Questions about DDR RAM and the FSB

[Croz]

Hey ~
I've heard a lot of different things about how important the FSB is, and how important it is for your memory to match your FSB in speed.

Now, I just wanted to know a couple of things:

1. I heard that if you DDR speed matches your FSB original speed (before the multipliers) then THAT is the speed you are looking for, i.e. - my FSB is 100mhz with a 4x multiplier, and my ram is DDR PC2100 (or 167mhz before the 2x multiplier) does that mean my system performance should be up to par?

2. A good website or other place I can get this kind of nitty-gritty tech info from...

Thanks,
Croz

 

[o1die]

Pc2100 is designed to run with the regular athlon xp or a 533 p4. My board has a 266 setting (for pc2100) which enables me to run it with an 800 p4, but at that setting, the memory would be slowing the system down somewhat. It would still work, but performance would be compromised. It's better to use the lower memory setting with pc3200 (or higher) to permit higher cpu fsb settings. I'm currently running at 260 fsb (1040 net with a "c" series p4) on the cpu, with 333 setting for the memory (pc3500). The net memory speed is about 413, which is within the range of my memory. Some boards use numerical ratios (1:1, 5:4, etc) instead of numbers for the memory settings.

 

[Crashman]

In dealing with the P4 you have a problem: The CPU has a Quad Data Rate bus (the 4x you mentioned), while your RAM is Double Data Rate (the 2x you mentioned).

The memory controller functions best with both at the same speed. But the CPU functions best with both at the same BANDWIDTH. PC1600 (DDR200) would be a clock-for-clock match to the 400 bus P4, but with 1/2 the bandwidth. That's why modern boards use Dual Channel configuration, with the modules paired up to make the bus twice as wide, and the bandwidth twice as high.

OK, going back to the old tech: The BEST single channel configuration for the P4 with 400 bus was actually DDR400 (PC3200) in single channel. Even though that's 200MHz RAM to 100MHz CPU bus, the bandwidth match is more important.

PC2100 is DDR266, runs at 133MHz. 133MHz still provides better bandwidth than 100MHz, but either are a handicap compared to today's tech. The P4 wants more bandwidth, which again is why today's boards offer dual-channel.

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[jammydodger]

As crashman said nowadays Mhz ratings are so confusing that it is better to look at bandwidth. The Pentium4A has a max bandwidth of 3.2GB/s which is perfect for PC3200 RAM. The pentium4c has a max bandwidth of 6.4GB/s which is perfect for PC3200 RAM in a dual channel configuration.