DDR2 and FSB Freqency Math - Techist - Tech Forum

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Old 10-26-2004, 01:29 PM   #1 (permalink)
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Default DDR2 and FSB Freqency Math

I've been looking around on Google and I can't seem to get a straight answer on this. Also its entirely possible that I'm wrong on my assumptions, so please feel free to correct me.

DDR frequencies run at half the DDR rating. For example, DDR-400 runs at 200MHz, DDR-533 runs at 266MHz.

The FSB rating is equal to 4 times the actual FSB. For example, FSB800 actually runs at 200MHz, FSB533 actually runs at 133MHz.

If the above is correct, what would be the frequency of say, DDR2-533 or DDR2-667? I've heard that the actual frequency can be obtained by dividing the DDR2 rating by 4. So for example, DDR2-533 actually runs at 133MHz, and DDR2-667 actually runs at 166MHz. I've also heard that DDR2-533 runs at 533MHz, and DDR2-667 actually runs at 667MHz. Can someone please clarify this for me?

The reason I ask is because I'm building a new system soon and I want the FSB to match the memory frequency. I'm not exactly sure how to go about this. If I have an FSB of 800MHz, should I have 4 sticks of DDR400 since each is 200MHz? Or does it not matter because FSB800 is actually 200MHz and so is DDR400?

What effect, if any, does dual channel have on this?

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Old 10-26-2004, 05:22 PM   #2 (permalink)
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im nota pro, but i have learned this over time from ppl helping me....

1) the fsb of the cpu is calculated differently depending on if your talking about INTEL or AMD.

AMD cpu's FSB is calculated just like DDR, divided by 2... ex.- Athlon64 400fsb is actually 200.

INTEL cpu's FSB is quad pumped.. ex.- pentium4 800fsb is actualy 200.
and.... AMD's cpu's run at the same frequency as DDR... however INTEL's cpu's are quad pumped because thy rely on Duel channel.

AMD ex.- cpu fsb = 400 ..... ddr400(pc3200)

INTEL ex.- cpu fsb = 800 ..... ddr400(pc3200) + ddr400 (in duel channel) = 800
at this point DDR2 runs the same as DDR1, DDR2 will be developed more in the near future when motherbourds come out that support higher fsbs.

DDR2 is calculated the same as DDR1.... ddr2 533 = ddr1 533... both divided by 2 = 266

thy run at the same speed but ddr2 just does it ina different way. ddr2's prefetch is 2x as big as ddr1... thy use the same fsb, but DDR2's actuall speed is divided by 4... 533/4= 133.

so the fsb #'s witch thy perform at are the same... but ddr1 is limited in speed at about 533(pc4200) i think. but ddr2 will beable to reach 800(pc6400) and higher.

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Old 10-26-2004, 05:27 PM   #3 (permalink)
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So is it the same with AMD boards? In other words, if I get an AMD board with a 266MHz FSB, what speed of DDR should I get?
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Old 10-26-2004, 05:58 PM   #4 (permalink)
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So is it the same with AMD boards? In other words, if I get an AMD board with a 266MHz FSB, what speed of DDR should I get?
You'd get RAM that's rated 266MHz which would be PC2100.....more than likely you'll see it listed like PC2100(266), PC2700(333), PC3200(400), just like that. So then it basically plainly tells you what you need. In this case you need RAM that has a FSB of 266 so then you'd see PC2100(266) and boom there's your RAM. This is all for AMD as I know nothing of Intel.

Mind you when you go into your BIOS with an AMD board the FSB for the CPU will be set at 133....because 133x2 = 266 so that's how they figure out their FSB Rating for the RAM. Whatever the CPU's FSB is at, you have to multiply it by two and you'll know your RAMs FSB. Be careful though because BIOS's are different and will show different settings. I've had a BIOS where you select the RAM as 133,166 or 200 which translates into the RAM being either 266,333,or 400. The other board I had asks you for a ratio from the CPURAM so then you have to get the right ratio to get the right speed. If you got a 266FSB boad and CPU then your ratio would be 1:1.....I'll just stop at this point cause I don't want to confuse you. Let us know if you require more assistance.
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Old 10-26-2004, 06:08 PM   #5 (permalink)
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