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Does the x86/x64 cpu emulation work for JIT code or other self-modifying code? (Copy protected games, for example?) In the linked presentation they just briefly mention doing a load time(?) transpile of x86 to arm64 and caching the result on disk. What about exe packers that map memory rw then rx after unpacking, such as UPX?

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They say that it's a dynamic translation layer, so presumably it doesn't make a distinction about where the code came for (except maybe you'll lose some performance optimization if they are caching translated code)

Most dynamic emulators don't do instruction by instruction emulation - they take in a whole block of instructions and translate that block, caching chunks that may be used without any inference (eg. branching). Once you hit a similar instruction block you don't have to do any translation; just execute from the cache. This is why you have a load time for reading from an x86 image.

It's actually only x86 emulation. x64 is not supported according to the video.

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