[atari7800] Re: DMA TIMING

  • From: Christopher Tumber <christophertumber@xxxxxxxxxx>
  • To: atari7800@xxxxxxxxxxxxx
  • Date: Mon, 08 Nov 2004 17:54:53 -0500

>Picture looks pretty much the same as the one posted on atariage that someone 
>posted of a screen capture.  Pressing fire changes the color of everything 
>from redish to purplish.  The sprites are not visible at all.

Thanks Guys...!


Well, that's very... unfortunate.

I think we can conclude a few things:

- There's no difference in DMA timining if bitmap data is in RAM or ROM.
- If you write a full scanline at 320x2 that pretty much takes up ALL your DMA 
time (You're not going to be able to overlay any sprites).
- 80 bytes of bitmap data is about all you have time to DMA in a scanline.
  - You may be able to squeeze a little more out (by optimising headers a 
little better) but not much.


Which is really too bad as it means that using 320x2 backgrounds is basically 
impossible. So 320x2 use is going to be pretty limited - title screens and cut 
screens and the like. If we had the RAM we go just use 320x2 as a full bitmap 
mode, however being limited to 4 colours per scanline would be problematic.

You could still use 320x2 (or 320x1) pretty effectively for something like the 
radar in Defender or Battlezone but as far as being your main graphics mode 
that's just not going to happen *if you want a complex background*.

Which leaves us with 160x2 and 160x4 which is what all the original 7800 games 
used. Which is barely an improvement over the Intellivision and 2600 and a STEP 
BACKWARDS from the Colecovision, let alone the NES. How Atari thought (160,192) 
resolution was anywhere good enough is beyond me and what's even worse is that 
the 320x2 mode is actually ideally suited to games without backgrounds, games 
like Robotron, Joust, Ms Pac-Man and Galaga and yet THEY DIDN'T USE IT! Instead 
they used the ultra-crappy 160x modes.

The one and only aspect of the system that blows away the competion and Atari 
ignores it. Bah!

Yeh, I'm ranting, but check the attached and you'll see what I mean.

Colours look okay in MESS (but are not 100%), may be completely pooched in a 
CC2. Neither actually does anything other than display a static screen.

I really wanted to get away from ports, but I'm really, really tempted to "redo 
them right". Yeah, you'd need a little trickery to deal with the colour depth 
(mixing chars and sprites), particularly in Robotron with how fluid positioning 
is, but whatever...


Chris...

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