Harpertown Front Side Bus

[SDAVE]

Hi guys
was reading about the Harpertown CPU's which are in the 2008 Mac Pro and was wondering why the FSB is at 1600 but the Mac Pro runs at 800?

http://ark.intel.com/products/33088/Intel-Xeon-Processor-X5482-12M-Cache-3_20-GHz-1600-MHz-FSB

Also it seems the max you can go with these processors (if you ever need to upgrade) is the X5492, which is 3.4Ghz...I have dual 3.2's

 

[Baunkjaer]

Well, the Pro runs with Dual Channel RAM. Hence 800MHz equals 1600Mhz

 

[SDAVE]

doh! thanks for clearing that up.

 

[666sheep]

Well, the Pro runs with Dual Channel RAM. Hence 800MHz equals 1600Mhz

It does not work like this. Using Dual Channel increases memory bandwidth, not the bus speed. I.e. you're pushing twice as data in that same time than via single channel, but @that same speed per channel. Single channel is 64-bit wide. X5400 chipset (and X5000 too) do have 4 memory channels, BTW. Depending on slots population you can use 64, 128 and 256-bit wide data path.

"800 vs 1600 thing" comes from way how DDR memory bus works. Double Data Rate allows 2 transactions per clock cycle what doubles real speed. Hence PC2-6400 RAM (rated 800 MHz) runs @effective 1600 MT/s.

It's a little more complex when you'd want to dig deeper in details, but you should get the idea from above.

X5492 is no go in MP due to lack of microcode for it in MP EFI. It was confirmed here on MR by hyram (AFAIR).

 

[SDAVE]

It does not work like this. Using Dual Channel increases memory bandwidth, not the bus speed. I.e. you're pushing twice as data in that same time than via single channel, but @that same speed per channel. Single channel is 64-bit wide. X5400 chipset (and X5000 too) do have 4 memory channels, BTW. Depending on slots population you can use 64, 128 and 256-bit wide data path.

"800 vs 1600 thing" comes from way how DDR memory bus works. Double Data Rate allows 2 transactions per clock cycle what doubles real speed. Hence PC2-6400 RAM (rated 800 MHz) runs @effective 1600 MT/s.

It's a little more complex when you'd want to dig deeper in details, but you should get the idea from above.

X5492 is no go in MP due to lack of microcode for it in MP EFI. It was confirmed here on MR by hyram (AFAIR).

Not to go any deeper into this, just wanting to make sure, 800Mhz is pretty much the maximum this computer can run as far as memory, correct?

I'm still happy with this machine, just added an SSD for OS X and mountain lion and it's super fast. I was feeling a bit bad when my 2011 MBP felt much faster than this machine before the SSD upgrade

 

[666sheep]

Not to go any deeper into this, just wanting to make sure, 800Mhz is pretty much the maximum this computer can run as far as memory, correct?

Correct. If you want to get full 256 bit data path, you need to use 4 or 8 identical sticks. This will max out memory performance. Real world difference depends on application. Some can benefit from this, some can't.