cycle

7
SCASB SCAN STRING INSTRUCTION

• Scan a string for a specified value. Continue to
  Compare bytes AL from ESDI while the
  comparison is not equal or until CX is 0
• (Similar to indexOf in java or strstr in
  C)ESDI - AL
• set flags according to resultOF, 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_cxif CX ltgt 0 then
• do following chain instruction
• CX CX - 1
• if ZF 0 then
• go back to check_cx
• else
• exit from REPNE cycle
• 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.h
  tml
• 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 32 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

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

• 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_TITLE16
• LEA DI, WORK_SPACE16
• MOV CX, 17
• REP MOVSB

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
• MOV AH, 09H
• LEA DX, EQUALPROMPT
• INT 21H

17
Binary Data

• ADC
• CBW
• NEG
• SBB

18
ADC

• Add with Carry.Algorithmoperand1 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). AlgorithmInvert 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. Algorithmoperand1
  operand1 - operand2 - CF

25
Example

• STC Set CF1
• 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
• 11110110 246 -10
• 10001001 37 -119
• (1)01111111 127 (invalid) -129
  (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 13290147H, 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.h
  tml
• 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 32 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

32
Practice

• 4. Define data
• BIN1 DW 0147H
• DW 1329H
• BIN2 DW 02B3H
• DW 0241H
• SUM DW 0
• DW 0
• Write a program to add the doublewords beginning
  at BIN1 and BIN2 and store the result to
  doubleword beginning at SUM

SUM 156A03FAH