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DDR-2 Memory Investigation Guide - Making sense of memory settings - The Latency Issue

By: Cameron Johnson | DDR-2 Memory in RAM | Posted: Apr 21, 2005 4:00 am

The Latency Issue - Timings vs. Bandwidth


Now we get to what has caused all the uproar with DDR-2 - higher latencies. DDR1 memory modules are well ahead of DDR-2 in terms of latency. DDR operates at 2.0ns, 2.5ns and with super high speed modules 3ns. 2ns are the modules of choice for AMD Athlon users as memory latency really takes a huge wack out of the AMD CPU's however; Intel doesn't follow this general rule. DDR-2 with the double buffer rate has pushed latencies (otherwise known as timings) to 3ns, 4ns and even 5ns. AMD Athlon 64 with its on-die memory controller uses a much faster crossbar to access the memory modules which means that the faster the modules can synchronise with the controller, the better the performance.


Due to the traditional FSB nature of the Intel Pentium 4 and the Netburst architecture's higher bandwidth capabilities, latency doesn't play a huge role, as the latency of the FSB alignment is already there, memory modules that can run faster than this alignment simply don't help.


The large pipeline of the Intel Pentium 4 requires a huge amount of data per clock cycle in order to keep the Pentium 4 from stalling; this means that the memory has to have a lot of throughput in order to keep the data path happy. Pentium 4 uses an 800MHz FSB for its Pentium 4 with a 1066FSB for the faster Extreme Edition CPU. This means that an 800 FSB P4 can chew up to 6.4GB/s and an Extreme Edition up to 8.5GB/s. This simply is beyond what the DDR standard can produce; however, DDR-2 can do this. DDR-2 running at higher than CPU clock speeds is also possible with Asynchronous bus speeds.


Traditionally this has led to latency problems, however, with the nature of DDR-2 and the already higher than traditional latencies, Asynchronous DDR-2 on paper actually would benefit the Pentium 4 previously than before.


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