Introduction
FB-DIMM
(Fully Buffered DIMM)
is a memory module technology targeted to servers developed recently
created in order to increase the memory speed and the maximum memory
capacity of a server. In this tutorial we will explain what are the
main features of FB-DIMM modules and the difference between DIMM and
FB-DIMM modules.
The
main difference between
FB-DIMM
modules and regular DIMM modules is that on FB-DIMM the communication
between the memory controller (chipset) and the module is serial, in
the same way that occurs with PCI Express, while on standard DIMM
modules this communication is parallel.
Using
serial communication the
number
of wires needed to connect the chipset to the memory module is lower
and also allows the creating of more memory channels, what increases
memory performance. With FB-DIMM technology it is possible to have up
to eight modules per channel and up to six memory channels. So this
technology increases both memory capacity and speed.
Each
extra memory channel that
is
added to the system increases the memory subsystem transfer rate. For
example, if you use a single DDR2-533 channel, the memory transfer rate
will be of 4,264 MB/s. If you use two DDR2-533 channels, as it happens
on dual channel configuration available today, the memory transfer rate
will be of 8,528 MB/s. With four channels, the memory transfer rate
jumps to 17,056 MB/s, and so on.
FB-DIMM
serial communication
uses 10
pairs of wires between the chipset and the memory sockets and 12 or 14
pairs of wires between the memory sockets and the chipset. Each pair of
wires use differential transmission, i.e. the signal is transmitted on
a wire and the same signal but inverted is transmitted on the other
wire from the pair, using the same idea used on twisted pair networking
cables. This is done to ensure that the receiver is getting the data
correctly. Read
this tutorial to understand how this kind of transmission works.
Another
important aspect of
FB-DIMM is
that it uses different paths for data transmission and data reception.
Standard DIMM modules use the same path to both transmit and receive
data. The system used on FB-DIMM modules helps to increase the
performance of the memory subsystem.
click to enlarge
Figure 1: How FB-DIMM modules work.
Initially
FB-DIMM modules will
use
DDR2 memory, migrating to DDR3 when this kind of memory becomes
available. It is very important to notice that FB-DIMM modules will
have the same physical size as DDR2-DIMM modules and will keep this
same size when DDR3 memories are released.
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