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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
