About six years ago I built a computer based around an Abit AV8-3rd Eye motherboard. I’d like to add more memory, but the motherboard is a bit stroppy about RAM configurations. I currently have two 512MB sticks of Samsung 400MHz DDR RAM running in dual-channel mode, so any additional 400MHz RAM I add must be “single-rank” memory – which appears to be about as rare as rocking horse droppings. I can add double-rank 333MHz RAM, but this would mean mixing RAM of different speeds, in which case I think the motherboard would simply run all the RAM at the speed of the slowest module.
What is the best option? Does dropping RAM speed really make such a huge difference that it outweighs an extra gigabyte of capacity? I’ve trawled various internet forums and there are lots of contributors who are evangelical about never mixing different speeds.
I must admit I don’t really understand this single/double-rank issue. Is it something fundamental to the design of the motherboard, or is it an issue that can be overcome with a BIOS update?
Memory “ranking” refers to how the chips on a DIMM are arranged electronically. In a single-rank arrangement, all the chips can be accessed directly by the memory controller. In a multi-rank arrangement, they’re divided up into two or more groups, of which only one can be active at a time. This has an effect on how quickly data can be read and written, but it isn’t normally something you need to worry about: for most purposes, a DIMM’s rated speed and timings will tell you everything you need to know.
Some chipsets, however, can’t operate at maximum speed if they mention the single/double-rank issue, so the only way to find out whether a newer BIOS might support double-rank DIMMs at full speed is to get some 400MHz modules and try it.
If you do end up running at 333MHz, that isn’t a big problem. We’ve tested DIMMs of various speeds in the past, and we’ve found that memory speed is only a small contributor to overall system performance. Having 2GB of memory running at 333MHz will definitely give you a much faster and smoother system than having 1GB at 400MHz.
And don’t worry too much about mixing DIMMs of different speeds. Your motherboard will probably run all your DIMMs at the speed of the slowest, so partnering fast DIMMs with slower ones means missing out on a little performance potential, but it should have no other detrimental effect.
Occasionally, though, we’ve seen motherboards try to apply the frequencies and timings of the fast modules to the slow ones, leading to glitches and system crashes. You can cure this by going into BIOS and manually setting memory speeds and timings to match the slowest DIMMs.