Datasheet

Identifying Purposes and Characteristics of Memory 47
The most commonly found range of actual clock speeds for DDR3 tends to be from
133MHz at the low end to 250MHz. Because double-pumping continues with DDR3 and
because four operations occur at each wave crest (8 operations per cycle), this frequency
range translates to common FSB implementations from 1066MHz to 2000MHz in DDR3
systems. Naming these memory devices follows the conventions established earlier. There-
fore, if you buy a motherboard with a 1600MHz FSB, you know immediately that you need
a memory module populated with DDR3-1600 chips because the chips are always named
for the FSB speed. Using the 8:1 module-to-chip/FSB naming rule, the modules you need
would be called PC3-12800, supporting a 12800MBps throughput.
The earliest DDR3 chips, however, were based on a 100MHz actual clock signal,
so we can build on our earlier example, which was also based on an actual clock rate
of 100MHz. With 8 operations per cycle, the FSB on DDR3 motherboards is rated at
800MHz, quite a lot of efficiency while still not needing to change the original clock our
examples began with. Applying the 8:1 rule again, the resulting RAM modules for this
motherboard are called PC3-6400 and support a throughput of 6400MBps, carrying chips
called DDR3-800, again named for the FSB speed.
Choosing the Right Memory for Your CPU
Let’s say you head down to your local computer store, where motherboards, CPUs,
memory, and other computer components are sold a la carte. You’re interested in putting
together your own system from scratch. Usually, you will have a CPU in mind that you
would like to use in your new system. Assuming you choose, for example, an Intel Core 2
Quad Q8200 processor, you discover you need a motherboard that has an LGA 775 socket
and supports a frontside bus of 1333MHz. You could assume you need DDR3 memory
because DDR2 tops out around 1066MHz (PC2-8500). If you’d rather not assume, you
can consult the chosen motherboard’s documentation or display slick and confirm your
suspicions. If you’re right, you’ll be buying at least one stick of your favorite capacity of
PC3-10666 (multiplying 1333 by 8), two sticks if you need to feed a hungry dual-channel
motherboard. In case you missed it, PC3-10666 modules are made using DDR3-1333 chips,
so named for the speed of the FSB. Recall the 8:1 module-to-chip/FSB naming convention.
DRDRAM
Direct Rambus DRAM (DRDRAM), named for Rambus, the company that designed
it, is a proprietary SDRAM technology, sometimes called RDRAM, dropping “direct.”
DRDRAM can be found in fewer new systems today than just a few years ago. This is
because Intel once had a contractual agreement with Rambus to create chipsets for the
motherboards of Intel and others that would primarily use DRDRAM in exchange for
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