Re: Ada and embedded applications

[Robert A Duff <bobduff@world.std.com>]

John English <je@brighton.ac.uk> writes:

> Hmm, the "obvious" way to handle exceptions involves one extra word
> in the stack frame (a pointer to the start of the exception handling
> code) which costs (typically) 4 bytes per handled block and 1 extra
> word to pop on exit, which may or may not cost nothing, but is certainly
> pretty cheap.

But the more desirable implementation is to save the small cost of
setting that pointer, and spend perhaps 10,000 instructions searching a
table of code addresses when an exception is raised.  If exceptions
being raised are rare, but blocks protected by exception handlers are
more common, that's a win.

> Does anyone have any references to alternative ways of implementing
> exception handling?

One variant of the code-table technique was decribed in the Ada 83
rationale.  The implementation you described above is essentially
equivalent to the VAX/VMS standard method.  I don't have other
references at hand, but surely a lot of this stuff is common lore, and
documented somewhere.

- Bob