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1000 MHz FSB - DDR400 Discrepancy

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1000 MHz FSB - DDR400 Discrepancy

Postby YasharF » Fri Dec 23, 2005 12:13 am

I am putting a dual Opteron 280 system together. From the CPU specs I can see that the FSB is running at 1GHz, but then from the motherboard specs (Tyan S2895)and other postings in this forum I see that DDR400 is the memory to use.

Isn't DDR400 for 800MHz FSB? I was under the impression that the new AMD architecture has the memory on the FSB with no external memory controllers.

The other way that I can see this working is that FSB is only for the link between the processors/HT, and the memory is being separate from the Front Side Bus.

Does anyone know how this works? I just want to make sure that I am not forcing the FSB to run at 800MHz by using DDR400.

Thanks,
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Re: 1000 MHz FSB - DDR400 Discrepancy

Postby The_Ghost » Fri Dec 23, 2005 2:05 am

I am putting a dual Opteron 280 system together. From the CPU specs I can see that the FSB is running at 1GHz, but then from the motherboard specs (Tyan S2895)and other postings in this forum I see that DDR400 is the memory to use.

the 1ghz is to things that are hooked up to the hypertransport , like going from one cpu to the other ,and other hypertransport connections

Isn't DDR400 for 800MHz FSB? I was under the impression that the new AMD architecture has the memory on the FSB with no external memory controllers.

ddr400 is actually 200mhz but it is a double data connection which doubles your 200mhz fsb

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Re: 1000 MHz FSB - DDR400 Discrepancy

Postby gobucks » Wed Feb 22, 2006 11:52 pm

Athlon 64s and Opterons have no "FSB." They use a Hypertransport link to connect to things like the north and south bridges, PCI/PCIe bus, etc. In a multi-CPU setup, inter-CPU communication takes place on its own dedicated hypertransport link, which is why multi-CPU opterons destroy Xeon setups. Hypertransport does not connect to the memory controller, because it is located on the chip itself, so this frees up the bus quite a bit. The base clock of this link is 200MHz, and usually has a multiplier of 4X with old chips and 5X with socket 939 and newer 940 ones. So "1000MHz FSB" really just means that the hypertransport link is 1GHz. This doesn't mean a whole lot though - 1GHz is WAAAAY more than is necessary for most tasks it will need. After all, an Intel Core Duo manages to do allright with 800MHz of FSB speed, and it has to shuttle data from its memory controller and communicate between the cores in addition to normal duties!
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Re: 1000 MHz FSB - DDR400 Discrepancy

Postby Indigo » Tue Mar 28, 2006 7:34 pm

Don't forget that the 1 GHz rating is one-direction only. The HT Bus operates as a highway - both sides go at the same speed. Mmm Mmm Good.
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