800 ram with 1333 CPU and 1066 mobo

victordilorenzo

Distinguished
Apr 17, 2008
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would the FSB of those run at 800 instead if I used 800 ram?

I am trying to be as economical as possible, I would like to go with less expensive 800 ram, but only if it won't slow my build down a mentionable amount.

This rig will be for games, I am also geting an 8800GT, and an E8400.

Thanks!
 

jaguarskx

Titan
Moderator
The key thing to remember is that the FSB is Quad Pumped and DDR2 RAM is "double pumped".

A CPU with a 1333MHz FSB is actually running at 333MHz. That means 1033MHz FSB will actually be running at 266MHz.

DDR1 and DDR2 RAM basically means Double Data Rate as opposed to the older SDR RAM, or Single Rate RAM. DDR2 800 RAM means that it can actually runs at 400MHz. Since the CPU's FSB is quad pumped due to Intel's architecture, it is designed to run with a CPU with a 1600MHz FSB.

The current Core 2 Duo CPUs (Penryn) and Core 2 Quad (Yorkfield) are CPUs with 1333MHz FSB. There DDR2 800 can be used to overclock these CPUs, but that also depends on if the motherboard is capable of overclocking as well.
 

Mondoman

Splendid
Jun 22, 2006
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vdl, the key thing is to match the *datarate* of the memory bus with the datarate of the front-side bus. For current Intel CPU designs, the data travels as follows:
RAM <-> memory bus <-> northbridge chip <-> FSB <-> CPU
From this we can see that the speed of the FSB can be adjusted separately from the speed of the memory bus, so running RAM at DDR2-800 speed will not affect the FSB, as long as the BIOS is not set to "synchronous" mode, which links the two bus speeds.
To figure out what memory speed you need to keep the FSB "full", start with the FSB datarate (1333MHz in your example). Assuming you will install the RAM in pairs, your memory bus will be running in "dual-channel" mode, which doubles the memory-bus datarate versus that from a single module. Thus, we need to divide the FSB datarate by 2 to figure out what speed a single module needs to run at. 1333MHz divided by 2 = 667MHz, or DDR2-667 memory bus datarate.
Similarly, a 1600MHz FSB datarate corresponds to DDR2-800 memory bus datarate in dual-channel mode. Thus, unless you need to OC the FSB past a 1600MHz datarate, DDR2-800 RAM will be more than fast enough.
 

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