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Closer Look at AMD Phenom II X4 810 CPU

Now that we know what to expect from the Socket AM3, it seems to really hard to surprise us with anything about a processor designed in this form-factor. However, it is not quite the case. Although new Phenom II processors are overall not very different from those launched by AMD a month ago, the Phenom II X4 810 model we received for our tests revealed a few unexpected characteristics.

First of all I have to say that this CPU got the 800-series model number for a reason. AMD uses these lower numbers to mark the CPUs with limited characteristics. In this case it was L3 cache memory: Phenom II X4 810 has only 4MB L3 cache, while the “fully-fledged” Phenom II processors have 6MB L3 cache.

In fact, it is quite logical that there appeared Phenom II processors with smaller L3 cache, as well as disabled cores. Although they use 45nm process to make monolithic Deneb dies, it is still pretty big in size – it measures 258sq.mm. Just for your reference: it is only a little smaller than the Intel Core i7 die, which means that both these processors cost about the same to make. However, if we compare the retail prices on both, it will evidently be not in Phenom’s favor, which means that Phenom II manufacturing is a considerably less profitable enterprise than Core i7 production. And keeping in mind that AMD doesn’t have any dies yet that could compete successfully against the best Intel solutions, it is absolutely understandable that they are trying to use all resources to get as much profit as possible. One of the ways to do it is to sell CPUs made of partially defective dies that for some reason couldn’t be used for Phenom II 900 series solutions.

In fact, Phenom II X4 810 is a great example of how this tactics works. This CPU is based on the same Deneb semiconductor die as Phenom II 900 series processors. However, one third of its L3 cache is disabled. This way AMD can put the dies with some defect in the L3 cache part to good use. If the defect is in the part with computational cores, these dies are suede for triple-core Phenom II 700 series processors that are also launching today.

The specifications of Phenom II X4 810 L3 cache memory look pretty strange:

If we believe the report from the diagnostic utility, this processor’s L3 cache has 64-way associativity, while 6MB L3 cache of normal Phenom II X4 900 had only 48-way associativity. The most logical explanation for this phenomenon is that there is some mistake in the CPU-Z report and the L3 cache of our Phenom II X4 810 processor actually has 32-way associativity. Otherwise, the L3 cache of 800 series processors should have higher latency than the top processor models, which is not what we see in reality.

However, L3 cache of Socket AM3 Phenom II CPUs is still faster than by their Socket AM2+ counterparts. However, the reasons are not buried deep in the microarchitecture: they are right there on the surface. The thing is that AMD has set higher frequency for the North Bridge integrated into their Socket AM3 processors that is used to clock the L3 cache-memory. L3 cache in Phenom II X4 810 works at 2.0GHz, just like in other CPUs for the new platform, while the L3 cache of their predecessors worked at 200MHz lower frequency.

As you can see from the screenshot above, this is also true for a Socket AM3 processor installed into a Socket AM2+ mainboard.

Despite the differences between our today’s Socket AM3 hero and its Socket AM2+ predecessors that we have already tested about a month ago, it is very hard to hide the fact that they are blood relatives. For example, Phenom II X4 810 uses the same C2 processor stepping that we saw in Phenom II X4 940 and 920 CPUs a month ago. And it means that the semiconductor dies used for Socket AM2+ and Socket AM3 Phenom II modifications are the same and the supported memory types are determined only when the CPU gets packaged.

Influence of L3 Cache Memory Size on Performance

The first question that comes to mind when you look at the Phenom II X4 810 specifications, is how the smaller L3 cache will tell on the performance. In search for a definite answer to this question we decided to compare the performance of a Phenom II X4 810 and Phenom II X4 910 processors. Both these CPUs are based on a 45nm Deneb core, work at the same 2.6GHz clock speed and differ only by the size of their L3 cache that also works at the same 2.0GHz frequency in both cases.

The results of our test session show that cutting off one third of the L3 cache doesn’t slow Phenom II X4 down significantly. Phenom II X4 810 lost 2% on average to its “fully-functional” counterpart. Even in the worst case scenario the performance difference never exceeded 5%.

So, it is quite logical that Phenom II X4 810 costs only $20 less than Phenom II X4 920. These two processors evidently don’t differ dramatically in their practical performance. And the major drawback of the junior model is not the smaller L3 cache, but lower clock frequency.

By the way, don’t forget that L3 cache memory of Phenom II X4 810 processor works at 200MHz higher frequency than the L3 cache of the top Phenom II X4 940 and 920 models. And it can be regarded as a compensation for smaller cache. As we have found out before, 200MHz frequency increase of the North Bridge built into the processor raises its performance by about 1.5%.

 
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