Author: Sune Fischer
Date: 03:18:41 02/01/02
Go up one level in this thread
On February 01, 2002 at 05:54:20, David Rasmussen wrote: >On January 31, 2002 at 15:25:55, Severi Salminen wrote: > >>Hi! >> >>This is what I got from intel about bsf/bsr instructions. I asked this: "In what >>situations bsf/bsr eax,VARIABLE changes the contents of eax, if VARIABLE is 0?" >> >>This is what they say: >> >>"Although the value appears unchanged in some cases, the definition of this >>instruction applies to all IA-32 processors starting with the Intel 386. That >>stated, the BSR and BSF instructions both work as defined on all processors >>previously noted. So even though it might be deterministic in a specific >>implementation, no information will be provided on special cases." >> >>So practically I got no info at all. Still it is just speculation to try to >>guess if there are systems where eax might be changed. In Requiem I make a small >>test (which of course can't cover all situations) to see if bsf/bsr changes eax >>in above situation and exit if this is that case. >> >>Severi > >If you ever encounter a system where eax _is_ changed, your program will surely >crash very quickly, so you will know instantly. Why don't you just assume what >everybody else is assuming, and then if you ever experience yourself, or get a >user report that the program crashes instantly and deterministically, you can >deal with it. It will never happen :) > >Alternatively, you could check at the beginning of your program, whether it >works or not. If it doesn't, you could write a very nice error message and quit. >That way, it will be 100% clear what have happened. Again, it will never happen, >but if you want to feel safe. > >What will not happen, is that you encounter a system where it doesn't work, and >then your program just plays worse, without you knowing. > >/David I would go for the small test as part of the initialization :) Reason is, that 5 years from now, the chips may change and he will have forgotten all abut this little problem, and then he has 100k lines to debug :( -S.
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