I've been reading some of the reviews of this new P4 core, and unfortunately it looks to be overall a bit slower than the Northwood 'C' type P4s are. It also seems to be a very hot (temperature wise
) CPU. Even though the Prescott doesn't seem to really offer much (if anything) over Northwood, I did notice some very interesting things about it. Take this with a grain of salt, as these are only my opinions. I don't have any hard evidence to back any of these predictions up, but I figured some of you would be interested. (Besides, if I make a prediction that turns out to be right in the future I can pull this page up and say "I told you so."
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I was able to find a die photo (which is a picture of the actual layout of the chip itself) of Prescott, and comparing it to Northwood I've come to the conclusion (and this is my opinion only, so I may be wrong) that Prescott actually contains 2 seperate sets of execution units! In otherwords, it looks like Prescott has, in essence, two CPUs sharing some components (Level 2 cache mainly) on one die. The trace cache, the P4s equiv. of the conventional Level 1 instruction cache, also seems bigger than it should be given what Intel has published for Prescott.
Why is this interesting? Mainly because if I'm right, Intel may have 64 bit capabilities that are disabled but present in the Prescott. It looks to me that, if I'm right, they have acheived this essentially by gluing 2 slightly modified P4s together. In the past other CPUs, mainly microcontrollers, did something similar in what was called a "bit slice" design. Essentially what that means is that you have 2 32 bit CPUs, one processing the least significant 32 bits, and the other processing the most significant 32 bits. (In other words, the lower 32 bits on one core, the upper 32 bits on the other.) This would be an interesting approach for Intel to add 64 bit (like AMDs X86-64) support to the P4 design. The fact that Intel has not publically confirmed the existance of 64 bit support in Prescott, even though it is certainly disabled for now, would seem to indicate to me that it is there mainly as a sort of insurance policy against the success of AMD's Athlon 64 CPUs. More likely Intel doesn't want to admit it, because IMO, that would probably severely impact the sales of their IA-64 Itanium line in the high end server market. (Itanium definately doesn't need to be selling any slower than it already is. I won't get into the whole "Itanic" joke...
) I'm wondering if Intel is concerned with the success of AMD's Opteron in the server market and wants to have the ability to enable a sort of AMD x86-64 compatibility in their future Prescott based Xeons to avoid losing face to AMD in the server market. I doubt they enable 64 bit support for the standard Pentium line in the next 6 months or so, as isn't hasn't been too long since they brushed off AMD's 64 bit Athlons as unnecessary. Enabling 64 bit support now would seem to be to be an admission of an error on their part in that regard.
Some other evidence I see of possible 64 bit support in Prescott is that its transistor count is really about double what it should be, as compared to Northwood. The fact that the pipeline was extended from 20 to 31 stages, which is one of the reasons why it is slower than Northwood - even with the doubled L2 cache, may be that some of the extra stages are necessary for proper scheduling of instructions to make best use of both cores if or when 64 bit support is enabled. (The other reason why it is slower is that even though the L2 cache is larger, it is also much slower.)
Obviously this is all speculation on my part at this point until Intel actually publically admits something to either confirm or deny the existance of 64 bit support in Prescott, and at least for now that doesn't change the fact that Prescott in its current form is somewhat disappointing to those who were hoping for better performance than Northwood.
(Interestingly, I remember a discussion I had when I was still at Oregon State that the original design of went on to become the Pentium 4 was a lot different than what ended up being released as Willamette. Willamette, and even Northwood, seemed to have some serious compromises made to the original design plans for the P4, probably to save in manufacturing costs. Prescott seems a LOT closer to what the original P4 design. If I recall correctly, the original design for what became the Pentium 4 was started as a 64 bit CPU, but that was cancelled when Itanium was released.)
