[cc65] Interrupt <-> C runtime environment

Hi,

I tried to invoke a C function within an interrupt.
As the cc65 run time environment does not allow this my question is:

Which memory adresses do I have to save to restore the state before the 
interrupt?
Does not matter if it is 1k ... just interested in making a clean 
context switch for an own OS with C.

Kind regards marcas

P.S.: Rectangle of TGI needs optimization :-)
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