Subject: Re: [stella] Meditations on the pause button From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx> Date: Tue, 7 Sep 2004 15:14:56 +0200 |
> >Unless there's some case when the 2600's data bus would not contain $0f? > > I like the idea. But it is possible for the 2600 to get rubbish or '0's > floating on the databus too. > It depends very much on the hardware pluged in and is not very reliable. > That the last byte still floats on the databus has something to do with how > fast a rom can change its output state and how much load there is on databus Also not all 7800s will zero out the undefined bits on the TIA read registers. This is just a side effect of a hardware fix that Atari build into later models of the 7800. This helped with some problems that the games with cartridge RAM had, but broke compatibility with Activision's FE bankswitching and the Supercharger. The origial 7800's would react just like a 2600 on a TIA read. Also some people disabled the extra hardware (only requires removing a certain capacitor) to make their 7800 compatible with the Supercharger and the Activision games again. So this is not a very reliable method of detecting 7800s. BTW, if pause is such an important feature, couldn't you use some other switch to enable it? Or are they all used for something else? > I've also been looking on the schematic for a possible 7800 hardware > detection. But found nothing concrete yet. I'm wondering though: > > The RIOT is addressed using the MARIA chip would there be any change on > small incompatibility in the memorymap mirroring ? No. In 2600 mode the memory map is exactly like on a real VCS. > Memory configuration is externally locked. But MARIA seems not locked be it > will some MARIA fuctions still be avialable? > NMI line is controlled by MARIA chip. Is there a possibility to trigger a > NMI ? The only way to trigger an interrupt on the MARIA is to have the DMA find an "interrrupt bit" in the display data for a certain scanline region. I think DMA is disabled in 2600 mode, so you shouldn't be able to trigger a NMI. And since the DMA is the only way to make the MARIA do anything, you shouldn't be able to get a reaction from the MARIA, even if some of it's write registers are still active in 2600 mode. BTW, How does Video Pinball play on your EPROM cart? That game intentionally relies on the state of the undefiened bits in the TIA read registers. If they don't stay like the address of the register, then the ball shouldn't bounce off the flippers anymore. We had this problem in the emulators untill we simulated the TIA read registers properly. Ciao, Eckhard Stolberg
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