Messages in this thread | | | Date | Sun, 04 May 2008 14:07:24 -0700 | From | "H. Peter Anvin" <> | Subject | Re: i387/FPU init issues... |
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Arjan van de Ven wrote: >> >> It has retroactively been added to the documented list for all P6 core >> chips - that should mean it works on all of them. The most common >> reason for not documenting something (other than various Pure Evil NDA >> schemes) is that it hasn't been properly verified. However, >> verification can be done a posteori. > > the other reason is that certain groups of "unknown" opcodes will act as > NOP. > Which for this purpose is .. exactly the right thing retroactive >
Specifically, I believe the P6 added a whole class of instructions which would act as NOP's if used on older chips. It has been used for prefetch instructions, etc. At some point, the 0F 1F /0 group was declared to be NOP, and nothing else, for all future.
-hpa
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