Sunday, 25 June 2017

2600 for a day and IIGS video

A little diversion; someone posted on an Atari-related FB group about tinkering with Ms Pacman and not having much luck getting it 'loaded into a disassembler' for more in-depth hacking. I couldn't help myself and started asking questions, and of course ended up doing it myself to satisfy my own curiosity, having never done anything with 2600 before.

The complication is this case is that the 2600 only maps 4KB of cartridge space, and Ms Pacman is 8KB. There are a handful of different banking schemes implemented in various cartridges, Ms Pacman being one of the simplest. Despite that, DiStella for example, doesn't support banked cartridges though it is forgivable and not really surprising. Also worth nothing that Dan Boris is one of the authors, and there's bound to be a good reason if he elected not to support banking.

After another wildly unsuccessful attempt to understand if/how banking is supported in IDAPro, I forged ahead with Ms Pacman only to discover that the first code bank actually executes at $D000, and not $1000-$1FFF as is documented as the reserved cartridge address space. Of course with the higher address lines missing from the 2600's 6507 CPU, the machine's 8KB of addressable device/memory space is mirrored every $2000.

I then turned my attention to the second bank, loading it into a second IDAPro session - until it was revealed that this bank actually executed at $F000! No doubt making development much, much less painful, it also allowed both banks to be loaded into the same IDAPro disassembly and the banking issue all-but-ignored. I added a few other segments, notably the TIA registers, the zero page area and the PIA registers, and as a result have a ready-to-go base for reverse-engineering.

However, I should note that there's very little chance I'll be tempted to work on this at the expense of Asteroids, or even anytime soon after I'm done! From what very little perusing I did of the source, it really didn't look enticing at all, especially in light of my limited knowledge (having read Racing The Beam) that suggests programming the 2600 is just as much about coding a video hardware controller as it is about coding game logic. And although I briefly mused about porting a 2600 title to another platform, I also quickly realised that much of the code wouldn't resemble the original in the slightest.

So about Asteroids; I've done further reading on the subject of IIGS architecture, and the video memory in particular. At best it looks like you can only write to the video memory at 1MHz, though you can read back at 2.8MHz. I've also read a few interesting articles on optimisation techniques - some specifically for the IIGS - and suspect I'll be employing at least some of them down the track. But for now, I think I'm across the technical aspects enough to choose a tact and begin work on it next session.

So it's time to fire up the 65816 assembler - or rather, 65816 switch on CA65 - and see if I can manage not to crash the IIGS in 4 lines or less...

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