These are a pair of routine I use quite often. I know MemSet is virtualy the same as ZeroMemory in kernel32.dll, but there may be a time I want to use these in an embedded system where this dll won't be avaliable.


; Many times routines need local variables that are usually saved on the stack.  At times it
; is convenient to have this space intialized to nulls.

;	ENTRY:	EBP 	= Top of procedure frame
;		ESP + 4 = Bottom of frame

;	LEAVE:	EAX	= 0
;		ECX 	= Size of procedure frame
;		EDX	= Base pointer to fill area.

  InitFrame	xor	eax, eax
		mov	ecx, ebp		; Top of area
		mov	edx, esp		; Bottom of area
		add	edx, byte 4		; Allow for callers return
		sub	ecx, edx		; Calculate size of frame

; Now we can just fall into MemSet to fill procedure frame.

; ============================================================================================
; 					*** MEMORY SET ***

;	ENTRY:	EAX = Fill pattern (32 bit).
;		ECX = Total number of bytes to initialize.
;		EDX = Base pointer to fill area.

;	LEAVE:	All registers preserved.

; NOTE: If EAX is not evenly divisible by 4 then all 4 bytes of EAX must be the same,
;	otherwise last 1, 2 or 3 bytes of buffer will only be fill with AL.

  MemSet	push	edi
		push	ecx			; Preserve registers
		push	edx

		mov	edi, edx		; Set destination index.
		mov	 dl,  cl		; Save LSB of counter
		shr	ecx, 2			; Number of dwords that can be written
		cld				; Assure EDI will auto increment with STOS
		rep	stosd
		mov	 cl,  dl
		and	 cl,  3			; Is buffer evenly divisible by 4
		jz	.Done			; ZR = 1, if so.

		rep	stosb			; Write odd numbered bytes

  .Done		pop	edx
		pop	ecx			; Restore registers
		pop	edi
About the Author

I've been a computer enthusiast since 1978 and enjoy assembly programming as a hobby.

nice to see the Grandfather of it all appear!! Thanks!