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To: Ali Chen who wrote (72796)	3/1/2002 3:59:37 AM
From: Petz	Respond to of 275872

Ali (answering for Gopher's "...does not mean that there were no hyperthreading-related optimisations that were enabled in the Northwood core" Which optimizations ??? I think it is either "enable" or "disable" the feature. I think we are referring to things like allowing execution of both targets of a conditional branch or dynamically creating very small "threads" in branch-less code. Doing this requires hardware resources in the CPU like additional registers which are similar to running simultaneous threads known to the operating system, so optimizations like these may be considered part of "Hyperthreading" according to Intel's definition. However they may already be implemented in the Northwood core. Petz

To: Ali Chen who wrote (72796)	3/1/2002 4:25:13 AM
From: Gopher Broke	Respond to of 275872

What do mean by "how much HT was enabled", and what the "old core" means? By old core perhaps I should have said pre-Prestonia versions of the P4 core, where hyperthreading was present but not fully activated. (It seems like Prestonia is more that just a "stepping" but I don't know the correct term.) What I meant by "how much was enabled" is that while the flag in the BIOS might be a boolean, that does not mean that the flag in the core is a boolean. For example, the BIOS might provide a flag to disabling SSE, but presumably the core actually supports selective disabling of individual SSE instructions. Hyperthreading is a huge function and I would guess that there are many flags to enable/disable selected portions of it. So, activating full hyperthreading might involve activating core functions A, B and C. Disabling hyperthreading through the BIOS might turn off all those functions. The point is that disabling function C alone would also give you a processor with hyperthreading disabled, but by leaving A and B active you might still get a performance boost. Who can say that Northwood does not still have some hyperthreading functions enabled that would also be disabled by setting hyperthreading off in the bios?