cc65 generated programs with the default setup run with the kernal and basic banked out. This gives a usable memory range of $1000 - $FD00. Having the kernal and basic ROMs banked out means, that no ROM entry points may be called directly from user code.
Special locations:
The text screen is located at $C00 (as in the standard setup).
The color RAM is located at $800 (standard location).
The C runtime stack is located at $FCFF and growing downwards.
The C heap is located at the end of the program and grows towards the C runtime stack.