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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