If any bits of n are non-zero, pop the topmost exception frame from the exception stack, along with everything on the return stack above that frame. Then restore the input source specification in use before the corresponding CATCH and adjust the depths of all stacks defined by this standard so that they are the same as the depths saved in the exception frame (i is the same number as the i in the input arguments to the corresponding CATCH), put n on top of the data stack, and transfer control to a point just after the CATCH that pushed that exception frame.
If the top of the stack is non zero and there is no exception frame on the exception stack, the behavior is as follows:
Otherwise, the system may display an implementation-dependent message giving information about the condition associated with the THROW code n. Subsequently, the system shall perform the function of 6.1.0670 ABORT (the version of ABORT in the Core word set).
KEY DUP [CHAR] Q = IF 1 THROW THEN ;
?DUP IF ( exc# ) \ 0 THROW is no-op
HANDLER @ RP! ( exc# ) \ restore prev return stack
R> HANDLER ! ( exc# ) \ restore prev handler
R> SWAP >R ( saved-sp ) \ exc# on return stack
SP! DROP R> ( exc# ) \ restore stack
\ Return to the caller of CATCH because return
\ stack is restored to the state that existed
\ when CATCH began execution