Re: [stella] some questions about the a2600

Subject: Re: [stella] some questions about the a2600
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Wed, 26 May 1999 11:31:54 -0400 (EDT)
On Wed, 26 May 1999, Erik Mooney wrote:

> >> > There are no interrupts on the 2600.
> >
> >but dont games usually have interrupt vectors stored at the end of the roms?
> 
> They have vectors, yes, but they're not really for interrupts.  The word
> at address $FFFC is the address where the CPU will transfer control at
> powerup, and the word at address $FFFA is the address where the CPU will
> transfer control after hitting a BRK instruction.

Actually, $FFFE is the BRK vector.  MOst games simply use this vector to store
2 more bytes of data.  But it seems like there was one that actually uses a BRK.
Can anyone help out here?

> They'll start displaying right at powerup (actually, the TV even before
> powerup - static, or whatever happens to be on channel 3).  But it doesn't
> matter if it displays garbage or blankness for a few sixtieths of a second
> before you get your VSYNCs rolling.

Rolling VSYNCs?  Hmmm...seems like that's self contradictory.... ;)
Sorry, I'm just feeling silly today! :P

> >well, i thought maybe i could use the f8 method of bankswitching by
> >essentially
> >creating two 4K files with code that could in essence be seperate roms, append
> >one
> >on top of the other, and then access locations $FFF9 and $FFF8 (since $1000
> >and $F000 are the same to the 6507) to switch back and forth between 4K
> >sections...
> >thats the way i see it...you think that would work?
> 
> Sounds about like the standard method of writing bankswitched games.

Yes, but if you want to implement this physically, a single rom is both
simpler, and less power hungry.  Just think of the chip as 2 separate 4k
roms, selected by the A12 line.

Actually, if you need a physical implementation, I know this guy.... :)

-Chris


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