Datasheet

Identifying Purposes and Characteristics of Memory

Single- and Double-Sided Memory

Ask just about anyone who doesn’t manufacture memory for a living what the terms single-

sided memory and double-sided memory mean, and you’ll be treated to a blank stare or

a short diatribe on how some memory modules have chips on one side, while others have

chips on both sides. In fact, these terms have nothing to do with the physical attachment of

chips to the modules. Either style can have chips on one or both sides of the module.

Double-sided memory is essentially treated by the system as two separate memory

modules. Motherboards that support such memory have memory controllers that must

switch between the two “sides” of the modules and, at any particular moment, can only

access the side they have switched to. For the state-of-the-art memory at the time, double-

sided memory allows more memory to be inserted into a computer using half the physical

space of single-sided memory, which requires no switching by the memory controller.

Single- and Dual-Channel Memory

Standard memory controllers manage access to memory in chunks of the same size as the

FSB’s data width. This is considered communicating over a single channel. Most modern

processors have a 64-bit system data bus. This means a standard memory controller can

transfer exactly 64 bits of information at a time. Communicating over a single channel is a

bottleneck in an environment where the CPU and memory can both operate faster than the

conduit between them. Up to a point, every channel added in parallel between the CPU and

RAM serves to ease this constriction.

Memory controllers that support or require dual-channel memory implementation were

developed in an effort to alleviate the bottleneck between the CPU and RAM. Dual-channel

memory is the memory controller’s coordination of two memory banks to work as a synchro-

nized set during communication with the CPU, doubling the speciﬁed system bus width, from

the memory’s perspective. Because today’s processors largely have 64-bit external data buses,

and because one stick of memory satisﬁes this bus width, there is a 1:1 ratio between banks

and modules. This means that implementing dual-channel memory in today’s most popular

computer systems generally requires that pairs of memory modules be installed at a time.

Note, however, that it’s the motherboard, not the memory that implements dual-channel

memory (more on this in a moment). Single-channel memory, in contrast, is the classic mem-

ory model that dictates only that a complete bank be satisﬁed whenever memory is initially

installed or added. One bank supplies only half the width of the effective bus created by dual-

channel support, which, by deﬁnition, pairs two banks at a time.

Because of the special tricks that are played with memory subsystems to improve over-

all system performance, care must be taken during the installation of disparate memory

modules. In the worst case, the computer will cease to function when modules of different

speeds, different capacities, or different numbers of sides are placed together in slots of

the same channel. If all of these parameters are identical, there should be no problem with

pairing modules. Nevertheless, problems could still occur when modules from two differ-

ent manufacturers or certain unsupported manufacturers are installed, all other param-

eters being the same. Technical support or documentation from the manufacturer of your

mother board should be able to help with such issues.

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