X-bit labs

Fastest Pentium 4 Platform: Performance of i925, i915, i875 and i865 with DDR2-533, DDR2-400, DDR533 and DDR400 SDRAM (page 2)

Category: Chipsets

by Ilya Gavrichenkov

[ 08/04/2004 | 12:38 PM ]

Comparison of Intel’s Modern Chipsets Specs

Let’s look for any differences in the formal characteristics of the investigated chipsets:

So, without taking the different packaging and positioning of the chipsets’ North and South Bridges into consideration, the new i9xx series has the following distinguishing traits:

• Supports the new graphics interface PCI Express x16;
• Supports DDR2 SDRAM (the i915 is also compatible with DDR SDRAM);
• Supports the peripheral PCI Express x1 bus;
• Offers enhanced opportunities in terms of attaching SerialATA devices and uniting them into RAID arrays.

We already discussed the characteristics of the new chipsets, with emphasis on the i925, in our article called LGA775: New CPUs and Chipsets . Today I’m going to dwell on the i915 that targets the mass market.

Intel 915P Express Chipset

The i915P Express is positioned by Intel as a mass-market discrete chipset with support of PCI Express and DDR2 SDRAM. Thus, it costs much less than the top-end i925X Express – we saw a similar difference between the last-generation chipsets (the top-end i875P and the mainstream i865PE). What features will be unavailable for the users of i915P-based mainboards?

The i915P Express works with the memory slower than its top-end mate. As you know, the i875P featured Performance Acceleration Technology (PAT) that improved the performance of its memory controller, and the i925X also has a number of optimizations, missing in the cheaper i915P chipset.

The architecture of the Intel 925X Express chipset allows for a higher speed due to an improved pipelined scheme of working with the memory and a more efficient use of each memory channel. This technique is different compared to PAT. Instead of optimizing the data path from the chipset to the memory and back, Intel chose another strategy. Particularly, the i925X places memory regeneration (maintenance) commands into the standard data stream and regroups the data in the memory to optimize the access.

You may remember that mainboard manufacturers used to “outsmart” Intel and enable PAT in their i865-based products, but now this trick seems impossible. At least at the moment i915P-based mainboards are always slower than their i925X-based analogs under any conditions, while the manufacturers themselves deny the possibility of improving their i915P products to the level of the i925X ones.

Besides the higher speed, the memory controller of the i925X chipset boasts ECC support – the i915P doesn’t. However, this is no great loss. Firstly, ECC is only needed in servers and high-performance workstations and, secondly, an error in the current revision of the i925X leaves ECC only as a formal capability, which you cannot enable in practice yet.

I have been talking about the features that the memory controller of the i915P lacks, but it has something that the i925X has not – compatibility with ordinary DDR SDRAM. Well, it doesn’t mean that i915-based mainboards can work with DDR2 and DDR SDRAM simultaneously. The i915 series chipsets use the same group of pins for communication with both types of memory, so the mainboard can work with only one memory type at a time. Intel warns the manufacturers against developing mainboards with DIMM slots for both memory types – this may cause stability-related problems. That is, the manufacturers are offered to choose a particular memory type for each particular mainboard. Well, you couldn’t have expected anything more from Intel – the company’s support of the older memory type is already a nice display of care about the users.

Then, I should note that the memory controller of Intel’s new chipsets gives more configuration flexibility than the one in the i875/i865 series. As a result, i915P-based mainboards allow you to enable dual-channel memory access (both with DDR SDRAM and DDR2) more often i865-based ones.

Thus, thanks to the realization of Flex Memory Technology, the most advantageous symmetrical dual-channel memory access mode can be used in more cases than with the last-generation chipsets, which required the use of the same number of same-organization modules in each channel. Now, the i925/i915P chipsets only require that you install the same amount of memory into each channel. That is, even if you use one memory module in one channel and two modules in another, you can still enable the symmetrical dual-channel configuration. However, if the amount of memory per channel is different, the performance goes down to the level of single-channel chipsets – keep this fact in mind!

There are no other differences between the i925 and the i915 except those in the memory controller. However, this alone was enough to make the two North Bridges pin-incompatible, although they use the same packaging.