Chris Wilkson wrote:
> 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?
if the instruction BRK really does transfer control to the address indicated at
$FFFE, and the address at $FFFE is altered, perhaps that could be used for
a quick way to branch to some new instructions that can be used all the time, like
say, switching over to a bank-switching routine...or like if you hit the reset
button,
maybe it could do a BRK, fly to a routine (not on wings of course), and if you are
programming a bank-switched game, switch back to the first bank, then JMP to
the beginning (or just put $F000 as the vector and have it start over)...if that
could
be done, there are potential uses for this
> Rolling VSYNCs? Hmmm...seems like that's self contradictory.... ;)
> Sorry, I'm just feeling silly today! :P
arent we all :)
>
> 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.... :)
thanks all the same, but i am just beginning to understand the mess
that we call 6502 assembly :) and i know about as much about
electronics as i do fixing cars, so i dont know if i could ever make
an actual rom/eprom/whatever of any game without either
figuring out how the 2600 handles cartridges hardware-wise or
getting some outside help :P
dee
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