Thursday, 24 August 2017

Coco, Xevious and... Atari Jaguar now!?!

A few different topics today.

Firstly, the Apple II/IIGS version is temporarily on ice, but rest assured I will get back to it in the not-too-distant future. The current status is that it's all-but complete - if quite flickery - but I haven't figured out banking for direct page and stack registers yet, which is preventing the use of PEI slamming for the IIGS. I (also) plan on returning to legacy hires mode, and doing an accelerated II/IIC+ version, but this will probably come later.

In the mean time I started on the Coco3 port whilst waiting for some assistance on the aforementioned IIGS issue, and as a result I'm now on a roll and loathe to put it aside. Looking purely at code volume, I'd estimate it's about 30% complete now, with all text, asteroids and saucer rendering complete (saucer movement is a bit off). You can coin-up but not a lot else happens.

Rendering is far from optimal; the aim of the exercise is to complete the porting of the arcade 6502 core with the simplest and smallest amount of graphics data required.

Oh and together with a possible Vectrex port (proof-of-concept at least) I'm also considering a port to the arcade Star Wars hardware - another 6809-based vector platform!

The C port will probably progress in step with the Coco3 port, and they will probably both be used to debug the other at various points. Again, that's probably for Neo Geo and Amiga.

And now for something completely different...

I don't recall exactly how, but I recently stumbled across reports of a port of the arcade game Xevious to the Atari Jaguar. From what I can gather, it's close to completion and will be released on cartridge for sale. It is particularly interesting to me because Xevious is my all-time favourite arcade game! I do have some knowledge of its internals, as I have in the dim dark past done some preliminary reverse-engineering for the purposes of both software (MAME) and hardware (FPGA) emulation. In both cases I was beaten to the punch by someone else, but it's not for nought as it is knowledge that I plan to use again one day.

There's scant information on the technical details of the port; I have no idea to what extent - if any - the original arcade code has been reverse-engineered, nor whether the port involves any sort of emulation (doubtful) or how faithful the Jaguar code is to the original. In any case, I'd be very interested in learning about the process, and getting my hands on any RE work already done. Not sure how likely any of that will be. For now, I plan on buying the cart.

Of course I started to look into the specifications of the Jaguar, and the resources and toolchains available for homebrew development. To my surprise the homebrew scene is quite active and, compared to similar platforms, the output is quite prolific - and that is due in no small part to the comparatively large number of Atari ST games ported to the platform!

As it turns out, a decent number of Atari ST games lend themselves to being quite easily patched to at least run on the Jaguar, and the architecture of the console - with no less than 3 decent CPU's - gives it the capability to screen-scrape the (largely incompatible format of) ST video memory!

[This is of course exactly what I have recently done with Asteroids on the Apple IIGS; patching as little as two instructions allows the core 6502 code to run on the IIGS, and the processing of the display list is perfectly analogous to screen-scraping!]

Developing for the Jaguar appears to be a tad more complicated than other platforms, though it is suggested that most homebrew development primarily commandeers the 68K 'management' CPU and the pair of purpose-built GPU/DSP devices are under-utilised. Also the toolchain involves the installation of a complete Ubuntu distribution - a far cry from simply having a makefile and AS6809.EXE and ASLINK.EXE in your path for the Coco3, for example!

Of course now I want to get my hands on some Jaguar hardware (not to mention Xevious!) Unfortunately - not unlike other platforms - the so-called SkunkBoard (basically a flash cartridge designed for homebrew developers on the Jaguar) doesn't appear to be currently in production. It's going to be an expensive foray...

The retro gaming scene is alive and never ceases to amaze me!

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