[stella] nop and wrap around

Subject: [stella] nop and wrap around
From: Mark De Smet <desmet@xxxxxxxxxxxx>
Date: Tue, 12 Jan 1999 17:35:11 -0600 (CST)
Now that I've finished my last project, I've been thinking about my newest
plans, and was wondering somethings.(The last one was that RAM-cart that
was programable with a dos copy command.  Finally got it all working)  My
newest will be a small embedded system connected to the atari.  It will
have an LCD screen that allows the user to select a game from it's large
ROM(and simultaniously display instructions while playing), load one from
a pc via serial, load one from actual cart(and send to pc), setup
bankswitching, and any other number of things that I can dream up along
the way.

But anyway, what I was trying to figure out.

My original thoughts to run it would be that my system would be able to
switch on and off the power to the VCS so that once a game was choosen, it
would power cycle the VCS to play the new game.  But while working I came
across an idea.  What if my system just swaped the rom while that last
game was running?  What I was thinking is swapping it to a rom that is all
nop's, and allowing the VCS to run those nops until it got to a certain
address(startup vector?) which would cause some of my hardware to switch
to the new game.  Or perhaps to a dummy program which would just display
some text, or maybe a screen saver of some sort until the user selects the
next game.  It's kinda like a bankswitch without having a software call to
do it.

Will this work?  Is there a common startup address(as in, is there a
location in memory where the 6507 fetches the very first instruction upon
startup?)? What is it?  Will the program counter in the 6507 wrap around
from the max to the minimum address?

Anybody have any ideas of how else to be able to get from a game running
to another one when you have no control over how either program was
written or where in it's execution it is?

Mark De Smet

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