This question need help for Assembly Language for x86 processors. Description: W

Question

This question need help for Assembly Language for x86 processors.

Description: Write a procedure named Str_find that searches for the
first matching occurrence of a source string inside a target string
and returns the matching position. The input parameters should be a
pointer to the source string and a pointer to the target string. If
a match is found, the procedure sets the Zero flag and EAX points to
the matching position in the target string. Otherwise, the Zero
flag is clear.

TODO: Complte Str_find PROC
!

INCLUDE Irvine32.inc

Str_find PROTO, pTarget:PTR BYTE, pSource:PTR BYTE

.data

promptTargetStr BYTE "Enter target string: ",0
promptSourceStr BYTE "Enter source string: ",0

str1 BYTE "Source string found at position ",0
str2 BYTE " in Target string (counting from zero).",0Ah,0Ah,0Dh,0
str3 BYTE "Unable to find Source string in Target string.",0Ah,0Ah,0Dh,0

MAX = 80 ;max chars to read
target BYTE MAX+1 DUP (?)
source BYTE MAX+1 DUP (?)

stop DWORD ?
lenTarget DWORD ?
lenSource DWORD ?
position DWORD ?

.code
main PROC
   ; prompt input target string
   mov edx,OFFSET promptTargetStr
   call WriteString

   mov edx, OFFSET target
mov ecx,MAX ;buffer size - 1
call ReadString

   call Crlf

   ; prompt input source string
   mov edx,OFFSET promptSourceStr
   call WriteString

   mov edx, OFFSET source
mov ecx,MAX ;buffer size - 1
call ReadString

   call Crlf

   INVOKE Str_find,ADDR target, ADDR source
   mov position,eax
   jz wasfound           ; ZF=1 indicates string found

   mov edx,OFFSET str3   ; string not found
   call WriteString
   jmp   quit

wasfound:                   ; display message
   mov edx,OFFSET str1
   call WriteString
   mov eax,position       ; write position value
   call WriteDec
   mov edx,OFFSET str2
   call WriteString

quit:
   exit

main ENDP

;--------------------------------------------------------
Str_find PROC, pTarget:PTR BYTE, ;PTR to Target string
pSource:PTR BYTE ;PTR to Source string
;
; Searches for the first matching occurrence of a source
; string inside a target string.
; Receives: pointer to the source string and a pointer
;    to the target string.
; Returns: If a match is found, ZF=1 and EAX points to
; the offset of the match in the target string.
; IF ZF=0, no match was found.
;--------------------------------------------------------
   INVOKE Str_length,pTarget   ; get length of target
   mov lenTarget,eax
   INVOKE Str_length,pSource   ; get length of source
   mov lenSource,eax

   mov edi,OFFSET target       ; point to target
   mov esi,OFFSET source       ; point to source

; Compute place in target to stop search
   mov eax,edi        ; stop = (offset target)
   add eax,lenTarget        ; + (length of target)
   sub eax,lenSource        ; - (length of source)
   inc eax        ; + 1
   mov stop,eax               ; save the stopping position

   ; TODO: Compare source string to current target  

   ret
Str_find ENDP

END main

Explanation / Answer

Str_find PROTO, pTarget:PTR BYTE, pSource:PTR BYTE

.data
target BYTE "01ABAAAAAABABCC45ABC9012",0
source BYTE "AAABA",0

str1 BYTE "Source string found at position ",0
str2 BYTE " in Target string (counting from zero).",0Ah,0Ah,0Dh,0
str3 BYTE "Unable to find Source string in Target string.",0Ah,0Ah,0Dh,0

stop DWORD ?
lenTarget DWORD ?
lenSource DWORD ?
position DWORD ?

.code
main PROC

   INVOKE Str_find,ADDR target, ADDR source
   mov position,eax
   jz wasfound           ; ZF=1 indicates string found

   mov edx,OFFSET str3   ; string not found
   call WriteString
   jmp   quit

wasfound:                   ; display message
   mov edx,OFFSET str1
   call WriteString
   mov eax,position       ; write position value
   call WriteDec
   mov edx,OFFSET str2
   call WriteString

quit:
   exit

main ENDP

;--------------------------------------------------------
Str_find PROC, pTarget:PTR BYTE, ;PTR to Target string
pSource:PTR BYTE ;PTR to Source string
;
; Searches for the first matching occurrence of a source
; string inside a target string.
; Receives: pointer to the source string and a pointer
;    to the target string.
; Returns: If a match is found, ZF=1 and EAX points to
; the offset of the match in the target string.
; IF ZF=0, no match was found.
;--------------------------------------------------------
   INVOKE Str_length,pTarget   ; get length of target
   mov lenTarget,eax
   INVOKE Str_length,pSource   ; get length of source
   mov lenSource,eax

   mov edi,OFFSET target       ; point to target
   mov esi,OFFSET source       ; point to source

; Compute place in target to stop search
   mov eax,edi    ; stop = (offset target)
   add eax,lenTarget    ; + (length of target)
   sub eax,lenSource    ; - (length of source)
   inc eax    ; + 1
   mov stop,eax           ; save the stopping position

; Compare source string to current target
   cld
   mov ecx,lenSource    ; length of source string
L1:
   pushad
   repe cmpsb           ; compare all bytes
   popad
   je found           ; if found, exit now

   inc edi               ; move to next target position
   cmp edi,stop           ; has EDI reached stop position?
   jae notfound           ; yes: exit
   jmp L1               ; not: continue loop

notfound:                   ; string not found
   or eax,1           ; ZF=0 indicates failure
   jmp done

found:                   ; string found
   mov eax,edi           ; compute position in target of find
   sub eax,pTarget
   cmp eax,eax    ; ZF=1 indicates success

done:
   ret
Str_find ENDP

END main

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