Athlon 64 X2: New Memory Dividers and Multitasking Performance

AMD was faced with a tradeoff during the development of the dual core Athlon 64 X2. In order to maintain backwards compatibility with earlier Socket-939 motherboards, they could not change the pinout of their dual core processors. While maintaining the same pinout resulted in the ability to upgrade virtually any Socket-939 platform to a dual core Athlon 64 X2, it meant that the dual core processors were left with no more memory bandwidth than their single core counterparts. The single-core Socket-939 Athlon 64s feature a 128-bit wide DDR memory controller, which when operating at DDR400 speeds, it gives the A64 a maximum of 6.4GB/s of memory bandwidth. Sharing the same memory controller, the dual core Athlon 64 X2s also feature the same 6.4GB/s of memory bandwidth, despite the fact that there are now twice as many cores vying for the same amount of memory bandwidth.

Luckily for AMD, the single core Athlon 64 was not very memory bandwidth limited, and thus, the move to dual core still allowed AMD to scale relatively well. In fact, based on the results that we saw in our Athlon 64 X2 3800+ review, AMD continues to consistently scale better from one to two cores than Intel, despite the reduction in memory bandwidth per core.

Meanwhile, AMD quietly introduced a handful of new memory dividers in the latest revisions of their Athlon 64 and Athlon 64 X2 processors. These new memory dividers allow for memory clock speeds above DDR400 to be enabled without overclocking the Hyper Transport bus. The beauty of these new memory dividers is that owners of faster-than-DDR400 memory can take advantage of the extra bandwidth offered by their modules, without overclocking their CPUs or the rest of their system.

Last month, we took a look at the performance benefit, or honestly, the lack thereof with using higher bandwidth memory and Athlon 64/X2 processors. For the most part, we saw a 0 - 3% improvement in real world performance, with the vast majority of benchmarks showing us a 0 or 1% increase in performance, thanks to the higher bandwidth memory. There were some isolated cases where having more memory bandwidth translated into higher performance, in particular things like video encoding, gaming and heavy multitasking environments, but for the most part, the performance gains were negligible.

The performance gains in video encoding and gaming were to be expected, and we theorized that there would be some significant gains in multitasking environments. In a multitasking environment, particularly with an Athlon 64 X2, the overall memory bandwidth requirements of the two combined cores should be at their peak, well above and beyond the demands of a single-core Athlon 64. We saw this in our original article where one of our heavier multitasking tests yielded a 6.5% increase in performance when using DDR480 with an Athlon 64 X2 4800+. At the same time, some of our lighter multitasking tests yielded absolutely no performance increase when paired with higher bandwidth DDR memory. So, the point of this article is to find out if multitasking Athlon 64 X2 owners can benefit any more than single-core users from employing these new faster-than-DDR400 memory speeds.

Given the very specific nature of this article, we’re only going to be focusing on one processor - the Athlon 64 X2 4800+. As we found in our last piece, slower X2s weren’t impacted any differently than the fastest of the bunch, so anything we find here should be just as applicable in the real world to all other X2 processors.

We also only focused on two memory speeds: the base DDR400 and the fastest possible setting on the 4800+, DDR480. The details of how to select these speeds and the hardware we used to do so can be found in our first article .