Question for the memory Guru's

[PPRuNe Pop]

Why is it necessary to install RAM in double multiples? Why not one 512mb and one 256mb. For example: if you have only two slots?

[Bushfiva]

It's not always necessary. It's down to motherboard design.

Oversimplifying somewhat, CPUs have evolved from accessing memory in 16-bit wide chunks to 64-bit wide chunks.

Memory has evolved from 30-pin SIMMs (16 bits wide) & 72-pin SIMMs to 168-pin DIMMs (64 bits wide).

The evolution of memory has lagged the evolution of CPUs very slightly (in terms of price and availability), so at one point there would have been, say, Pentium motherboards (64 bits) that had 72-pin SIMMs (32 bits). To get the 64 bits, you needed two identical SIMMs so the CPU could pull 32 bits from one, and 32 bits from the other.

There are other reasons. For example, some motherboards used "dual channel" designs to provide a 128-bit memory path, so you have to use paired DIMMs in them, too.

(SIMM & DIMM are ways of building memory sticks.)

(I've actually oversimplified somewhat: it's ultimately the memory controller that determines the memory slot design. Right now, there are memory controllers that only address single slots, only address double slots, and those that can work out whether you're using matched pairs so choose the optimum solution automagically.)

[Cypherus]

It is NOT necessary to install multiples of RAM in any format, but to have them operate correctly, i.e. Speed and DDR they have to fulfil certain parameters, the best and most reliable way to estabish what will and WILL NOT run in any Mianboard is to go direct to the vendors support website and look it up, they will have listings of RAM they have tested and that will run correctly, also information on the configuration, which slots to use ect. Even so it is suprising what oddities will run quite happily together and also which supposedly compatible RAM, even from Brand names that will not.

[Saab Dastard]

It is NOT necessary to install multiples of RAM in any format

Cypherus, I think that you will find that what Bushfiva has written is absolutely correct. It may not be as common now as it once was, but it most certainly was necessary to install RAM in matched pairs in certain systems.

You are right that the specs for the RAM should be checked with the mobo manfr's website. Failure to do so can result in a non-working computer, or one unnecessarily crippled by running at the speed of the slowest module.

SD

[Mac the Knife]

Just to add (again) that many of the unstable systems that I see are caused by installing memory rated at the wrong speed or incorrect (overaggressive) memory timings set in the BIOS.

If the system won't run Memtest 86+ without errors on an overnight burn-in then it'll never be stable.