memchr.asm

C语言库函数的原型,有用的拿去

        page    ,132
        title   memchr - search memory for a given character
;***
;memchr.asm - search block of memory for a given character
;
;       Copyright (c) Microsoft Corporation. All rights reserved.
;
;Purpose:
;       defines memchr() - search memory until a character is
;       found or a limit is reached.
;
;*******************************************************************************

        .xlist
        include cruntime.inc
        .list

page
;***
;char *memchr(buf, chr, cnt) - search memory for given character.
;
;Purpose:
;       Searched at buf for the given character, stopping when chr is
;       first found or cnt bytes have been searched through.
;
;       Algorithm:
;       char *
;       memchr (buf, chr, cnt)
;               char *buf;
;               int chr;
;               unsigned cnt;
;       {
;               while (cnt && *buf++ != c)
;                       cnt--;
;               return(cnt ? --buf : NULL);
;       }
;
;Entry:
;       char *buf - memory buffer to be searched
;       char chr - character to search for
;       unsigned cnt - max number of bytes to search
;
;Exit:
;       returns pointer to first occurence of chr in buf
;       returns NULL if chr not found in the first cnt bytes
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************

        CODESEG

        public  memchr
memchr  proc \
        buf:ptr byte, \
        chr:byte, \
        cnt:dword

        OPTION PROLOGUE:NONE, EPILOGUE:NONE

        .FPO    ( 0, 1, 0, 0, 0, 0 )

        mov     eax,[esp+0ch]   ; eax = count
        push    ebx             ; Preserve ebx

        test    eax,eax         ; check if count=0
        jz      short retnull   ; if count=0, leave

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

        mov     bl,[esp+0ch]    ; bl = search char

        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,byte ptr [edx]
        add     edx,1
        xor     cl,bl
        je      short found
        sub     eax,1           ; counter--
        jz      short retnull
        test    edx,3           ; already aligned ?
        jne     short str_misaligned

main_loop_start:
        sub     eax,4
        jb      short tail_less_then_4

; set all 4 bytes of ebx to [value]
        push    edi             ; Preserve edi
        mov     edi,ebx         ; edi=0/0/0/char
        shl     ebx,8           ; ebx=0/0/char/0
        add     ebx,edi         ; ebx=0/0/char/char
        mov     edi,ebx         ; edi=0/0/char/char
        shl     ebx,10h         ; ebx=char/char/0/0
        add     ebx,edi         ; ebx = all 4 bytes = [search char]
        jmp     short main_loop_entry   ; ecx >=0

return_from_main:
        pop     edi

tail_less_then_4:
        add     eax,4
        jz      retnull

tail_loop:                      ; 0 < eax < 4
        mov     cl,byte ptr [edx]
        add     edx,1
        xor     cl,bl
        je      short found
        sub     eax,1
        jnz     short tail_loop
retnull:
        pop     ebx
        ret                     ; _cdecl return

main_loop:
        sub     eax,4
        jb      short return_from_main
main_loop_entry:
        mov     ecx,dword ptr [edx]     ; read 4 bytes

        xor     ecx,ebx         ; ebx is byte\byte\byte\byte
        mov     edi,7efefeffh

        add     edi,ecx
        xor     ecx,-1

        xor     ecx,edi
        add     edx,4

        and     ecx,81010100h
        je      short main_loop

; found zero byte in the loop?
char_is_found:
        mov     ecx,[edx - 4]
        xor     cl,bl           ; is it byte 0
        je      short byte_0
        xor     ch,bl           ; is it byte 1
        je      short byte_1
        shr     ecx,10h         ; is it byte 2
        xor     cl,bl
        je      short byte_2
        xor     ch,bl           ; is it byte 3
        je      short byte_3
        jmp     short main_loop ; taken if bits 24-30 are clear and bit
                                ; 31 is set

byte_3:
        pop     edi             ; restore edi
found:
        lea     eax,[edx - 1]
        pop     ebx             ; restore ebx
        ret                     ; _cdecl return

byte_2:
        lea     eax,[edx - 2]
        pop     edi
        pop     ebx
        ret                     ; _cdecl return

byte_1:
        lea     eax,[edx - 3]
        pop     edi
        pop     ebx
        ret                     ; _cdecl return

byte_0:
        lea     eax,[edx - 4]
        pop     edi             ; restore edi
        pop     ebx             ; restore ebx
        ret                     ; _cdecl return

memchr  endp
        end