Re: Multiply Overflow - when to use UMULH

Uncle George (gatgul@voicenet.com)
Sun, 24 Nov 1996 07:51:07 -0500

Uncle George wrote:

I try to mult two (possibly) 48 bit integers. Sometimes these nums will
overflow the 64 bit reg. I'd like to know if there is an easy way
to get at the overflow bit.
>From reading the asm manual, you have to wait for the pipeline to
clear, cause an exception, then get the status/overflow bit.

1) whis is faster ?
umul, then umulh ( forget the exception fault )
2) a) umul ( no exception to process )
b) umul -> exception -> do umulh ( only when truely needed )

I have no timeings for instruction sets. I have no timeings for
exception
faults.

I'd really like to test an overflow bit that did not involve an
exception, but that does not seem to be the case with alpha's ( maybe
all risc's )
gat

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