Re: [stella] Cart card edge connector

Subject: Re: [stella] Cart card edge connector
From: "B. Watson" <atari@xxxxxxxxxxxxxx>
Date: Mon, 3 Sep 2001 20:53:08 -0400 (EDT)
On Mon, 3 Sep 2001, Kevin Horton wrote:

> At 20:18 9/3/01 -0400, you wrote:
> >When I took the cart apart, the chip side of the board was facing up, so
> >looking at the card edge with the chip side up, and looking at the pinout
> >for the ROM chip (from, um, Kevin Horton's web site, I think?), I see that
> >the chip's A11 line is wired to the card edge's A12 line, and the card edge's
> >A11 line doesn't even have a pin... what gives?
> >
> >In fact, A0 thru A12 are present on the card edge connector... but this
> >would allow for 8K carts, without bank switching. So is the FAQ wrong?
> >
> >I'm sure this has been discussed before on this list, I'm just trying to get
> >clarification.. I *need* to understand this fully, before I break out the
> >soldering iron.
> Simple.  That's a 2K ROM, so it has no A11.  That "A11" on the chip is 
> really the positive enable.  If you connect it to A12, it will be enabled 
> when you are accessing the upper 4K of the total 8K address space.

D'oh! I was fooled by the fact that the rom dump of combat I have is 4k, but I
see it's just 2 copies of the same 2k of data now... I had just assumed it was
a 4k cart..

> A 4K chip has an A11, so you connect it to A11, and then A12 is still the 
> positive enable. (All EPROMs have negative enables, so you must use an 
> inverter (7404 or similar) to turn the positive enable into a negative one).

I knew about the inverters, got a pile of them.

> You can only use the upper 4K of address space, since the lower 4K holds 
> the RAM, stack, TIA registers, etc. and is therefore unusable for code.

Oh yeah, I was being stupid... or overly optimistic :)

Too bad the TIA, PIA, etc. decode as much address space as they do. If there
weren't `mirrors' of them all over the first 4k, it could have been possible to
have 6k or 7k carts, maybe. Too late to change that design decision now I
guess :)

Now, if only I could get my EPROMs to erase, I'd be ready to start soldering..
well, assuming I only wanted to use 2k of address space, since the missing pin
on the combat cart means I don't get to use 4k...

*sigh* I get to decide which of my priceless collection will be the next
sacrificial victim. Maybe Pac-man, the Atari version sucks, and it's actually
4k, from looking at my rom dump of it (vector at $FFFC points to $F000)



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