用汇编语言写一个查找字符串中的指定字符的程序

        page    ,132

        title   strchr - search string for given character

;***

;strchr.asm - search a string for a given character

;

;       Copyright (c) Microsoft Corporation. All rights reserved.

;

;Purpose:

;       defines strchr() - search a string for a character

;

;*******************************************************************************



        .xlist

        include cruntime.inc

        .list



page

;***

;char *strchr(string, chr) - search a string for a character

;

;Purpose:

;       Searches a string for a given character, which may be the

;       null character '\0'.

;

;       Algorithm:

;       char *

;       strchr (string, chr)

;       char *string, chr;

;       {

;         while (*string && *string != chr)

;             string++;

;         if (*string == chr)

;             return(string);

;         return((char *)0);

;       }

;

;Entry:

;       char *string - string to search in

;       char chr     - character to search for

;

;Exit:

;       returns pointer to the first occurence of c in string

;       returns NULL if chr does not occur in string

;

;Uses:

;

;Exceptions:

;

;*******************************************************************************



        CODESEG



found_bx:

        lea     eax,[edx - 1]

        pop     ebx                 ; restore ebx

        ret                         ; _cdecl return



        align   16

        public  strchr, __from_strstr_to_strchr

strchr  proc \

        string:ptr byte, \

        chr:byte



        OPTION PROLOGUE:NONE, EPILOGUE:NONE



        .FPO    ( 0, 2, 0, 0, 0, 0 )



        xor     eax,eax

        mov     al,[esp + 8]        ; al = chr (search char)



__from_strstr_to_strchr label proc



        push    ebx                 ; PRESERVE EBX

        mov     ebx,eax             ; ebx = 0/0/0/chr

        shl     eax,8               ; eax = 0/0/chr/0

        mov     edx,[esp + 8]       ; edx = buffer

        test    edx,3               ; test if string is aligned on 32 bits

        jz      short main_loop_start



str_misaligned:                     ; simple byte loop until string is aligned

        mov     cl,[edx]

        add     edx,1

        cmp     cl,bl

        je      short found_bx

        test    cl,cl

        jz      short retnull_bx

        test    edx,3               ; now aligned ?

        jne     short str_misaligned



main_loop_start:                    ; set all 4 bytes of ebx to [chr]

        or      ebx,eax             ; ebx = 0/0/chr/chr

        push    edi                 ; PRESERVE EDI

        mov     eax,ebx             ; eax = 0/0/chr/chr

        shl     ebx,10h             ; ebx = chr/chr/0/0

        push    esi                 ; PRESERVE ESI

        or      ebx,eax             ; ebx = all 4 bytes = [chr]



; in the main loop (below), we are looking for chr or for EOS (end of string)



main_loop:

        mov     ecx,[edx]           ; read  dword (4 bytes)

        mov     edi,7efefeffh       ; work with edi & ecx for looking for chr



        mov     eax,ecx             ; eax = dword

        mov     esi,edi             ; work with esi & eax for looking for EOS



        xor     ecx,ebx             ; eax = dword xor chr/chr/chr/chr

        add     esi,eax



        add     edi,ecx

        xor     ecx,-1



        xor     eax,-1

        xor     ecx,edi



        xor     eax,esi

        add     edx,4



        and     ecx,81010100h       ; test for chr

        jnz     short chr_is_found  ; chr probably has been found



        ; chr was not found, check for EOS



        and     eax,81010100h       ; is any flag set ??

        jz      short main_loop     ; EOS was not found, go get another dword



        and     eax,01010100h       ; is it in high byte?

        jnz     short retnull       ; no, definitely found EOS, return failure



        and     esi,80000000h       ; check was high byte 0 or 80h

        jnz     short main_loop     ; it just was 80h in high byte, go get

                                    ; another dword

retnull:

        pop     esi

        pop     edi

retnull_bx:

        pop     ebx

        xor     eax,eax

        ret                         ; _cdecl return



chr_is_found:

        mov     eax,[edx - 4]       ; let's look one more time on this dword

        cmp     al,bl               ; is chr in byte 0?

        je      short byte_0

        test    al,al               ; test if low byte is 0

        je      retnull

        cmp     ah,bl               ; is it byte 1

        je      short byte_1

        test    ah,ah               ; found EOS ?

        je      retnull

        shr     eax,10h             ; is it byte 2

        cmp     al,bl

        je      short byte_2

        test    al,al               ; if in al some bits were set, bl!=bh

        je      retnull

        cmp     ah,bl

        je      short byte_3

        test    ah,ah

        jz      retnull

        jmp     short main_loop     ; neither chr nor EOS found, go get

                                    ; another dword

byte_3:

        pop     esi

        pop     edi

        lea     eax,[edx - 1]

        pop     ebx                 ; restore ebx

        ret                         ; _cdecl return



byte_2:

        lea     eax,[edx - 2]

        pop     esi

        pop     edi

        pop     ebx

        ret                         ; _cdecl return



byte_1:

        lea     eax,[edx - 3]

        pop     esi

        pop     edi

        pop     ebx

        ret                         ; _cdecl return



byte_0:

        lea     eax,[edx - 4]

        pop     esi                 ; restore esi

        pop     edi                 ; restore edi

        pop     ebx                 ; restore ebx

        ret                         ; _cdecl return



strchr  endp

        end