Subject: Re: [stella] nop and wrap around From: Mark De Smet <desmet@xxxxxxxxxxxx> Date: Wed, 13 Jan 1999 00:16:37 -0600 (CST) |
> Cool... where do I buy one? :) Not done yet... > There is not a common startup address, but there is a startup vector at > $FFFC - on powerup, the 6507's PC is set to the word stored at locations > $FFFC and $FFFD. Is the vector stored in the lo, hi format the same as in a jmp?(from a quick search through existing roms, it seems so) If so, how about having a rom like this:(system relative addresses, not rom relative) $1000 - $1FFA = EA (nop) $1FFB = 4C (first part of a jmp, i think) $1FFC = FB (startup vector lo)(shouldn't actually be used) $1FFD = 1F (startup vector hi)(shouldn't actually be used) Assuming that I have hardware that switches from this rom to the new game after the address $1FFB appears and the ROM becomes deselected($0XXX)(i.e. after the 4C was loaded into the 6507). If I hot-swapped into the rom above(swapped in the middle of a games execution), would the system start up with the new game? I would think that the processor would proceed to execute nops until it got to $1FFB, where it would start a jmp, and then load the vector from the new game. Of course the screen update would be neglected, but I think that it would be much smoother a transition than power cycling the VCS. > Try experimenting with swapping in a ROM full of BRK instructions. I > *think* the BRK instruction acts as a soft reset, resetting the PC to the > startup vector. Anyone else know about the BRK? > As for wrapping around the PC... that's very unlikely to work. Within the > 2600's address space, $1000-$1fff is ROM space, then the very next byte at > $2000 wraps back around to $0, which is TIA input/output registers - > trying to execute code out of those registers is unpredictable, to say the > least. :) oops, forgot about that. Just curious, if the startup vector is at 1FFD:1FFC, then what is at 1FFE and 1FFF? Mark -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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