FSB Speed Mystery

[barryng]

In a few days I am going to replace the E6600 currently installed in my Abit AW9D-Max with an E8400. While doing a little planning ahead and trying to decide what bios settings to consider, I was stopped dead in my tracks trying to figure something out I always took for granted.

The bios, with the E6600 installed, is currently configured as follows:

External Clock: 356 Mhz
Multiplier: 9X
CPU Speed: 3204 MHz
Strap CPU as: PSB1066
CPURAM: DDR2 667 (4:5)
DRAM Clock: 445 MHz (DDR2-890)
Core voltage: Default 1.375 but CPUZ reports 1.410 Volts
FSB (per CPUZ): 1425 MHz

With these settings and PC2-8500 memory, my machine has been rock stable for almost two years now.

When I noticed the FSB speed this morning, I was surprised it is so high as the bios "Strap CPU as" is set as PSB1066. I always understood, obviously incorrectly, that the "Strap CPU" setting fixed the FSB to the specified value. Therefore, since it is strapped at PSB1066, I was expecting to see a 1066 MHz FSB, not 1425 MHz. What am I missing? What then does the "Strap CPU as" mean?

 

[Tempered81]

It's setting it to a FSB range: 800 - 1065, 1066 - 1332, 1333 - 1599, 1600 - 1999, 2000+
356 x 9 = 3204
346 x 4 = 1425

If you get your e8400 and use
400 x 9 = 3600
400 x 4 = 1600, 1600 will be your FSB speed, 400 will be your ram speed, and 3600 will be your cpu speed.

"your north bridge has an internal clock speed and latencies just like your CPU and memory. The FSB of your north bridge can be found by dividing your original CPU multiplier by your set CPU multiplier and then multiplying by your FSB.

So if you are running a E6600 (266 * 9) at 400Mhz x 8 your NB FSB is:

(9 / 8) x 400 = 450Mhz FSB (1800Mhz Total)

Just like your memory may be able to run at 4-4-4-12 at 1000Mhz but needs to run at 5-5-5-15 at 1200Mhz, your north bridge has a series of latencies which it must adjust in order to maintain stability at its FSB. These latencies seem to play a far more significant role in system performance than memory latencies.

Intel has predefined specific latencies at specific NB FSB speeds. They are referred to as straps. There is a strap for when the NB FSB is 1066Mhz and under, 1333Mhz FSB and under, 1600Mhz FSB and under, ect. When you go from the 1066Mhz FSB strap to the 1333Mhz FSB strap, the north bridge's internal latencies loosen to allow for greater stability.

ASUS has redefined the NB strap so that the 1333Mhz FSB strap does not come into effect until 401Mhz FSB (1604Mhz). Other perimeters of straps are someone unknown.

Tony, the guy who pretty much figured all this out and he works for OCZ. He is now a lobbyist trying to get ASUS and other major motherboard manufacturers to give the end user the control of when straps start.

There are 2 ways to beat the NB strap:

1. Boot to windows in the 1066Mhz strap and then use Clockgen to increase your CPU's FSB. You can then get to a much higher FSB while maintaining the 1066Mhz strap simply because the BIOS does not adjust the north bridge's latencies in real time.
2. Get a X6800 or QX6700 (or even a ES chip). To the north bridge, you are always at a default multiplier with a Extreme Edition processor. This allows you to set a much lower or higher multiplier without the NB FSB being effected."

http://www.xtremesystems.org/f...?p=1931740&postcount=1

 

[barryng]

Thank you for your quick response. It is, with out question, the best explanation I have seen on this subject and certainly corrected some significant misunderstandings I previously had. I appreciate your effort.