Tuesday, 19 November 2013

Back to Donkey Kong...

After playing around with the Phoenix emulator, and unsuccessfully trying to get it running on the MVS system, I've decided to leave that to the original author and get back on track with my Neo Geo BIOS and Donkey Kong projects.

To that end, I've now finished transcribing Sockmaster's comments into IDAPro which would mean that, technically, there should be enough information to start work on the 68K port. However, I'm loathe to leave the source half-commented (actually, considerably less than half) and have decided to press on with the reverse-engineering and comment as much of the code - aiming for 100% - as possible.

As it stands I've got a reasonable idea of the structure of the code; the program flow and the data structures involved. The code should also be almost completely re-locatable (at assembly time) now; later on in the piece I'll try moving code and data around and see if it still runs on Donkey Kong hardware! It might also be possible to port it to other, suitable, Z80-based hardware as well - any 8x8 tilemap system with 16x16 sprites should suffice. There is, for example, a port on Galaxian hardware.

Anyway, once I start the port my aim to is get the first level running, and release that as a playable demo. I'm also intending on coding both horizontal and vertical versions (selectable via soft dipswitch) - I guess it'll be the Neo Geo's 2nd ever vertical game!?!

Thinking more about the 68K port - it will perhaps be a little more work than the 6809 because of the 68K's 16-bit data bus. There will also be the temptation to simply re-write routines in 68K ASM with little regard to Z80 registers; an approach frought with danger when other routines expect certain values to be retained in certain registers. Must... resist... temptation...

So it's not going to end up as nice 68K code by any standard, unfortunately. But once it's running on the Neo Geo, other possibilities are interesting. An Amiga port almost goes without saying, but then there's also the Sega Genesis/Megadrive! Hmmm...

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