[gameprogrammer] Pushing on stack in assembly

I'm writing code to support variadic remote procedure calls in RakNet. I concatenate the parameter list of a function to an array of bytes. On the recipient, I read the length of the array, push this amount on the stack, and copy the memory onto the stack. I then call the function. 

Problem is, my assembly sucks.

C++:

struct MyStruct
{
        char a[50];
};
void _cdecl func2(MyStruct b)
{
}

Assembly:

// Bytes to pass, rounded up to 4
sub         esp,34h

// Loop counter (where does the 80 come from) ?
mov         ecx,0Ch

// Load esi with address of variable
lea         esi,[ebp-3Ch]

// Copy esp to edi
mov         edi,esp
// * Moves a byte, word or doubleword (8 bytes) from data segment and offset esi to extra segment and offset edi . 
//      * Increments/decrements both edi and esi :
//      ecx stores count, not sure how it is calculated yet
rep movs    dword ptr es:[edi],dword ptr [esi]

// Copy out remainder bytes
movs        word ptr es:[edi],word ptr [esi]

// Call function
00413734  call        func2 (4111EFh)

// Take bytes off stack
00413739  add         esp,34h

My questions are:
1. Will it even work at all, to concatenate input parameters, then just push that whole concatenated array onto the stack? 2. Where does the value of ecx come from? I'm pushing 50 bytes onto the stack 
3. How cross platform is this?

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