The MASM Forum Archive 2004 to 2012

C++ memcpy function...

Example:
include \masm32\include\masm32rt.inc

include _memcpy.asm

Main PROTO

.data
szSrc db 20h  dup(1)
szDst db 20h  dup(0)

.code

Start:
  Invoke Main
  Invoke ExitProcess,0

Main proc

push 10
push offset szSrc
push offset szDst
call _memcpy

ret
Main endp

end Start

 :bg
        mov     esi,offset szSrc
        mov     edi,offset szDst
        mov     ecx,sizeof szSrc
        rep     movsb

sure!  :bg
but the code I posted might be useful in some especial situations. as it was for me today  :wink

you can use c++ functions.................or you can just roll your own

.code
MemCopy PROC USES esi edi, Source:DWORD, Dest:DWORD, ln:DWORD

cld
mov esi, Source
mov edi, Dest
mov ecx, ln
shr ecx, 2
rep movsd

mov ecx, ln
and ecx, 3
rep movsb

ret
MemCopy ENDP
END

About using C++ functions you are right, but as I said it's not just matter of copying memory! I had to use exactly this code for a special case and I just shared it here maybe someday, someone find it useful. that's all ;)

if speed is need, the CRT (dynamical linked)  is probably the best choice. Using this, you application will also take speed gain of future instruction sets.

I have a reasonably simple view on memory copy, if its a terrabyte it matter, if its a megabyte it doesn't. If you have the registers to spare a simple REP MOVSB does the job in most instances.


mov esi, src
mov edi, dst
mov ecx, cnt
rep movsb


that's right
the real problem with trying to speed it up by using REP MOVSD is not dealing with the mod 3 count at the end
it is the fact that strings are not usually both dword aligned - that negates the speed advantage
you have a 1 in 16 chance of going really fast   :bg

Quote from: dedndave on May 10, 2011, 03:36:09 AM
that's right
the real problem with trying to speed it up by using REP MOVSD is not dealing with the mod 3 count at the end
it is the fact that strings are not usually both dword aligned - that negates the speed advantage
you have a 1 in 16 chance of going really fast   :bg

Dave,
In real life big buffers are 8- or 16-bit aligned - and then rep movsd is almost unbeatable. Remember the Code location sensitivity of timings (http://www.masm32.com/board/index.php?topic=11454.msg87608#msg87608) thread...? Look for MemCo1.

It looks exactly like if you have disassembled ms crt library memcpy function with IDA.
And invoked it as stdcall while the function is clearly cdecl.

Yes, that's what I did :p