Intel 865PE fast bypass path of memory
controller enabled when overclocked at FSB800! Bluetooth 10 June 2003
Intel 865PE fast bypass path of memory
controller enabled when overclocked at FSB800!
Every one has been wondering why some Springdale are faster than other 865PE
boards. Today we spoke to a good friend of us and he gave us an insight into
making i865PE board perform at 875P levels.
In the BIOS, I tested a few things to verify his
findings. Firstly there are a number of settings which you should take note.
there is the NB Strap setting, FSB and other settings in the BIOS
The experiment : We set the Performance to Ultra, CPC to enabled, CPU is
defaulted to P4 3.0C. We ran Sisoft Memory Benchmarks and scored around
5000/5000. Next we disabled the Performance and CPC and benchmark again. The
scores dip down to around 4400/4400 which is most 865PE boards are scoring.
Before we proceed, there is this setting which is
available in the BIOS which sets the cpu:mem ratios.
First of all, there is this FSA/FSB setting in the BIOS
if u want to set PSB400 to NB , you will only have CPU/DRAM 3:4 ratio worked
if u want to set PSB533 to NB , you will only have CPU/DRAM 1:1 and 4:5 ratio
worked
if u want to set PSB667 to NB , you will only have CPU/DRAM 1:1 and 5:4 ratio
worked
if u want to set PSB800 to NB , you will have CPU/DRAM 1:1 , 5:4 , 3:2 support.
Now we manually set the settings in the NB Strap to PSB667 and FSB is set to
200Mhz (3.0G). RAM voltage set to 2.7v. We booted up and found that memory
benchmark scores 5000+ which is similar to enabling the performance setting in
the BIOS. That is to say when NB Strap is not set to PSB800 (by default), it
seems that fast bypass path is enabled. It seems to be turned off when FSA/FSB =
PSB800.
If you are really running PSB 800, 3:2 ratio should work, but the memory
benchmarks remains at 5000+. This implies that 3:2 ratio does not work since 3:2
is only available for PSB800. This reveals that the system is not running PSB800
but something else e.g. PSB667.
Thus, to make it simple, to enable better performance on all 865PE, just trick
the system to run using NB Strap of PSB667 instead of PSB800. When that is done,
you lose the ratios.
PAT is only disabled on 865PE when NB receive FSA/FSB=PSB800
in order to make PAT worked on 865PE , you can not set FSA/FSB to PSB800.
