Like the ORG (or *=) in assembly, is there anything similar for C, to specify that from this point on, start compiling opcodes at this address, until it is specified again later?

(I am of course experimenting with C for 65xx)

If you’re using cc65 then it’s the linker which puts code at addresses - check your machine description or linker control script or command line.

There definitely is a method to bind a hard-coded address to an ISR or an I/O port using a somewhat convoluted cast, but I haven't done so in over 30 years, and the details elude me at the moment (the applicable program listings are stashed somewhere in my attic).

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

It might be helpful if you could tell us a bit more about what you're trying to do.

One possible solution is to use a trampoline written in a separate assembler file, where you can specify the origin, that simply jumps to the address of your C function; that way, you can just rely on the the linker to figure out where your code should go.

So if you have this code in, say, myfunc.c:

You could write something like this (from memory and untested, but hopefully you get the gist) in myfunc_trampoline.s:

The problem, however, is that defining the origin of any piece of code doesn't mean that it will be automatically relocated there when loaded from media; it just makes the compiler generate code that is pinned to a specific location for the purpose of calculating absolute addresses. It is then your responsibility (or your OS's, if it supports dynamic loading) to place it in a particular location in memory.

Hope this helps!