Saturday, 10 June 2017

A tale of two CPUs

Not much progress... well, none to be exact... on the reverse-engineering front but I have been researching IIGS architecture and development.

The plan for a first-pass demonstration is to leave the 6502 code more-or-less intact, changing only what is required to get it running on the IIGS. Aside from possibly changing some addressing, the only other changes that come to mind immediately are the interrupt, and handshaking with the digital vector generator (aka DVG).

The result of the above should be an area of IIGS memory that is periodically updated with a new display list for the DVG.

The intention then is to implement a crude emulation of the DVG, or more precisely, an emulation of what the DVG is doing on Asteroids, that reads the display list and renders to the IIGS super hires display. I'm not planning on drawing vectors on the screen, but rather using a crude lookup to determine what the DVG is supposed to draw, and then display pre-rendered bitmaps at the appropriate screen location. This is possible only because most of the objects are actually rendered via subroutines in the DVG ROM; once I know the subroutine address, I know what object is being rendered.

Once all that is going, there's plenty of work to do to optimise the whole process by more tightly integrating the main code with the rendering code. I have some ideas about that already.

My conundrum now is whether to develop the rendering routines in native 65C816, or remain in emulated 6502 mode. I'm leaning towards the former, but it does introduce some complexities to the process, such as assembling effectively two completely separate programs, for different CPUs. And likely different assemblers, as I'd probably stick to AS6502. OTOH, it will avoid the addressing limitations, and some of the slowdowns, of remaining in 6502 mode. Not to mention, well, 6502...

Time is ticking towards WOzFest so I'd like to get a start on this sooner rather than later. I still need to sort out the emulator I'll be using (preferably MAME), and exactly how to go about executing a binary built from a generic assembler on an emulated IIGS. Guessing I'll be learning something about Ciderpress soon enough.

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