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The String Instruction

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Direction Flag

y Its purpose is to determine the direction in which stringoperations will proceed.

y These operations are implemented by the two index registers SI

and DI.y Suppose : STRING1 DB AB

y Offset address content ASCII char

y 0200h 041h A

y 0201h 042h B

y If DF =0, SI and DI proceed in the direction of increasingmemory location.

y If DF =1, SI and DI proceed in the direction of decreasingmemory location.

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CLD and STDy To make DF =0 use the CLD instruction:

y CLD ; clear direction flag

y To make DF =1 use the STD instruction:y STD ; set direction flag

y CLD and STD have no effect on the other flags.

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Using the DUP Operatory Use DUP (duplicate) to allocate (create space for) an

array or string.

var1 db 20 DUP(0) ; 20 bytes, all equal to zero

var2 db 20 DUP(?) ; 20 bytes, uninitialized

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Moving a String

y In order to move a defined string, we use two instructions

in assembly.

y MOVSB

y MOVSW

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MOVSB

y The MOVSB instruction

y MOVSB

copies the contents of the byte addressed by DS:SI, to the byteaddressed by ES:DI

y The contents of the source byte are unchanged

y After the byte has been moved, both SI and DI are automatically

incremented if DF = 0 or decremented if DF = 1

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Suppose we have defined two strings as follows:

y

.DATA

y STRING1 DB HELLO

y STRING2 DB 5 DUP (?)

y To move the first two bytes of string1 to string2, we execute the following instructions:

y MOV AX, @DATA

y MOV DS, AX

y MOV ES, AX

y LEA SI, STRING1

y LEA DI, STRING2

y CLD ; clear DF

y MOVSB

y MOVSB

MOVSB

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y Before MOVSB

y STRING1

y STRING2

y After MOVSB

y STRING1

y STRING2

y After MOVSB

y STRING1

y STRING2

MOVSB

H E L L O

H

H E L L O

H E

H E L L O

SI

DI

SI

DI

SI

DI

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THE REPPrefixy MOVSB moves only a single byte from the source string to the destination

string

y To move the entire string, first initialize CX to the number N of bytes in the

source string and execute

y REP MOVSB

y The REP prefix causesMOVSB to be executed N times

y After eachMOVSB, CX is decremented until it becomes 0

y To copy STRING1 to STRING2, we execute

y CLD

y LEA SI, STRING1

y LEA DI, STRING2

y MOV CX, 5 ; no of characters in STRING1

y REP MOVSB

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MOVSW

y The MOVSW instruction

y MOVSW

Moves a word from the source string to the destination string

y It expects DS:SI to point to a source string word and ES:DI to point to a

destination string word

y After a string word has been moved, both SI and DI are automatically

increased by 2 if DF = 0 or decreased by 2 if DF = 1

y MOVSB and MOVSW have no effect on the flags

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Store Stringy In order to store a string, we use two instructions in assembly.

y STOSB

y

Moves the contents of the AL

register to the byte addressed by

ES:DI.

y DI is incremented if DF = 0 or decremented if DF = 1

y STOSW

y Moves the contents of the AX register to the byte addressed by

ES:DI and updates DI by 2, according to the direction flag setting

y STOSB and STOSW have no effect on the flags.

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y Example

y Storing two As in STRING1

y MOV AX,@DATA

MOV ES,AX

LEA DI, STRING1

CLD

MOV AL, A

STOSB

STOSB

Store String

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y Before STOSB

y STRING1

y

After STOSBy STRING1

y After STOSB

y STRING1

Store String

H E L L O

DI

A

A E L L O

DI

A

A

A

L L O

DI

A

AL

AL

AL

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Reading and storing a character Stringy Read and store characters in a string, until a carriage return is typed

y If the user makes a typing mistake and hits the backspace key, the previous

character is removed from the string

y Algorithm

y char_read = 0

Read a char

WHILE char is not a carriage return DO

IF char is a backspace

THEN

char_read = char_read 1

Remove previous char from string

ELSEStore char in string

Char_read = char_read + 1

END_IF

Read a char

END_WHILE

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y READ_STR PROC NEAR

; BX number of characters read

CLD

XOR BX, BX

MOV AH, 1

INT 21h

WHILE1:

CMP AL, 0DH

JE END_WHILE1

CMP AL, 8H ; backspace?

JNE ELSE1 ; no, store in stringDEC DI ; yes, mov str ptr

DEC BX ; back

JMP READ ;read again

Reading and storing a character String

ELSE1:

STOSB

INC BX

READ:INT 21h

JMP WHILE1

END_WHILE1:

RET

READ_STR ENDP

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Load String

y In order to load a string, we use two instructions in assembly.

y LODSB

y

Moves the byte addressed by DS:SI into AL

y SI is incremented if DF = 0 or decremented if DF = 1

y LODSW

y Moves the word addressed by DS:SI into AX

y SI is increased by 2 if DF = 0 or decreased by 2 if DF = 1

y LODSB and LODSW have no effect on the flags.

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y Example

y Load the first and second bytes ofSTRING1

STRING1 DB ABC

y Code

y MOV AX,@DATA

y MOV DS,AX

y LEA SI, STRING1

y CLD

y LODSB

y LODSB

Load String

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Load String

y Before LODSB

y STRING1

y

After LODSBy STRING1

y After LODSB

y STRING1

A B C

SI

A B C

SI

A

A

B C

SI

B

AL

AL

AL

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Displaying a character string

y Algorithm

y FOR count times DO

Load a string character into AL

Move it to DL

Output character

y END_FOR

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y DISP_STR PROC

PUSH AX

PUSH BX

PUSH CX

PUSH DX

PUSH DIMOV CX, BX

JCXZ P_EXIT

CLD

MOV AH,2

TOP:

LODSB

MOV DL, AL

INT 21h

LOOP TOP

Displaying a character string

P_EXIT:

POP SI

POP DX

POP CX

POP BX

POP AX

RET

DISP_STR ENDP

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Scan String

y In order to scan a string, we use two instructions in assembly.

y SCASB

y Can be used to examine a string for a target byte

y The target byte is contained in AL

y SCASB subtracts the string byte pointed to by ES:DI from the contents of AL anduses the result to set the flags

y The result is not stored

y Afterward, DI is incremented if DF = 0 or decremented if DF = 1

y SCASW

y Can be used to examine a string for a target wordy The target byte is contained in AX

y SCASB subtracts the word pointed to by ES:DI from the contents of AX and usesthe result to set the flags

y The result is not stored

y Afterward, DI is increased by 2 if DF = 0 or decreased by 2 if DF = 1

y All the flags are affected by SCASB and SCASW

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Scan Stringy Before SCASB

y STRING1

y

After SCASBy STRING1

y After SCASB

y STRING1

A B C

DI

B

A B C

DI

B

A

B C

DI

B

AL

AL

AL

ZF = 0

(not found)

ZF = 1

(found)

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Thank You