Subject: Re: [stella] some questions about the a2600 From: gentlben@xxxxxxxxxxxx Date: Wed, 26 May 1999 10:59:20 -0700 |
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 -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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