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PC2700 @ 266MHz by Default?

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CuriousOne

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Aug 14, 2002
Am I correct in assuming that if you're running a 533Mhz chip @ 133MHz FSB, even if the ram is PC2700 (rated @ 333MHz), it's still only running at 266MHz because of your FSB (ie. 133MHz x 2 = 266MHz)?

If this is true, then what would be the point of using PC2700 ram in such a system if you never have any intention of overclocking (eg. Dell systems), thus the power of the ram is never utilized? A marketing thing? Or does using PC2700 over PC2100 in a system like that still provide better peformance even if you never go over 133MHz FSB?
 

Stumpjumper5200

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Kinda...

Well, it'll technically still be running at 333, but in an AMD system, you'll only use it as fast as 266mhz.

In an Intel system, the bus is 133, but performs equivalent to a 533, therefore you would use all of the bandwidth.
 

attack

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May 23, 2002
I'm not sure what dells set thier RAM speeds at but I would guess that they use PC2100 in all there systems. P4A's would use a 4:3 divider to make them run at 133mhz ram when FSb is 100, and a 1:1 divider on P4B's.

With the KT333 and KT400 chipsets you can run the FSB at 133 for the chips and have the RAM at 166 (pc2700) or even 200 (PC3200) and everything will still be clocked at stock. The advantage of this is there is more memory bandwidth for the cpu to utilize. this is called ASYNC for a-synchronous (sp?). It's much faster to run your computer SYNC'ed with the RAM so your cpu can access the RAM just as fast as well as have it's bandwidth, but if your not OCing then you can't really do this.
 

UnseenMenace

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Apr 23, 2001
Re: Kinda...

Stumpjumper5200 said:
Well, it'll technically still be running at 333, but in an AMD system, you'll only use it as fast as 266mhz.

In an Intel system, the bus is 133, but performs equivalent to a 533, therefore you would use all of the bandwidth.

I was of the opinion that DDR stood for Double Data Rate which meant that the memery runs at twice the motherboard FSB speed.
The internal bus multiplication of the processor no matter if it belongs to a double pumped AMD or a quad pumped Intel P4 has very little to do with the memory bus speed at all.

The 226 mhz only refers to the internal bus speed of the AMD processor which is a double pumped mobo FSB (2 x 133 = 266 mhz ) likewise the early P4's ran on a 100 mhz mobo FSB which was quad pumped internally for the processor ( 4 x 100 = 400 mhz ) the latest P4's run on a 133 mhz mobo FSB which is also quad pumped internally for the processor ( 4 x 133 = 532 mhz, but i guess they round it up so it sounds nice )
DDR memory is rated by bandwidth and not bus speeds as was done with old SDRAM
 
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Stumpjumper5200

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Dell's use all kinds of ram now, PC800 and 1066RDRAM and DDR (2100 I think)

UnseenMenace - the "pumped" speed of the fsb means nothing to the CPU multiplier because it only takes the true clock and multiplies it. BUT, the pumped speed is a representation of the data rate which directly relates to how much memory bandwidth you have. For example, my computer is o/c to 145fsb and with a quad pumped bus I have an effective data rate of 580Mhz, which Sandra estimates to have about 4680Megs/second of bandwidth.

BTW, yes DDR stands for double data rate, because it transmits on both sides of the clock cycle (rising and fallling, basically just twice per clock). But it effectively runs at double whatever you have it set at. My memory is on a 4/5 setting and with a 145fsb that gives me a little over 181Mhz or about DDR363.

ATC9001 - I made that mistake before too when I did the math wrong :). A 4/3 divider will SLOW down the memory, a 4/5 or 4/6 will speed it up.
 

UnseenMenace

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Apr 23, 2001
Stumpjumper5200 said:
UnseenMenace - the "pumped" speed of the fsb means nothing to the CPU multiplier because it only takes the true clock and multiplies it. BUT, the pumped speed is a representation of the data rate which directly relates to how much memory bandwidth you have. For example, my computer is o/c to 145fsb and with a quad pumped bus I have an effective data rate of 580Mhz, which Sandra estimates to have about 4680Megs/second of bandwidth.

I am aware that the internal bus speed has nothing to do with the CPU multipler and I did not state either that I believed it did so. Perhaps you are mis-reading the comment about 'internal bus multiplication' as this refers to the double pumping ( ie :- multiply the mobo FSB twice )

I was also of the opinion that data rate is not measured in MHZ (Megahertz) and always believed that it was measured in KBS (kilobits per second) and as such fail to fully understand how you can have a data rate of 580Mhz.

Out of curiousity do you believe that 4680 megs/second of bandwidth is across your processor / memory / 33mhz PCI slots or the whole system ?

I will try to clarify my origional question regarding your statement further, take the AMD system for example :-

Stumpjumper5200 said:
Well, it'll technically still be running at 333, but in an AMD system, you'll only use it as fast as 266mhz.

I fail to see how memory running on a mobo FSB of a 133 mhz and double pumped ( aka DDR ) will technically be 333 just because its PC2700. DDR memory of any bandwidth rating from my understanding will run at twice the mobo speed within its limitations and as such PC2700 on a mobo running at 133 mhz will run at 266 mhz speed and not as you quoted 'technically still be running at 333' to get PC2700 running at 333 mhz you have to either run a 166 mhz mobo FSB or multiply a 133 mhz mobo FSB with the 33 mhz PCI clock speed.
 
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Stumpjumper5200

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Yeah, let's try to clear this up...

True, the data rate is not measured in Mhz, but in Kb or Mb. The bus transfers "x" amount of data per clock cycle. Now when the bus is sped up, it transfers the same amount of info per clock cycle, but there are more cycles in each second, therefore it transfers more each second. I hope that makes sense :)

The 4680Mbps is a bandwidth estimation (from sandra) by the fact that my front side bus effectively runs at 580Mhz. I should have mentioned the "front side" in my last post.

I am assuming that high fsb bandwidth would increase the performance of the whole system? If not, at least the components linked by the motherboard's north bridge (Processor, memory, and AGP)

And finally, the "AMD system ram problem" haha. The RAM would run at whatever you tell it to, based on what CPU:Memory ratio you have set in the BIOS. At 1:1, yes, a 133fsb would mean only DDR266 (PC2100). But, at a 4:5 CPU:Mem ratio, the Memory's clock would be sped up to 166Mhz (DDR333/PC2700) by the fact that:

(CPU stands for the true fsb clock speed)
CPU:Mem = 4/5
CPU = 4, Mem = 5
(133Mhz / 4) * 5 = 166Mhz

And if a 4/6 ratio was chosen, you would wind up with 200Mhz (DDR400/PC3200)

And the AGP/PCI clocks are taken from the true fsb with their own dividers. At 133mhz, a 1/2 divider is for the AGP (66Mhz) and a 1/4 divider is for the PCI (33Mhz). That's why people can only overclock so far....especially without a motherboard that has only 1 set of dividers.....because you overclock the entire system, and some components don't like that.

WHEW!! Please let me know if I left something out. :D