memset汇编实现，跟踪得到的

跟进new就能跟到这里

        page    ,132
        title   memset - set sections of memory all to one byte
;***
;memset.asm - set a section of memory to all one byte
;
;       Copyright (c) 1985-1997, Microsoft Corporation. All rights reserved.
;
;Purpose:
;       contains the memset() routine
;
;*******************************************************************************

        .xlist
        include cruntime.inc
        .list

page
;***
;char *memset(dst, value, count) - sets "count" bytes at "dst" to "value"
;
;Purpose:
;       Sets the first "count" bytes of the memory starting
;       at "dst" to the character value "value".
;
;       Algorithm:
;       char *
;       memset (dst, value, count)
;               char *dst;
;               char value;
;               unsigned int count;
;               {
;               char *start = dst;
;
;               while (count--)
;                       *dst++ = value;
;               return(start);
;               }
;
;Entry:
;       char *dst - pointer to memory to fill with value
;       char value - value to put in dst bytes
;       int count - number of bytes of dst to fill
;
;Exit:
;       returns dst, with filled bytes
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************

        CODESEG

        public  memset
memset proc

        .FPO    ( 0, 3, 0, 0, 0, 0 )

        mov     edx,[esp + 0ch] ; edx = "count"
        mov     ecx,[esp + 4]   ; ecx points to "dst"

        test    edx,edx         ; 0?
        jz      short toend     ; if so, nothing to do

        xor     eax,eax
        mov     al,[esp + 8]    ; the byte "value" to be stored


; Align address on dword boundary

        push    edi             ; preserve edi
        mov     edi,ecx         ; edi = dest pointer

        cmp     edx,4           ; if it's less then 4 bytes
        jb      tail            ; tail needs edi and edx to be initialized

        neg     ecx
        and     ecx,3           ; ecx = # bytes before dword boundary
        jz      short dwords    ; jump if address already aligned

        sub     edx,ecx         ; edx = adjusted count (for later)
adjust_loop:
        mov     [edi],al
        inc     edi
        dec     ecx
        jnz     adjust_loop

dwords:
; set all 4 bytes of eax to [value]
        mov     ecx,eax         ; ecx=0/0/0/value
        shl     eax,8           ; eax=0/0/value/0

        add     eax,ecx         ; eax=0/0val/val

        mov     ecx,eax         ; ecx=0/0/val/val

        shl     eax,10h         ; eax=val/val/0/0

        add     eax,ecx         ; eax = all 4 bytes = [value]

; Set dword-sized blocks
        mov     ecx,edx         ; move original count to ecx
        and     edx,3           ; prepare in edx byte count (for tail loop)
        shr     ecx,2           ; adjust ecx to be dword count
        jz      tail            ; jump if it was less then 4 bytes

        rep     stosd
main_loop_tail:
        test    edx,edx         ; if there is no tail bytes,
        jz      finish          ; we finish, and it's time to leave
; Set remaining bytes

tail:
        mov     [edi],al        ; set remaining bytes
        inc     edi

        dec     edx             ; if there is some more bytes
        jnz     tail            ; continue to fill them

; Done
finish:
        mov     eax,[esp + 8]   ; return dest pointer
        pop     edi             ; restore edi

        ret

toend:
        mov     eax,[esp + 4]   ; return dest pointer

        ret

memset  endp

        end