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this is for a small research I'm doing in one of my classes
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What is the question you have been asked? If the question is to compare external processors to soft-core processors in FPGAs, then that is a very specific question which can be more easily answered. If the question is to compare CPUs to FPGAs, then its a very open-ended question.
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i don't know where to find the needed information
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The 'needed information' is very application specific, so you will not find one source of information.
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can you give me a quick recap or comparison between the two ?
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There is no 'quick recap'. For example, what sort of CPU are you being asked to compare? An ARM core, an ARM microcontroller, an ARM processor, an x86 processor, an AVR microcontroller, a GPU, etc?
Try to narrow the question, and you'll be able to answer it easier.
Cheers,
Dave