Is it possible to play games from other platforms on PPSSPP?

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No. Although these consoles are more similar than say a PC and an Android phone, they have very different:

* Operating systems
* Memory cards hardware interface
* Controller hardware interface
* Graphics rendering
* CPU coprocessor capabilities
* Memory mapping
* Secondary DSP / processing components

These are all technical things, but they were very very different between these consoles (and even more is different on the PS3.) PPSSPP only emulates the way these things work on a PSP.

If you can imagine the insides of an emulator as a bunch of complicated components all plugged into each other with wires, not only would many of those components be vastly different between a PSP, PS1, and PS2 emulator - but the wires would be connected in pretty different ways (especially for the PS2.)

PPSSPP does not run anything but PSP games. It may however run emulators designed to run on real PSPs, which in turn emulate other weaker systems (but not a PS2, it's not powerful enough.) No one has written an open source PS1 emulator for the PSP, but it's theoretically possible (and very time consuming.) It would be faster to use a PS1 emulator that already exists.

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