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New Low-Latency DDR2

So, the weakest aspect of the DDR2 technology is high latency which led to DDR2-based systems having a smaller performance compared to DDR-based ones. And this was the problem the memory manufacturers interested in promoting DDR2 SDRAM in the market have been trying to solve. And they have made some success!

Early DDR2-533 SDRAM modules available at the time of the announcement of i925 and i915 chipsets had 4-4-4 timings (CAS Latency - RAS to CAS Delay - RAS Precharge Time). Today, however, many memory manufacturers, especially the developers of advanced modules for PC enthusiasts like Corsair or OCZ, are offering DDR2 SDRAM capable of working at 533MHz with 3-3-3 timings. These modules are not overclocked, but fully comply with JEDEC’s appropriate standards (the official specification describes a DDR2-533 modification with 3-3-3 timings and a voltage increased to 1.9v).

Intel also approved of DDR2-533 SDRAM with 3-3-3 timings in LGA775 systems. The company has officially confirmed its new chipsets support such memory and even specifically emphasized that such memory would be the best choice for enthusiastic users. Intel’s reasoning is easy to grasp: theoretically, the reduction of the CAS latency of DDR2-533 to 3 cycles is a significant improvement of the parameters of DDR2 SDRAM:

Memory

Timings

Latency

Bandwidth in dual-channel mode

DDR400 SDRAM

2.5–3–3

12.5 ns

6.4 GB/sec

DDR400 SDRAM

2–3–2

10 ns

6.4 GB/sec

DDR533 SDRAM

3–4–4

11.2 ns

8.5 GB/sec

DDR533 SDRAM

2.5–3–3

9.4 ns

8.5 GB/sec

DDR2-533 SDRAM

5–5–5

18.8 ns

8.5 GB/sec

DDR2-533 SDRAM

4–4–4

15 ns

8.5 GB/sec

DDR2-533 SDRAM

3–3–3

11.2 ns

8.5 GB/sec

DDR2-600 SDRAM

5–5–5

16.6 ns

9.6 GB/sec

DDR2-600 SDRAM

4–4–4

13.3 ns

9.6 GB/sec

You can see that DDR2-533 SDRAM with 4-4-4 timings has a half worse latency compared to the widespread DDR400 SDRAM; the 30-percent growth of the bandwidth will hardly compensate such a worsening of the access time. The latency of DDR2-533 improves considerably at 3-3-3 timings, however, and is only 12 percent worse than the latency of 2-3-2 DDR400 SDRAM. As the result, considering that DDR2-533 SDRAM has a higher bandwidth, we can expect DDR2-based systems with reduced timings to be as fast as DDR1-based platforms.

We want to add that the 3-3-3 timings formula is not the evolution end of DDR2-533. For example, OCZ has started to output DDR2 memory modules with even more aggressive timings: the new product, PC2 4200 Enhanced Bandwidth Platinum, has a timings formula of 3-2-2! LGA775 platforms equipped with such memory are going to offer tough competition to platforms with ordinary DDR SDRAM.

Besides that, mainboard makers have also moved one step closer towards a faster LGA775 as many i915/i925-based mainboards for overclockers from ASUS, ABIT and other companies allow to clock DDR2 memory at 600MHz instead of 533MHz. This capability is realized through the use of undocumented memory frequency ratios available in i925/i915 chipsets. Although there’s no DDR2-600 standard ratified, this memory mode is going to increase the memory subsystem bandwidth as well as to reduce its latency a little compared to DDR2-533 with 4-4-4 timings.

Since the manufacturers of memory for enthusiasts are offering not only DDR2-533 memory with low timings, but also DDR2-667 modules capable of working at 600MHz and 667MHz, the use of 600MHz memory in LGA775 systems may be of some practical interest.

In this review we are going to estimate the speed of LGA775 systems equipped with a faster memory than we used in our earlier tests (DDR2-533 SDRAM with 4-4-4 timings). The main question we’re going to answer is if the more progressive DDR2 SDRAM can lift the performance level of i915/i925-based systems to that of i875/i865-based computers. We will also examine some particular DDR2 SDRAM modules which helped us to carry our tests through.

 
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