Original Link: http://www.anandtech.com/show/1362


Almost as soon as the 875/865 chipsets were introduced last year, we were dealing with news stories about PAT being implemented on the 865 chipset. This Intel "875-only" feature was quickly and cleverly being introduced on the mainstream 865 chipsets by the Taiwanese motherboard makers. This was not what Intel had intended, but it was a testament to the engineering capabilities of the major board manufacturers.

The just-introduced Intel 925X and 915 chipsets seem destined to continue the tradition of Intel trying to limit the chipset features. This round, the hot topic is Intel's attempt to limit or lock-out overclocking on the new chipsets, and the clever engineering that companies, like Asus and Abit, are doing to get around this new issue.

Our early testing confirms what you may have already heard -- Intel has limited the overclocking on their new chipset to about 10%. In looking at boards from smaller or less savvy board makers, you simply can't overclock beyond 10% over specification. We first became suspicious of a lock because some Prescott chips are already overclocking as much as 50% and higher on Intel 875P systems. Given that fact, why should these same cores suddenly be such poor overclockers on 925X/915? The answer, of course, is that the chipset is different and it handles overclocking differently.

When an attempt is made to boot at more than about 10% on a 925X or 915, the system simply reboots or shuts-down. We have not been able to get any kind of official explanation from Intel, but board makers tell us that Intel has added an overclocking limiter that resets a PLL and reboots or shuts down the system if overclock attempts are made at speeds over about 110% of specification.

So, are we stuck with 10% overclock limits if we choose the new Intel chipsets? Fortunately, some of the major manufacturers have found ways around this issue.

UPDATE: Intel's Reply. Intel declined to officially answer our question about whether they have implemented a 10% Overclock Lock on the 915 and 925X chipsets. Intel provided the following official statement:
"Intel has done extensive evaluation of our 915G/P and 925X chipset products and have designed them to run robustly at 800FSB. Any use or operation of these chipsets beyond their specifications, including overclocking, has not been evaluated or validated by Intel. If a board provider decides to overclock their platform, they do so at their own risk, since this action will void the warranty for the Intel products. Additionally, from time to time, Intel may choose to implement functionality that helps to ensure that the Intel product experience is not degraded by those who may try to run our products out of spec."

Asus Breaks the Lock . . .

Asus is one of the largest motherboard makers in the world, and they were also the company that first brought PAT-like features to the 865 chipset. It is probably apropos that Asus is also one of the companies that has apparently broken the Intel Overclock Lock. We say "apparently" because there are still questions after our tests as to whether the lock is completely bypassed by Asus or anyone else.

To complicate matters further, the rumor mill has it that the overclock lock only affects retail locked chips, and not the unlocked Engineering Samples that Intel supplied with their press evaluation kits. Fortunately, we have a retail 3.2 chip in addition to the 3.6 ES supplied by Intel. This allowed us to determine if this rumor has any merit, or if it is just speculation.

To test whether or not Asus has broken the lock, we benchmarked both the 3.2E and the 3.6 ES on the Asus P5AD2 Premium motherboard. The 3.2E was overclocked to the highest overclock that we could achieve adding no more than 0.1V to the stock 1.3875V voltage. We then found the highest overclock we could achieve with the 3.6 ES at the base speed of 3.6 (18x200). We also tested for the highest overclocks at lower multipliers to see if the overclock lock is truly bypassed.

 Asus P5AD2 (Intel 925X) - Highest Stable Overclock - Retail 3.2
 CPU  Multiplier  Voltage  Highest FSB  CPU Speed  % Overclock
3.2 Retail 16 1.425V 992 (248) 3968 24%

Asus certainly cracked the 10% overclock lock with their P5AD2. In fact, at first glance, it appears that taking a 3.2 to over 4Ghz is allowing the CPU to do all it is capable of doing with air cooling.

. . . or Do They?

However, further tests with the 3.6 Engineering Sample shed more light on the question.

Asus P5AD2 (Intel 925X) - Highest Stable Overclock - 3.6 ES
CPU Multiplier Voltage Highest FSB CPU Speed % Overclock
3.6 ES 14 Default 992 (248) 3472 24%
3.6 ES 15 Default 992 (248) 3720 24%
3.6 ES 16 Default 992 (248) 3968 24%
3.6 ES 17 1.425V 952 (238) 4046 19%
3.6 ES 18 1.425V 896 (224) 4032 12%

While the 3.6 ES was also able to reach around 4GHz when overclocked, this represents only a 12% overclock for this chip. It is not unusual for top-of-the-line processors to achieve a more modest overclock percentage than lower clocked CPUs that probably are off the same line. The advantage here is that the ES chip allows the setting of lower multipliers to determine the maximum overclock potential of this motherboard.

We should be able to achieve the same approximate 4GHz at lower multipliers - but with higher FSB. At a 14 multiplier, 288x14 would give us about the same CPU speed as 18x224. However, we find that the highest FSB that we can achieve is the same 248 maximum we found on the 3.2GHz. In fact, whatever the multiplier, 248 was the highest FSB setting that we could achieve.

While we can't be certain whether 248 is the limit of the Asus P5AD2 motherboard or an artificial limit of the chipset, we suspect that we are dealing with chipset limitations. It appears that Asus has effectively bypassed the Intel Overclock Lock, but the design of the lock cannot be completely bypassed. Even a clever design like the Asus still seems to be overclocking limited to around 25% above rated speed.

Our Take

There certainly appears to be a built-in overclock lock with the new 925X and 915 Intel chipsets. The design limit appears to be around 10% over the rated speed of the CPU. Asus has found a way around this artificial limit, but it appears that they have not been able to completely bypass the Overclock Lock, since overclocks on the Asus still seem limited to about 24%.

Others are also reported to have solutions that bypass the Intel OC lock. We are told Abit, Gigabyte, and MSI all have designs which bypass the OC lock. We do not yet know if these designs are also limited to overclocks in the 20% to 25% range or if these designs have truly broken the OC lock. As we review the new motherboards, we will report more on the effectiveness of the designs for overclocking.

There are certainly readers who will wonder if we have rocks in our heads for complaining about a 24% overclock, or even a 10% overclock. We would only say that it is a matter of perspective. We have easily reached 3.6GHz with a Prescott 2.4 CPU on an Intel 875 chipset, an overclock of 50%. By comparison, 24% seems a very low overclock limit. We also know that the 875P does not limit overclocks as Intel has attempted to do on the new 925X and 915 chipsets. In fact, we have recently heard of new motherboards from overclocking-savvy companies that will combine Socket T (775) with the Intel 875P chipset. The only reason for this marriage would be massive overclocking. Combine fast DDR memory, the new Prescott 775 chips, and an 875 chipset without an artificial Overclock limit, and you will achieve record-breaking overclocks.

There is no doubt that OEMs will not likely care if overclocks are limited to 10% on the new 925X and 915 chipsets, and many of our readers won't care at all. However, the enthusiasts who do care about overclocking and who want to buy Intel will need to be especially careful to find a motherboard that can bypass the limits of the Intel Overclock Lock. For some overclockers out there, this may be the push that persuades them to buy an Athlon 64 for this round.

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