The VS Dualsystem thread

Discuss emulation of the Nintendo Entertainment System and Famicom.

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oRBIT2002
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Location: Gothenburg/Sweden

The VS Dualsystem thread

Post by oRBIT2002 » Thu Mar 18, 2021 11:09 am

I'm a bit curious about the Dualsystem, perhaps someone knows this..
1) It's actually two systems connected together with two screens... I understand they share the same memory (at $6000-$7FFF), however can both systems access this RAM at the same time?
2) I've been disassembling some code here and there and see quite some IRQ-related stuff, what's it doing? Does it force the other CPU to run or switch CPU or something?
3) Does a Dualsystem-game consists of two (slightly different) games or does both systems have the same ROM? Most dumped ROMs floating around the net suggest it's mostly one ROM..?

lidnariq
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Re: The VS Dualsystem thread

Post by lidnariq » Thu Mar 18, 2021 11:51 am

oRBIT2002 wrote:
Thu Mar 18, 2021 11:09 am
1) It's actually two systems connected together with two screens... I understand they share the same memory (at $6000-$7FFF), however can both systems access this RAM at the same time?
They can't.

One CPU ("M") can control which CPU has access to the RAM.
2) I've been disassembling some code here and there and see quite some IRQ-related stuff, what's it doing? Does it force the other CPU to run or switch CPU or something?
Both CPUs can assert an IRQ on the other CPU. This is the only way to communicate about which CPU is going to get access to the shared RAM.

Rainwarrior helped me to rewrite the relevant section of the wiki recently:
nesdevwiki:Vs. System § $4016 write
3) Does a Dualsystem-game consists of two (slightly different) games or does both systems have the same ROM? Most dumped ROMs floating around the net suggest it's mostly one ROM..?
It varies, but it tentatively seems that it's usually two slightly different copies.

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