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Topic Title: Questions about FSB, Hypertransport, RAM ...
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Created On: 04/28/2005 12:55 PM
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 04/28/2005 12:55 PM
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Martin Cracauer
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Joined: 04/28/2005

I am missing some pieces in my understanding of the interaction between CPU, RAM, the northbridge and the implications of the integrated memory controller in the Athlon 64 CPUs.

I think it's best to just throw in a bunch of random questions:

The newer Athlon 64s have 800 MHz FSB, 1000 MHz Hyptertransport and an integrated RAM controller which makes the RAM independent of the FSB. Still, the RAM to be used is specified to be 400 MHz DDR PC3200 or dual-channel 800MHz.

I am a little lost in figuring out where the term FSB applies in that case and how the specification for PC3200 comes together. Is that a mainboard/chipset, not CPU, thing? Where exactly is the FSB at 800 MHz and what does it control that is neither on Hypertransport nor on the integrated memory controller?

Some related questions, not strictly AMD-related, but might help in understanding the speed ratings of memory busses and front side busses.

With Pentium-Ms you have 533 MHz FSB, but some newer Pentium-M setups claim to do dual-channel memory at 2x 2x 200MHz = 800 MHz. Is there any benefit to that and where does it apply? Is that between the RAM and the Northbridge?

Some people also throw DDR2 533 MHz = 1066 in dual channel mode at Pentium-4s with 800 MHz FSB. Is there any benefit in that?

 04/28/2005 01:45 PM
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first of all, here are a couple links to help you understand amd architecture. first, all a64's run at a 200 HTT (fsb).' ">

pc3200 is the name given to ddr 400. it has a max theoretical bandwidth of 3.2gB/s at 400mhz (since it's ddr this means 2x200mhz)
since most a64 chips have built in dual channel memory controllers(as apposed to a memory controller on the NB like intel has) the max theoretical bandwidth in a dual channel configuration is 6.4 gB/s at 200mhz HTT' ">

ddr2 ram has higher bandwidth, but has much slower timings.
couple things, though, ddr2 is starting to come with lower timings, but ddr is starting to come with higher bandwidth. ddr2 and ddr require completely different mobos/systems. p4's are using ddr2, amd uses ddr but are possibly switching to ddr2 in the future or switching to ddr500 for the internal HTT. meaning that the internal HTT would be running at 250mhz as apposed to 200mhz.

there are lots of links in the tech guides and tips section of this forum that will help you understand it better
 04/28/2005 02:30 PM
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Martin Cracauer
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Thanks, flippin.

The links only contained what I already knew. The exact places where which component introduces which limitation are still unclear.
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