How are exceptions actually implemented in assembly?

[Aahz Maruch]

In article <mailman.1011809616.25217.python-list at python.org>,
hungjunglu <hungjunglu at yahoo.com> wrote:
>--- In python-list at y..., Steven Majewski <sdm7g at V...> wrote:
>>
>> C doesn't really have exceptions in the language, but the equivalent
>> functionality is usually coded in C using setjmp & longjmp (which 
>are
>
>Thanks to all that have replied. I was more looking into really low 
>level implementation (i.e., single CPU instruction, one level further 
>down from setjump/longjmp. That is, all I wanted to know was whether 
>it was an interrupt or a jump), because all of a sudden I realized 
>that exception handling can have an appreciable cost in resources, 
>when routines call routines that call routines, etc. Python exception 
>handling is at much higher level, and it charges this cost at that 
>level. Not too bad, in a sense, since the underlying C code does not 
>necessarily have to propagate the exception handling all the way down 
>to assembly. But, it also means that at Python level, exception is 
>built into everywhere. That's OK, I guess... compared to other things 
>(e.g: variable name look up), exception handling is probably pretty 
>cheap in resources/performance.

Note carefully that the cost comes almost entirely during the handling
of exceptions, not in setting up exception blocks.  So if as a
percentage of instructions executing exceptions are rare, it's a very
small penalty.
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