String Instructions

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String Instructions

• SI always points to the source operand which is
  located in the data segment.
• DI always point to the destination operand which
  is located in the extra segment.
• ES must be initialized prior to using these
  instructions.
• data is stored in consecutive memory locations,

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• The Direction flag (DF) is used to increment or
  decrement the data pointers
• The instruction CLD resets DF (DF 0) for
  autoincrement.
• The instruction STD sets DF (DF 1) for
  autodecrement.
• The REP prefix allows a string instruction to
  perform the operation repeatedly.
• Register CX holds the number of times that the
  instruction should be repeated - It is
  automatically decrement after each REP.

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(No Transcript)
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• This program transfer 20 bytes of data using MOVS
• in the data segment
• data1 DB abcdefghijklmnopqrst
• ORG 30H
• data2 DB 20 DUP(?)
• in the code segment
• ASSUME CSCDSEG, DSDTSEG, ESDTSEG, SSSTSEG
• MOV AX, DTSEG
• MOV DS, AX initialize the data segment
• MOV ES, AX initialize the extra segment
• CLD clear direction flag for autoincrement
• MOV SI, OFFSET data1 load the source pointer
• MOV DI, OFFSET data2 load destination pointer
• MOV CX, 20 load the counter
• REP MOVSB repeat until CX becomes zero

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• This program stores the byte AAH in 100 memory
  locations using STOS
• put pattern AAAAH in 50 word locations
• MOV AX, DTSEG initialize
• MOV DS, AX DS register
• MOV ES, AX and ES register
• CLD clear direction flag for autoincrement
• MOV CX, 50 load the counter (50 words)
• MOV DI, OFFSET mem_area load the pointer for
  destination
• MOV AX , AAAAH load the pattern
• REP STOSW repeat until CX 0

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• This program checks the spelling of the word
  EUROPE and inform the user
• data_dict DB Europe
• data_typed DB Euorope
• MESSAGE1 DB The spelling is correct
• MESSAGE2 DB Wrong spelling
• from the code segment
• CLD clear DF for increment
• MOV SI, OFFSET dat_dict offset for data1
• MOV DIlt OFFSET dat_typed offset for data2
• MOV CX, 6 load the counter
• PEPE CMPSB repeat as long as equal or until
  CX0
• JE OVER IF ZF0 Then display message1
• JMP DISPLAY otherwise display message2

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• This program scans the name Mr. Gones and
  replaces the G with J
• in the data segment
• DATA1 DB Mr. Gones
• in the code segment
• MOV AX, DTSEG
• MOV DS, AX
• MOV ES, AX
• CLD DF 0 for increment
  
• MOV DI, OFFSET DATA1 ESDI array offset
• MOV CX, 9 length of the data array
• MOV AL, G prepare for scanning for the
  letter G
• REPNE SCASB repeat if not equal or until CX0
• JNE OVER jump if Z 0
• DEC DI decrement to point at G
• MOV BYTE PTR DI, J replace G with J