6. For the following exercises, assume that the data segment contains: source BY

ID: 3831640 • Letter: 6

Question

6. For the following exercises, assume that the data segment contains: source BYTE brown" dest BYTE "brine" a) Suppose that the following instructions are executed: mov al, 'n' lea edi, dest cld mov ecx, 5 repne scasb Assuming that 00417005 is loaded in EDI, what will be the values stored in EDI b) Suppose that the following instructions are executed: lea esi, source +4 lea edi dest+4 std mov ecx, 3 rep movs b Assuming that 00417000 is the address loaded in ESI and 00417005 is loaded in EDI, what will be the values stored in ESI and EDI following the rep movsb instruction? What will be stored in ECX? What will be stored in the destination string? (2 points)

Explanation / Answer

String - a byte or word array.

Operations that can be performed with string instructions:

Direction Flag

CLD - clears the DF; sets DF = 0
STD - sets DF = 1

Moving (Copying) Strings

Instructions:

    MOVSB - copies contents of BYTE given by DS:SI into ES:DI
    MOVSW - copies contents of WORD given by DS:SI into ES:DI
    MOVSD - copies contenst of DOUBLE WORD given by DS:SI into ES:DI

Notes:

Example:

.DATA
STRING1 DB 'HELLO'
STRING2 DB 5 DUP (0)

.CODE
        MOV AX, @DATA
        MOV DS, AX            ; init DS
        MOV ES, AX            ; init ES

        LEA SI, STRING1     ; source
        LEA DI, STRING2      ; destination
        CLD                   ; DF = 0

MOVSB ; mov 1st byte
MOVSB ; mov 2nd byte

Note: MOVSB moves only 1 byte at a time. Set CX = count and use the REP prefix to move a specified number of bytes.

Example:

        CLD                   ; forward direction
        PUSH DS               ; set ES = DS
        POP ES
        LEA SI, STRING1       ; set SI to source
        LEA DI, STRING2       ; set DI to destination
        MOV CX, 5

REP MOVSB ; copies 5 chars

Note: both SI and DI are incremented for each byte that is copied from SI to DI (in the forward direction)

Reverse:

        STD                        ; reverse direction
        PUSH DS
        POP ES                     ; ES = DS
        LEA SI, STRING1+4          ; end of string
        LEA DI, STRING2+4          ; end of string
        MOV CX, 5

REP MOVSB ; copy 5 chars

Note: both SI and DI are decremented by one for each byte that is copied from SI to DI (in the reverse direction).

Storing Strings

Instructions:

    STOSB - copies contents of AL to BYTE address given by ES:DI. DI is incremented/decremented by 1.
    STOSW - copies the contents of AX to the WORD address given by ES:DI. DI is incremented/decremented by 2.
    STOSD - copies contents of EAX to the DOUBLE WORD address given by ES:DI. DI is incremented/decremented by 4.

Example:

MOV AX, @DATA
MOV ES, AX ; initialize ES

    LEA DI, STRING1         ; assume BYTE string
    CLD
    MOV AL, 'A'

STOSB ; store 1st byte of A
STOSB ; store 2nd byte of A

Load String

Instructions:

    LODSB - moves the BYTE at address DS:SI into AL. SI is incremented/decremented by 1.
    LODSW - moves the WORD at address DS: SI into AX. SI is incremented/decremented by 2.
    LODSD - moves the DOUBLE WORD at address DS:SI into EAX. SI is incremented/decremented by 4.

Example:

        MOV AX, @DATA
        MOV DS, AX
        LEA SI, STRING1
        CLD

LODSB
LODSB

Loads two bytes from STRING1 into AL (second byte overwrites the first).

Scan String

Instructions:

    SCASB - compares BYTE at ES:DI with AL and sets flags according to result.
    SCASW - compares WORD at ES:DI with AX and sets flags.
    SCASD - compares DOUBLE WORD at ES:DI with EAX and sets flags.

Example:

.DATA
STRING1 DB 'ABC'

.CODE
MOV AX, @DATA
MOV AX, ES ; initialize ES

        CLD ; left to right
        LEA DI, STRING1
        MOV AL, 'B' ; target character

SCASB ; scan first byte
SCASB ; scan 2nd byte

Note: when the target ("B") is found, ZF = 1 and DI points to the byte following the target since DI is automatically incremented by SCASB.

Also, set CX = count and use:

REPNE SCASB
REPNZ SCASB

to repeat the scan until the target byte is found, or until the entire string has been searched (i.e., CX = 0).

Compare String

Instructions:

        CMPSB - compares BYTE at ES:DI with BYTE at DS:SI and sets flags.
        CMPSW - compares WORD at ES:DI with WORD at DS:SI and sets flags.
        CMPSD - compares DOUBLE WORD at ES:DI with WORD at DS:SI and sets flags.

Example:

.DATA
STRING1 DB 'ACD'
STRING2 DB 'ABC'

.CODE
        MOV AX, @DATA
        MOV DS, AX
        MOV ES, AX

        CLD
        LEA SI, STRING1
        LEA DI, STRING2
        MOV CX, 3            ; string length

REPE CMPSB ; repeat while strings match

Increments (or decrements) each string pointer and successively compares bytes until there is a mismatch between the bytes being compared, or until CX = 0.

CMPSB can be used to determine whether two strings match, or whether one string is a substring of another string.

BYTE, WORD, and DOUBLE WORD form of string instructions:

The operands are IMPLICIT.

GENERAL FORM of STRING instructions (optional):

MOVS destination, source
CMPS destination, source
STOS destination, source
LODS destination, source
SCAS destination, source

INSTRUCTION DEST SOURCE BYTE WORD DOUBLE
WORD Move String ES:DI DS:SI MOVSB MOVSW MOVSW Compare String ES:DI DS:SI CMPSB CMPSW CMPSW Store string ES:DI AL/AX/EAX STOSB STOSW STOSW Load string AL/AX/EAX DS:SI LODSB LODSW LODSW Scan string ES:DI AL/AX/EAX SCASB SCASW SCASW

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6. For the following exercises, assume that the data segment contains: source BY

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