Tuesday, 30 May 2017

Between a rock and a hard place

OK, without further ado, the mystery project is... Asteroids - the arcade version of course!

Image from playertheory.com

Nobert Kehrer did much the same first for the Atari 800XL, then for the Commodore 64. So I won't be breaking any new ground, but rather leveraging off the work done by Lonnie Howell in the original reverse-engineering effort and of course Norbert's so-called emulators. I've managed to advance Lonnie's work so that - as it stands - a good portion of the code is now commented, and Norbert has been kind enough to answer technical questions and even permit me to use his rasterized graphics data.

I'm confident now that I know enough already about Asteroids itself to port to other platforms; the task now is to learn enough about each of the target machines to effect the ports.

I've decided that I'll tackle the Apple IIGS first - a machine I currently know very little about. I've done some reading up on it tonight, and learned at least that it's possible to use the enhanced graphics modes (with sane memory maps) with the CPU in 'emulation' (6502) mode at 2.8MHz. 6502 mode does, however, place some restrictions on both resources and performance, so it remains to be seen whether or not I do indeed need to switch to native 65816 mode.

I was hoping to keep the code as legacy 6502 for two reasons. One, it negates the need to port the bulk of the code to a new CPU and two, it would leave an option to produce a version for Apple II/+/e machines with an accelerator card, requiring only rework of the graphics routines. Only time will tell...

The main impetus behind doing the IIGS port first is the fact that the next WOzFest meeting is slated for a IIGS theme and will coincide with the 2017 KansasFest (possibly with a live hookup). So I thought being able to demonstrate a new IIGS game on the day would be pretty cool.

As for Coco3, that's probably a lot more straightforward for me as I've done three ports to it now (Lode Runner, Knight Lore and Space Invaders), but of course requires translation from 6502 to 6809.

At first glance, having a 6809 version would make the Vectrex an obvious target, but unfortunately the stock console has insufficient RAM. However there exist several multi-carts that include additional system RAM, so a port may yet be worthwhile. The technical challenge there is to squeeze out enough performance from the 1.5MHz 6809 to emulate both the 1.5MHz 6502 and the Atari Digital Vector Generator (DVG). Perhaps a tall ask, but relatively little effort for a proof-of-concept demo at the least.

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