Re: [stella] Supercharger RAM question

Subject: Re: [stella] Supercharger RAM question
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Sat, 14 Mar 1998 15:51:57 +0100
At 11:59 13.03.98 -0500, you wrote:

>One SC question:  Is there any part of the SC that I need to worry about 
>that is _not_ RAM?  I'm not anywhere close to messing with the SC, but it 
>has in the past sounded like you could rewrite part of your scan line 
>routine (for example) during overscan with the SC.  That is just about 
>perfect for achieving special effects for this game.  I was thinking 
>about writing a program that actually had commands in the Atari's RAM, 
>just to see if that would work...  This is pretty much what writing code 
>for the SC would be like, correct?  (knowing, of course, that I'd have 
>TONS more RAM!)

My Domino program changes it's display code in SC RAM, so it is
possible.

Address $FFF8 contains the control byte for the SC. Any writes to
it would switch RAM banks or turn on and off SC writing. Therefore
you can't use it to store data. 

Accesses to addresses between $F000 and $F0FF would cause SC writes.
You can't read from, write to or run code in this page, when SC
writing is enabled, or your program might crash, because your code
got changed accidentely. If you don't want to waste these 256 bytes,
you can put some code or data there, that is not needed for writing.
Then you can enable SC writing for the routines, where you need it,
and disable it afterwards.

All other addresses can be used to write too. But you should
remember, that SC writes take quite some cycles and if you also
have to use complicated calculations to find the correct address
to write to in your display code, there is not that much time
for doing many writes during Vblank.

I think it would be better, if you would start with a simple display
kernal first and add effects like this later.

BTW, the 'prizes' in Jumpman are supposed to be bombs. :-)

Ciao, Eckhard Stolberg



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