绝对经典的汇编源代码

inurlcn 2014-02-07 05:28:55

        page    ,132

        title   strcmp.asm - compare two strings

;***

;strcmp.asm - routine to compare two strings (for equal, less, or greater)

;

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

;

;Purpose:

;       STRCMP compares two strings and returns an integer

;       to indicate whether the first is less than the second, the two are

;       equal, or whether the first is greater than the second, respectively.

;       Comparison is done byte by byte on an UNSIGNED basis, which is to

;       say that Null (0) is less than any other character (1-255).

;

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



        .xlist

        include cruntime.inc

        .list



page

;***

;strcmp - compare two strings, returning less than, equal to, or greater than

;

;Purpose:

;       Compares two string, determining their lexical order.  Unsigned

;       comparison is used.

;

;       Algorithm:

;          int strcmp ( src , dst )

;                  unsigned char *src;

;                  unsigned char *dst;

;          {

;                  int ret = 0 ;

;

;                  while( ! (ret = *src - *dst) && *dst)

;                          ++src, ++dst;

;

;                  if ( ret < 0 )

;                          ret = -1 ;

;                  else if ( ret > 0 )

;                          ret = 1 ;

;

;                  return( ret );

;          }

;

;Entry:

;       const char * src - string for left-hand side of comparison

;       const char * dst - string for right-hand side of comparison

;

;Exit:

;       AX < 0, 0, or >0, indicating whether the first string is

;       Less than, Equal to, or Greater than the second string.

;

;Uses:

;       CX, DX

;

;Exceptions:

;

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



        CODESEG



        public  strcmp

strcmp  proc \

        str1:ptr byte, \

        str2:ptr byte



        OPTION PROLOGUE:NONE, EPILOGUE:NONE



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



        mov     edx,[esp + 4]   ; edx = src

        mov     ecx,[esp + 8]   ; ecx = dst



        test    edx,3

        jnz     short dopartial



        align   4

dodwords:

        mov     eax,[edx]



        cmp     al,[ecx]

        jne     short donene

        or      al,al

        jz      short doneeq

        cmp     ah,[ecx + 1]

        jne     short donene

        or      ah,ah

        jz      short doneeq



        shr     eax,16



        cmp     al,[ecx + 2]

        jne     short donene

        or      al,al

        jz      short doneeq

        cmp     ah,[ecx + 3]

        jne     short donene

        add     ecx,4

        add     edx,4

        or      ah,ah

        jnz     short dodwords



        align   4

doneeq:

        xor     eax,eax

        ret



        align   4

donene:

        ; The instructions below should place -1 in eax if src < dst,

        ; and 1 in eax if src > dst.



        sbb     eax,eax

        sal     eax,1

        add     eax,1

        ret



        align   4

dopartial:

        test    edx,1

        jz      short doword



        mov     al,[edx]

        add     edx,1

        cmp     al,[ecx]

        jne     short donene

        add     ecx,1

        or      al,al

        jz      short doneeq



        test    edx,2

        jz      short dodwords





        align   4

doword:

        mov     ax,[edx]

        add     edx,2

        cmp     al,[ecx]

        jne     short donene

        or      al,al

        jz      short doneeq

        cmp     ah,[ecx + 1]

        jne     short donene

        or      ah,ah

        jz      short doneeq

        add     ecx,2

        jmp     short dodwords



strcmp  endp



        end