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From: cfchiesa@bsu-cs.UUCP (Christopher Chiesa)
Newsgroups: comp.sys.atari.8bit
Subject: 16K cartridge - How?
Message-ID: <2806@bsu-cs.UUCP>
Date: 29 Apr 88 17:42:37 GMT
Organization: CS Dept, Ball St U, Muncie, Indiana
Lines: 62
Keywords: Cartridge 16K ROM RAM


I've recently been trying to learn about the innards of one of my game cart-
ridges (CENTIPEDE, to be specific).  I find that I can go into BUG/65, move the
display list and display data "out of the way" of the Cartridge memory areas,
then snap the cartridge into the slot and have its ROM suddenly appear in the
appropriate address space. 

Using this arrangement, and assuming for convenience that I plug CENTIPEDE into
the LEFT cartridge slot on my 800, I find that 8K of address space switches from
RAM to Cartridge ROM.  I can then copy this 8K of data down into RAM, unplug 
CENTIPEDE, and copy the data back to its original addresses which are now RAM
again.  The same bytes are now in the same addresses as when CENTIPEDE was there
"in person."  So far, so good.

If I execute at the "cartridge run address," with CENTIPEDE plugged in, it 
works.  I can play CENTIPEDE.  But when I go through the above procedure, and
try executing the SAME code in RAM, it DOESN'T work!  I get a display of the  
title screen, and the "demo mode" makes a fair attempt at execution, but the 
P/M graphics are totally garbled, and you can't get beyond the title screen to
try playing the game.
 
I have several theories about this phenomenon, which I'd like to bounce off
you of the Net.

1) Centipede is a 16K cartridge and I'm just not getting all the code.

    Support: opening the cartridge, there are TWO chips inside, as opposed to
    ONE chip inside Atari BASIC; Atari BASIC DOES work properly under the ROM-
    to-RAM copy scheme described above.

    Questions:

       How do I get at the other 8K ?  Is there a "switch" address in the ROM
       that toggles in the second half "upon request"?  Or does it appear in
       some other, hidden memory block that I haven't found yet?

2) Centipede is an 8K cartridge but detects RAM-vs-ROM execution and bungs up
   if you've moved it to RAM.

    No real proof OR disproof of this one, except that "that's something I'd
    probably do if I were Atari." 


There ARE a few places in the code where an LSR (logical shift right) is per-
formed on memory addresses WITHIN the Cartridge ROM.  I haven't been able to 
determine whether these operations are part of "detecting ROM vs RAM" or "swap-
ping in a second 8K".  I also haven't looked at the WHOLE 8K yet, of course;
it's an ongoing effort and your responses here will be helpful in "pruning" 
paths of inquiry that are in the wrong direction.

Please post or e-mail anything you may know on this type of thing; the manuals
are amazingly sparse.  The only comment I've ever even SEEN about 16K cart-
ridges was something to the effect that "...Atari designed a second slot into
the 800 with the intent of marketing larger-than-8K programs as TWO cartridges,
but then they discovered a way of putting 16K programs into ONE cartridge, ..."

This implies that I'm on the right track, but this is ALL the documentation 
there seems to be.  Thanks in advance for any info you Netpeople can provide!

Chris Chiesa  
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