Why is it so difficult to emulate PS2 games in a PS3?

Question

I know that PS2 games can only be played in a 'fat' PS3 with the right hardware to run it. But why is so difficult to emulate a PS2 game in the PS3 without that hardware? Is the processor, the architecture, both? Thanks in advance.

Answer

Short Answer

PS2 games were made for an older CPU. They cannot run on the CPU designed for the PS3.

Long Answer

PS2 games were created to work on Sony's Emotion Engine CPU and their Graphics Synthesizer GPU. This CPU/GPU combo was put in all PS2 units and in the first variants of the PS3 (the NTSC 20GB and 60GB models) to make them backward-compatible. These models can play almost all PS2 games.

The 80GB Metal Gear Solid IV bundle had the GPU as well, but replaced the Emotion Engine chip with software emulation of the chip. It can still play most PS2 games, but support isn't as good as the earlier models with the actual EE CPU.

Models since the MSG:IV bundle had neither the CPU nor the software emulation of the CPU required to play older games.

Sony has a knowledgebase article that explains which three versions can play PS2 games. They also have a compatibility tool that you can use to see if specific games can play on those specific models.

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