4.6.1 Calling convention

If the the function does not have variable arguments, and the first argument is not a struct or union and has 8 or 16 bits, it is passed in xl (8 bits) or y (16 bits). All other arguments are passed on the stack right-to-left. Return values are in xl (8 bit), x (16 bit), xxl (24 bit), yx (32 bit) or use a hidden extra pointer parameter pointing to the location (anything wider than 32 bit, and all struct / union). The stack is not adjusted for the parameters by the callee (thus the caller has to do this instead).