Near-term Athlon and Coppermine Overclocking

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I wrote the initial Athlon article after one of you sent me a statement from AMD with some ambiguous language. Lawyers love ambiguity, and I was afraid some overzealous legal beagle could read that and get the idea that it was OK to play overclocking Nazi to those who advocated overclocking or those who actually did it. There’s a big difference between saying “you voided your warranty” and “you violated our trademark, or encouraged others to do so.”

In a world where we hear comments like “I did not have sexual relations with that woman,” you want to get matters straight right away. So I wrote the article, and Joe Citarella sent a note to AMD, who, to their credit, made it immediately clear that they were not interested in individual overclocking. So don’t send AMD notes telling them they are tramping on you, because they’ve said they wouldn’t.

We now have some folks telling you that this is nothing at all, that you can just use Golden Fingers to do all the overclocking you want. Wrong and I’m going to tell you why.

An Athlon’s L2 memory setup is just like an Intel PII/Katmai PIII setup. You have separate memory chips rated at a certain speed. Run them more than 10-15% above that speed, and they usually stop working and so do you.

The initial Athlons, like the PIIs and Katmai PIIIs were set to run memory at half the speed of the processor. They often came with cache chips that were a good deal faster than the rated speed of the chip called for (just as the later PIII 450s came with faster cache than it needed to run at 450Mhz).

Unlike the PII/Katmais, though, the fully modified Athlons let you change the cache ratio, so if a chip didn’t run at 750Mhz with L2 at half the CPU speed, you just set the cache ratio to 1/3, and went on your way. That’s why you heard of Athlons getting to 800 or even 900Mhz. If they came with 3.1ns L2 cache, you could set the ratio to 1/3, and the cache memory wouldn’t be overtaxed until it got up close to 900Mhz (at which point massive heat would be stopping most people anyway).

The newest Athlons are not set to run at fifty percent of the CPU speed; they are set to run at forty percent. This tells me AMD is going to be putting L2 cache memory a lot closer to the rated speed of the processor than was the case with the initial Athlons.

No doubt this is due to expense; I would bet about 3ns cache RAM doesn’t cost more than slower cache RAM nowadays, but is a lot cheaper than 2.5 or 2ns cache RAM. So it looks like you’ll be using the same (or a little faster) L2 cache on the newest Athlons that you had in the old Athlons. This means you can’t expect much more from the newest Athlons than you are seeing from the old Athlons so long as the memory chips are about the same.

This little chart should demonstrate the most you can expect under normal conditions (no extreme cooling or overvolting) Per the old Athlons, 3.1ns cache was about the fastest AMD ever put into the CPUs, some Athlons came with cache as slow as 4.0ns. If you are thinking about buying a .25 micron Athlon now, you get one of those, and you can’t change the cache ratio, you aren’t overclocking very much.

Memory Bottleneck due to Overclocking*

Athlon Processor L2 Cache Speed L2 @ ½ Ratio – MHz L2 @ 2/5 Ratio – MHz L2 @ 1/3 Ratio – MHz
0.25 Micron 3.1 ns 700-750 850-900 950+
750 MHz 3.1 ns NA 850-900 950+
850 MHz 2.8 ns NA 950-1000 1050
1 GHz 2.5 ns NA 1100-1150 1300+

*Not considering heat and other issues.

Obviously the speed of the L2 cache makes a big difference in the maximum speed of the chip. I suspect this is the reason why AMD is in no rush to get to 1Ghz; they have to use 2.5ns chips just to get to the unoverclocked speed reliably. Notice also you aren’t seeing any big overclocks either, especially with the slower cache you are bound to see any time soon.

Also notice that if you can’t change the cache ratio, you are limited to the “L2 at 2/5 ratio” rather than the “L2 at 1/3 ratio.” Now if you can change the cache ratio with your motherboard’s BIOS, then this is not a problem. If you don’t mind using software to change it, then this is not a problem.

If someone comes out with a Golden Finger that can change the cache ratio (and I haven’t heard of one yet), it is not a problem. However, if you want to crank one of these new Athlons up any time soon, and you can’t or won’t do any of the above, then you have a problem. If on top of that AMD is putting 3.1 ns chips into the Athlon-750s, then you have a real problem.

A few months down the road, you may well see AMD using 2.5ns caches on all its chips, and motherboards will all have cache divisors in the BIOS, and a cheap 750 or 800Mhz Athlon may be a good choice for 1Ghz+ overclocking. Not until then, though.

I would be amiss if I didn’t compare and update the Coppermine situation.

Coppermines (and Celerons) have a different memory arrangement. (Future Athlons will have the same). They have ondie cache RAM designed to run at the same speed as the processor. Due to how people make CPUs, Intel tries to design a chip that should ideally run as fast as the fastest member of that family should run. Intel has announced the Coppermine will get to at least 933Mhz, so that’s a rough benchmark as to how fast these chips should be able to run eventually.

Right now, the fastest unoverclocked Coppermine can run at 800Mhz, and the overclocked chips aren’t doing much more above that. It looks like about 840Mhz is the most you can expect without extreme measures. So don’t assume that just because a 500E gets to 750Mhz or a 550E gets to 825Mhz pretty easily, that a 600E or 650E will get to 900 or 975Mhz easily.

Initial indications are that they will not, at least not at the moment, under normal circumstances (i.e. good air cooling, no peltiers or Kryotechs). Eventually, I think they will, just as the Celeron 366 didn’t usually hit 550Mhz initially, but did later on. Intel is still in the cA2 stepping at which it introduced the products. If you really want 900Mhz or better, wait until Intel revises the chip.

I’m sure some of you are saying, “Doesn’t this guy like anything? All he tells us to do is wait!” Believe me, I’m just as frustrated as you are, and I’m just as on hold as you are. We’re just very early in the development cycle, and Intel has uncharacteristically been screwing up a lot with motherboards and CPU supplies while AMD, which is really performing heroics in implementing an incredibly frantic schedule, has left a gap for overclockers which will get filled in a few months.

If you can handle the motherboard and memory issues, you can get to 750-800Mhz now. If you don’t want to deal with that, you can wait and let Intel figure out how to make enough chips and good motherboards and introduce .18 micron Celerons. Or you can wait a few months more until AMD pushes the Athlon to 1Ghz and pushes the cache speed up and the price down on its Athlon 750s and see what the DDR SDRAM motherboards will do.

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