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        .listpage;***;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  memsetmemset 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_loopdwords:; 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     stosdmain_loop_tail:        test    edx,edx         ; if there is no tail bytes,        jz      finish          ; we finish, and it's time to leave; Set remaining bytestail:        mov     [edi],al        ; set remaining bytes        inc     edi        dec     edx             ; if there is some more bytes        jnz     tail            ; continue to fill them; Donefinish:        mov     eax,[esp + 8]   ; return dest pointer        pop     edi             ; restore edi        rettoend:        mov     eax,[esp + 4]   ; return dest pointer        retmemset  endp        end