7800 demo

[EricBall]

I've created the following demo for the 7800. It may form the basis for a game, but right now it's just an experiment to figure out how to do 4-way scrolling backgrounds on the 7800.

 

The demo currently shows some blocky clouds drifting over a fixed blocky landscape. (Use your imagination.) Both are done using character maps (tiles).

 

Although it runs fine via emulation, it may be more than MARIA can do in 456 cycles. I also haven't implemented wrap-around yet, so you may see some discontinuities, and it might crash or black out if left running long enough.

 

The file is 8K and signed, so simply strip the 128 byte A78 header if you want to burn it to an EEPROM.

78scroll.zip

[+Mitch]

Cool! I'll try to find time to test it on my RAM cart tonight.

 

Mitch

[Eckhard Stolberg]

I've been running the demo for a couple of minutes on both, my PAL and my NTSC 7800. It is running pretty well on both of them. I didn't notice any wrapping problems or crashes. You seem to be doing not enough scanlines though. Even on the NTSC system there are a couple of scanlines with changing garbage data at the very bottom of the screen. This is of course even worse on PAL. If you like, you can have a look at the backdoor example program on my website, which also demonatrates how to make your program compatible with PAL and NTSC at the same time.

 

Also it seems that you have set the border to have the same colour as the background. Therefore there are two blue stripes on the left and on the right of the screen. That's probably not a good idea for a scrolling game. If you want to scroll up and down too, you need to set up more active lines on the top and bottom too. Right now there are some small black stripes. You should fill the entire screen to make the landscape scroll without popups.

 

 

Ciao, Eckhard Stolberg

[EricBall]

Thanks for the feedback Eckhard. I think it crashes after 15 minutes or more, when the tile list goes over the page boundary.

 

This is my first attempt at a 7800 game/demo, and I'm flying by emulator only, so I'm not surprised that there are a few quirks. I'll check out your example program. Currently the DLL generates 192 active lines, with some top and bottom boundary lines.

 

I'll check the border control bit. Maybe the sense is reversed from the documentation I have (Character Width was, at least). Something emulators don't show.

 

My next challenge is to work on vertical scrolling, which means messing with DLL offsets and charbase for the top and bottom zones. More work for the NMI routine.

[+Mitch]

It looks like Eckhard beat me to it.

And my results were the same as his were.

 

Mitch

[EricBall]

Question for the PAL folks - does the 7800 have the same PAL palette as the 2600, or is it closer to NTSC? In my demo the ground plane should be blue, two shades of green, and a tan.

 

And I am working on a revised version with NTSC/PAL autodetect too.

[Eckhard Stolberg]

It's nice to hear that your game will be supporting PAL mode.

 

I have compared the palettes on my PAL and NTSC 7800, and they are very similar in 7800 mode. I think they are supposed to be identical. However the factory setting for the colour adjustment potentiometer seems to be very different. The NTSC 7800 has a slight red touch, while the PAL 7800 goes more into the green. This makes some borderline colours like turquise or gold seem to shift by some spots in the palette. Therefore Atari's programmers seem to have adjusted the colour values in the PAL versions of the games.

 

I can't really suggest a colour mapping table. I think it would be best to leave the NTSC colours in and then have someone with a PAL setup test your game to find the colours that are looking too wierd.

 

 

Ciao, Eckhard Stolberg