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VIA Teams Up Pentium 4 With DDR-Memory

Pentium 4 Chipsets, Continued

Intel's upcoming 'low-cost' solution for Pentium 4, the i845-chipset, will initially only support PC133 SDRAM, which cuts its available memory bandwidth down to a mere 1033 MB/s. We have shown in a previous article that this low memory performance slows down Pentium 4 quite badly. By the end of this year or at the beginning of 2002 Intel will also allow i845 with DDR-SDRAM. This will double its memory bandwidth and should finally enable a reasonable performance of i845 platforms. The official reason for this delay are validation issues, as Intel seems rather displeased with the current chaos in the DDR-memory market, which is missing tight specifications that would ensure reliable operation of each DDR-SDRAM module (Intel: "Neither AMD nor VIA tried to do anything about it"). There's also still the old Rambus conspiracy theory, which claims that Intel is unable to supply i845 with DDR-support because of license agreements with our friend Rambus Inc.

VIA's P4X266 supports any kind of SDRAM or DDR-SDRAM. Once teamed up with PC2100 DDR-SDRAM, the available memory bandwidth is with 2133 MB/s still lower than what Intel's 850/dual-channel RDRAM solution is able to offer. It is rather questionable though, if the mere memory bandwidth number is indeed able to tell the whole story. Most applications require low latency as well, and that is where RDRAM hasn't been able to outdo SDRAM or DDR-SDRAM solutions so far.

Here's a table that shows the memory bandwidth numbers of the different memory types.

Speed GradeMemory TypeClockClock Edges UsedChannelsBus WidthPeak Bandwidth (clock x edges x bus width x channels)
PC100SDRAM100 MHz1Single64 bit800 MB/s
PC600RDRAM266 MHz2Single16 bit1066 MB/s
PC133SDRAM133 MHz1Single64 bit1066 MB/s
PC150SDRAM150 MHz1Single64 bit1200 MB/s
PC166SDRAM166 MHz1Single64 bit1333 MB/s
PC800RDRAM400 MHz2Single16 bit1600 MB/s
PC1600DDR SDRAM100 MHz2Single64 bit1600 MB/s
PC600RDRAM266 MHz2Dual2 x 16 bit2133 MB/s
PC2100DDR SDRAM133 MHz2Single64 bit2133 MB/s
PC800RDRAM400 MHz2Dual2 x 16 bit3200 MB/s
PC1600DDR-SDRAM100 MHz2Dual2 x 64 bit3200 MB/s
PC2100DDR-SDRAM133 MHz2Dual2 x 64 bit4266 MB/s

The table shows quite nicely, that of all single-channel memories, PC2100 DDR-SDRAM is the one with the highest bandwidth. However, we should not forget that one SDRAM or DDR-SDRAM channel requires 64 data lines, while RDRAM only requires 16. That is why a dual-channel Rambus solution requires 32 lines only, which is easier to implement, as long as there aren't any problems to deal with the very high memory clock of 400 MHz. The dual-channel DDR-SDRAM solutions listed at the bottom of the chart are currently only realistic with NVIDIA's nForce chipset. The nForce chipset as well the nForce reference boards show that dual-channel DDR-SDRAM is indeed doable. A dual-channel PC2100 DDR-SDRAM solution beats the bandwidth numbers of all other memory configurations.