All byte arguments are promoted to 16-bits.

Arguments are passed in R2–R7 (or R2–R_max if there are fewer than 8 registers).

Results are returned on the same registers used to pass arguments.

Varargs are always passed on the stack.

A good guideline is that approximately one third of unallocated registers should be caller saved, although that can increase to one half where there are plenty of registers. The following registers (if present) are caller saved: R10, R13, R16, R19, R22, R25, R28, R31, R33, R35, R37, R39, R41, R43, R45, R47, R49, R51, R53, R55, R57, R59, R61 and R63.