Setup three separate entry and exit points to the OS. Heavy-weight, medium-weight, and light-weight. The heavy-weight entry point stacks all the registers except a0 (the return value register), as if a ‘C’ routine were to be called and it is unknown which registers could be modified. The light-weight entry point only saves a few register (nine total) and is meant for small functions written in assembler. There are currently about a dozen OS functions that qualify. The medium-weight entry point is somewhere in-between. It saves enough registers to do a more complex routine.