[stella] byte 000h; atari models

Subject: [stella] byte 000h; atari models
From: Mark De Smet <desmet@xxxxxxxxxxxx>
Date: Sun, 27 Dec 1998 17:50:54 -0600 (CST)
If you recall, I have been working on a RAM based cart that can be
programmed via a file copy(In the future I'll write a nice UI) to a
parallel port.  I've got it mostly working, but have 2 questions that
hopefully someone knows something about.

What is stored at 000h(in the ROM)?  I mean is it something specific like
a startup vector or anything?  What would happen if it was changed?  
Specifically what would happen if it was changed to be identical to byte
FFFh?  I have not done any programming(in 6502 anyway) yet, so I am not
familiar with the exact locations of data in the ROM address space for
the 2600.  I belive my cart circuit is doing 4097 writes which is causing
the counter to wrap around and rewrite the first bit.

I've encountered what I think is a noise issue.  Games will come up and
run fine, but depending on what system I use, there will be a substantial
amount of noise and loss of color.  I've primarily used my 2600 jr.
(because it is such a nice system), but with it the cart has these noise
problems.  I've noticed that on a EPROM-cart I made the jr. sometimes has
color loss depending on the way the antenna line is routed in the room.  
This does not occur on normal carts.  I've never quite figured this one
out. I've measured and the EPROM-cart uses much less current than some of
the carts out there, and my RAM-cart uses even less (being CMOS and LS
logic. It uses about 160mA when running) so I don't think it is a lack of
power issue.  I've switched to an original 6 switch console, and I have
none of these problems.  Has anyone had such problems?  Where there any
differences in the consoles design that would cause this, or is my 2600jr
just screwed up?

Mark De Smet


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