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MESS 0.160 out... update to the 7800 driver


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Heads up that MESS 0.160 is out, and contains an update to the 7800 driver. In a nutshell, the upper memory mirrors that were part of the official docs but aren't present in real hardware have been removed.


This shouldn't affect well behaved programs, but ill behaved ones that hit the formerly mirrored addresses should break in the same way as real hardware now.


Big thanks to CPUWIZ for confirmation of the actual state of the upper memory mirrors, and Trebor for submitting the change to the MESS team! :thumbsup:

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  • 3 weeks later...

This shouldn't affect well behaved programs, but ill behaved ones that hit the formerly mirrored addresses should break in the same way as real hardware now.


Before I go rearrange my current emulator setups, can you give an example or two of non-well behaved programs this might affect?



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If you're looking for an official or finished homebrew rom that will be affected by the fix in 0.160, there isn't one. Its more of a WIP developer thing. If MESS didn't have the code to catch the error and a game hits it, eventually the game's bug would be caught during real hardware tests, but at that point it would be more of a pain to fix. That's why I posted about this update in programming forum, instead of the general 7800 forum.


The timing of your post is pretty good, though, as I needed to update this thread. It seems a change to some underlying MESS timer code introduced some timing issues in the 0.160 7800 driver. Until that issue is fixed in a later version, I'd have to say that the 0.159 driver is the gold standard in 7800 emulation accuracy right now.


I'm not sure what your current emulation setups are, but if they're non-MESS, then they're likely lacking >144k support (homebrew demos like softcell, multi lockon, and Frankdragon's WIP that CPUWIZ linked to), don't have accurate linecount, DMA cycle stealing, DMA cycle limiting. (This results in emulation line glitches in various roms. summer games, xenophobe, commando, among many others), pokey@450 support (various homebrew), 16-bit pokey mode (homebrews?), XM support (upcoming homebrews), proper 2 button support (WORM! homebrew), etc.

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Thank you for that insight.


I don't currently use MESS for 7800 emulation (I'm using different builds of ProSystem for OS X and on my Raspberry Pi 2). I guess it's time to download SLDMESS for my Mac and see if I can spot the differences for my usual gameplay.

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  • 1 month later...

Hey smart code-aware people!

Thanks to a thread in the Emulation sub-forum, it seems that there is a bug in MESS in how POKEY audio is detected and implemented. Specifically, in the current 0.161 version, POKEY audio does not play in Commando, while it does get detected and plays properly for Ballblazer.


By contrast, the now-geriatric ProSystem emulator detects and plays POKEY audio audio perfectly for the very same ROM, albeit with a bit of some graphical corruption at the bottom of the title screen. Does anyone (Trebor?) have an "in" with the MESS team to post a bug?

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Are you sure you don't have a Commando A78 file with a bad header? Try this one: http://www.atari7800.org/bin/a7800/Commando.zip




Well, THAT ROM works great, thanks! I have no idea where I got the original one that was causing trouble, but I also tried downloading one from AA and it has to same problem with POKEY audio. Maybe you could see about getting that one pulled and replaced.

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