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First of all: many people believes that since the main CPU (Emotion Engine) runs at a clock speed of 294Mhz (299Mhz on later revisions), it would be fast to emulate on recent hardwares. Far from it. The clock speed of a CPU it’s not and indicative factor that makes emulation easier or harder. The PS2 contains a multitude of custom CPUs and chips (main 128bit CPU Emotion Engine housing a FPU co-processor , 2 Vector Units, IOP, SPU2, Graphics Synthesizer and SIF) that work asynchronously and emulating them perfectly requires enormous power. Also the PS2, just like PS1, uses MIPS architecture instead of standard x86 code, thus making emulation slower.<ref>https://forums.pcsx2.net/Thread-Why-is-PCSX2-slow</ref>
Another big problem is the emulation of PS2’s own floating point unit (FPU), since it doesn’t follow the IEEE standard. To keep it simple, just changing a couple of numbers would lead to glitches in the game’s graphic (VU) and logic (EE) (broken AI, odd behaviors or graphical bugs). PCSX2 allows to change clamping/rounding on both VU and EE as a solution to fix these glitches, but it’s not the most accurate way to emulate the PS2 FPU.<ref>https://wiki.pcsx2.net/PCSX2_Documentation/Nightmare_on_Floating-Point_Street</ref><ref>https://github.com/PSI-Rockin/DobieStation/issues/51</ref>
To conclude the problems with PS2 emulation, we come to hardware rendering. The PS2’s graphic pipeline acts very differently than modern GPU cards and emulating it in HW mode with a certain degree of accuracy it’s hard: due of the versatility of PS2, and the fact that it doesn’t use fixed shaders, games don’t follow a precise formula to how achieve different effects. Various type of enanchments like scaling leads to the typical “black lines glitch” because of using a non-integrer resolution. The OpenGL backend on PCSX2 greatly improved these issues, but most games still requires “software rendering” to fix most of the glitches. Games using mip-mapping (Ratchet & Clank, Ace Combat) and games running on the Snowblind Engine are playable in OGL HW mode with minimal problems.
