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VU Add Hack
#1
As a matter of curiosity, what does this gamefix do? I've read that it increases emulation accuracy regardless of the game it's used with.
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#2
Configuration Guide Wrote:Game fixes

As the name implies, these are special game fixes for some games which for some reason emulating them requires something special. Note that the team has no intention of keeping hacks for specific games, these are there temporarily until a proper fix is found so they can be removed.


* FPU Compare hack: This hack is needed because of the limited range of floating point numbers defined by the IEEE standard that the PS2 doesn't follow. Enable it only for 'Digimon Rumble Arena 2'.

* VU Add/Sub hack: This hack is also needed because the PS2 doesn't follow the IEEE standard on floats.
Enable it only for 'Star Ocean 3','Valkyrie Profile 2' and 'Radiata Stories'.

Note that game fixes are emulation hacks. They are just as likely to cause problems as they are to fix them.
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#3
I think the VU Add Hack is known to cause problems in one other game, when enabled -- but that's not the main reason we made it a gamefix. The problem with that particular gamefix is that it has the potential to be very slow. The accurate emulation of the Add instruction is several times slower than the regular version, and the only place it's known to be a positive is for booting Tri-Ace games.

Most games don't use the instruction much, if at all, so chances are you won't really notice. But I think we did run into at least one game that loses several fps if the hack is enabled.
Jake Stine (Air) - Programmer - PCSX2 Dev Team
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#4
Yep, keep the hack off for all games other than Tri-Ace ones.
It's true it does one particular instruction more faithfull to the ps2, but that doesn't mean much usually.
There's still 100+ other, more important instructions. And those are also off from the ps2 results Tongue2
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