Fully populated RAM slots.

[SilthDraeth]

Why is it, that most motherboards can not run RAM at full speed with all the RAM slots populated.

For instance my motherboard supports DDR3 1866 RAM, but only if it is in two slots, otherwise it has to run the RAM at 1600.

Is this an Intel limitation, or does AMD also suffer from this? Is it a limitation of the motherboard designers?

Thanks.

 

[Cerb]

It's always been a limitation, when there have been several RAM slots (3, 4, more...), at least for SDRAM. It was a problem, from my recollection, as far back as 133MHz P3 and Athlon CPUs, sometimes needing to run RAM at 100MHz to support higher capacities (which, at the time, might have been 1GB 🙂).

With unbuffered DIMMs (UDIMM), the RAM controller is passing signals through every chip in every DIMM. The chips on each DIMM are not intelligent in the least. They are basically some amplifiers, with data arrays. The signal will degrade a little from the load presented by the memory chips, and the wires going between it all. I'm not an expert on the subject, by a long shot, but given the nature of DRAM, I suspect the biggest problem is capacitance.

Servers, which need higher and higher RAM capacities, have helped to alleviate the problem by way of registered RAM (RDIMM), which buffers commands, with FB-DIMMs (Fully Buffered), which buffered all access, but in a way that added lots of latency and power, and more recently with LR-DIMMs, which do the same sort of thing, but more sensibly.

 

[tweakboy]

NOPE the above is not true for me at least with my rig.