Plugin or Overlay system

[medzes]

Several suggestions in the issue list require either more C64 code space than currently is space for or more ARM code than is currently space for in the 48kB ARM / 16kB C64 area. It would be great to find some consensus how to approach such a plugin system and how this eases development.

A plugin or overlay system would be useful to allow easier development of addons/plugins/menu replacement.

I have prepared a first shot of this, as a prototype, where a file extension ARM is supported in the menu that loads 64kB of statically compiled ARM code into the dat_buffer and jumps to the start address while keeping the usb and/or c64 interface interrupt running. I am using this approach to run sd2iec code after an initial ATN signal is detected from the ARM's idle loop (using the 100 pin DEVEBOX). I anticipate for ease of development no re-use of code or symbol reuse between the plugin and the main code, just a jump subroutine to dat_buffer address and a return at the end of the code to go back into the main_loop. The return exit code would then allow the main loop to do a restart to menu or other actions as already defined in the code. It is needed however to pass somehow which part of the bss/data sections are freely available for the plugin without affecting the main loop. I don't have a solution for that yet. It would be possible to link the ARM code statically to not use the TCM RAM at all, except for the stack, that is a simple but somewhat inefficient solution.

C64 code support may already be possible through the EasyFlash API I am not aware of that. But it may help development to be able to replace the 16kB of C64 menu code by an external file, or bank switch that with a loaded file.