Assembly Language Solution

; Searching a substring from a Main string

[org 0x0100]

jmp start



mstr: db 'aaab', 0

sstr: db 'aab', 0

sub_len: dw ; Used to contain length of sub string

start: dw ; Place to resume search

stop: dw ; Last place to begin search



; subroutine to calculate the length of a string

; takes the segment and offset of a string as parameters

strlen: push bp

mov bp,sp

push es

push cx

push di

les di, [bp+4] ; point es:di to string

mov cx, 0xffff ; load maximum number in cx

xor al, al ; load a zero in al

repne scasb ; find zero in the string

mov ax, 0xffff ; load maximum number in ax

sub ax, cx ; find change in cx

dec ax ; exclude null from length

pop di

pop cx

pop es

pop bp

ret 4






; subroutine to compare two strings

; takes segment and offset pairs of two strings to compare

; returns 1 in ax if they match and 0 other wise

strcmp: push bp

mov bp,sp

push cx

push si

push di

push es

push ds



lds si, [bp+4] ; point ds:si to first sstr

les di, [bp+8] ; point es:di to second mstr



push es ; push segment of mstr

push di ; push offset of mstr

call strlen ; calculate string length

mov bx, ax ; save length in bx



push ds ; push segment of sstr

push si ; push offset of sstr

call strlen ; calculate string length

; Now length of sstr is in ax register

; See if mstr/sstr are null or sstr longer than mstr



or bx,bx ; mstr null?

JE exitfalse

cmp ax,0 ; sstr null?

JE exitfalse

cmp ax, bx ; sstr>mstr?

JE exitfalse



mov sub_len, ax ; Move length of sstr in sub_len



; see if the sstr is substring of mstr



LEA SI, sstr ; SI points to sstr

LEA DI, mstr ; DI points to mstr

Cld ; Left to right processing



; Compute STOP



mov stop, DI ; stop has mstr address

add stop, bx

mov cx,sub_len

sub stop,cx ; subtract substring length



; Initialize start



mov start, DI

REPEAT:

; compare characters

mov cx, sub_len

mov DI, start ; reset DI

LEA SI, sstr ; reset SI

Repe cmpsb ; compare characters



JE exitsimple



; substring not found yet



INC start

; see if the start<=stop



mov ax, start

cmp ax, stop

JNLE exitfalse

JMP REPEAT

mov ax,1

JMP exitsimple

exitfalse: mov ax, 0 ; store 0 to mark unequal



exitsimple: pop ds

pop es

pop di

pop si

pop cx

pop bp

ret 8



start: push ds ; push segment of first string

mov ax, mstr

push ax ; push offset of first string

push ds ; push segment of second string

mov ax, sstr

push ax ; push offset of second string

call strcmp ; call strcmp subroutine



mov ax, 0x4c00 ; terminate program

int 0x21

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