Wednesday, 5 July 2017

Standing on the shoulders of giants...

Undecided on how best to proceed with the remaining rendering tasks, today at lunchtime I downloaded Norbert's (Atari 800XL) emulator and fired it up in MAME, intending to plot the 'thrust' pixel in each of the 24 player ship bitmaps based purely on observation.

I've documented 12/24 but not surprisingly, it got old pretty quickly and, curiosity getting the better of me, I dumped the first 16KB of the Atari's memory into a binary file and loaded it up into IDAPro.

Before transferring control to the arcade code, Norbert's emulator patches a bunch of addresses in the ROM. Aside from those critical for running on non-Asteroids hardware (ie. the same patches I made) it also patches routines such as the 'CUR' (current) DVG command, character display, display of extra ships, etc.

It also installs a hook in the main game loop. The hook itself calls three subroutines; one to read the Atari joystick inputs and seed the memory-mapped Asteroids inputs, one to play the sounds based on the memory-mapped outputs, and the third I'm yet to ascertain.

Most importantly though, I'm still yet to determine how the emulator goes about rendering the display. From what little I've seen, the display list is somewhat 'corrupted' by the patched routines. However there are other unpatched routines that must still provide data to the emulator via the display list - so I'm not sure how it all works just yet. The waters could also be further muddied by the Atari 800XL's unique display hardware... I'm sure I'll work it all out next session.

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