`get_isr_handler` returns the address of the handler currently configured to handle the exception specified by `vector_number`.
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`set_isr_handler()` sets a handler for the exception specified by `vector_number`. It returns the address of the handler that it replaced.
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`set_isr_handler()` sets a handler for the exception specified by `vector_number`. It returns the address of the handler that it replaced, or _0xffffffff_ if the `vector_number` is out of range.