If you are writing in assembly language there are some extra rules to keep in mind in addition to those listed above. * Never use the TAS instruction on the Amiga. System DMA can conflict with this instruction's special indivisible read-modify-write cycle. * System functions must be called with register A6 containing the library or device base. Libraries and devices assume A6 is valid at the time of any function call. Even if a particular function does not currently require its base register, you must provide it for compatibility with future system software releases. * Except as noted, system library functions use registers D0, D1, A0, and A1 as scratch registers and you must consider their former contents to be lost after a system library call. The contents of all other registers will be preserved. System functions that provide a result will return the result in D0. * Never depend on processor condition codes after a system call. The caller must test the returned value before acting on a condition code. This is usually done with a TST or MOVE instruction.