Function return values for integers are stored in D0 and D1. A7 has a reserved use. Registers, A0, A1, D0, D1, F0, and F1, can be used for temporary values.

When a function is compiled with the default options, it must return with registers, D2 through D7, and registers, A2 through A6, unchanged.

If you have floating-point registers, then registers, F2 through F7, must also be unchanged.
 

Note:
Functions compiled with different calling conventions cannot be run together without some care.