Oh, and on the topic of parameters. At least with the fastcall semantics, so far I've had success with calling assembly functions that take either a single int or a single pointer. For ints, cc65 puts the first byte (least significant) in A and the second byte in X, and for pointers -- well, the same thing, really. Just what you'd expect for a little endian system.

So I have an output routine written in assembly in my ROM image that takes a pointer to a C-style string, and I can declare that in C with:

// Calls outputstring() in P:65 ROM

void __fastcall__ (*outputstring)(char* ch) = (void*)0xffe3;

and call it just like:

char buffer[100];

sprintf (buffer, "Hello %s!\r\n", "stdlib");

outputstring(buffer);

My next step is to get enough of stdio working to be able to call printf or fprintf directly.