MASM how to make desired function call - function

Question

I'd like to know, how to do the following.
I have an array, where i have to summ numbers (easy)
but the twist is, that i have to have a function call for it,
that get's is params through specific registers. How do i implement that?
In this case, the function needs to get the array (offset) through ESI, and the length of it through ECX.
please educate me
EDIT:
in the meantime i've conjured up this. No idea if this works to as my MASM compliling just broken itself for no reason
.data
intarray DWORD 10000h,20000h,30000h,40000h
.code
szummer proc uses esi ecx,
ptrArray:PTR DWORD, ;points to the array
szArray: Dword ;array size
mov esi, ptrArray ;address of the array
mov ecx, szArray ;szize
mov eax, 0 ;set to 0
AS1:
add eax, [esi] ;add each int to sum
add esi, 4 ;point to next int
loop AS1 ;reapet for array size
ret;
szummer endp
main proc
mov ecx, OFFSET intarray
mov esi, LENGHTOF intarray
INVOKE ArraySum,ecx,esi
invoke ExitProcess,0
main endp
end main

Answer 1

The MASM directive INVOKE works only with the calling conventions C (cdecl), STDCALL, BASIC, FORTRAN and PASCAL. All of these conventions pass the arguments on the stack. Thus, you can't use INVOKE for passing the arguments in registers. You can use the Assembly instruction CALL instead. Your program - slightly modified ;-) - with MASM32 library included (because of "ExitProcess"):
INCLUDE \masm32\include\masm32rt.inc
.DATA
intarray DWORD 10000h,20000h,30000h,40000h
.CODE
szummer proc uses esi ecx
mov eax, 0 ;set to 0
AS1:
add eax, [esi] ;add each int to sum
add esi, 4 ;point to next int
loop AS1 ;reapet for array size
ret;
szummer endp
main proc
mov esi, OFFSET intarray
mov ecx, LENGTHOF intarray
call szummer
invoke ExitProcess,0
main ENDP
END main