Stack pointer problem: C code calls to assembly routine

Hello,

From C code I call the assmebly routine: my_memcpy  (int *src, int *dest, int size)

R12 holds address of src

R13 holds address of dest

R14 holds the size.

Now I want to copy 16bits in src to dest:

MOV @R12+, R5

How can I copy data in R5 to address pointed by R13 ?

The code:

MOV R5,@R12 is illigal.

Can you help ?

Thanks.