I know that this is an old motherboard but I figured that I would ask the community for help anyway. I tried upgrading this little warrior to 16GB however although it boots just fine the system is extremely slow compared to its usual speed with only two DIMMs (8GB) in the blue slots. The Windows Memory Diagnostics Tool extended tests (Win7 Ultimate x64) do not find any error.
Single threaded performance is down only by 2% but multi-threaded performance is down by 32%! The overall boot time went from 40 sec to a whopping 5 min 35 sec!
It seems to be the same issue as reported here https://communities.intel.com/message/14531#14531 however that was back when 8GB was being installed using all the slots and the fix was a BIOS update which I can't really hope for now, can I? . I tried downgrading the BIOS by increments without any success from version 2006 down to version 1900 at which point the system wouldn't POST unless I replaced the memory with lower density DIMMs.
The system is configured as follow:
QX9770 @ stock multiplier & voltage
4 x 4GB Crucial Ballistix Elite BLE46301608DE1TX0 DDR3-1600 8-8-8-24 @ 1.9V
FSB @ 1.325V (+10%)
MCH @ 1.375V (+10%)
Apparently others successfully did it with other X48 motherboards http://www.overclock.net/t/1231879/x48-and-lga775-does-support-16gb-of-ram so it seems possible. However they don't say much about the end result as far as performance.
As a side note I have another totally identical system which also exhibit the same behavior.
I have two theories:
1. A RAM cacheability issue? Remember that the QX9770 is a dual dual-core so maybe 16GB is just too much for each dual-core packages' own 6MB of 24-way set associative L2 cache. I remember when the chipsets controlled the L2 cache they used to have a maximum amount of supported memory and a maximum amount of cacheable memory which meant that the system would be slower with too much RAM (ex. the i430TX which could support 256MB of memory but only cache 64MB) but nowadays Intel only report a maximum amount of supported memory. Unfortunately the processor datasheet does not specify anything about the maximum amount of cacheable memory.
2. A chipset memory prefetch buffer issue? The X48 chipset supports both DDR2 and DDR3 memory but Intel states that the maximum memory is 16GB for DDR2 and 8GB for DDR3. This could have been because at the time (2008) the memory density just didn't exist to put 16GB of DDR3 on a motherboard with only four slots however it could also be due to the fact that DDR2 only need a 4-bit prefetch buffer while DDR3 need an 8-bit prefetch buffer. If the chipset is limited in that aspect then I'm trying to address twice the amount of memory that the X48 prefetch buffer is designed to handle which slows it down considerably. Another theory that I cannot prove because the X48 datasheet does not mention anything about that.
In this particular case I'm only interested in understanding the issue and if at all possible, figuring out a way around it. I know very well that replacing the hardware would fix everything but that's beside the point. If there's any techie out there willing to give it a shot I would greatly appreciate.