MARCH 2004 MAXIMUMPC 3
DDR (double data-rate) SDRAM, now
the most common type of memory, per-
forms two operations per clock cycle, so
it can transfer data much more quickly
than SDRAM. For example, DDR400 runs
at a clock speed of 200MHz, but with
two operations per clock cycle the effec-
tive speed of the memory is 400MHz.
DIMM modules that use DDR chips are
usually referred to by their throughput.
For example, DDR400 memory has a
throughput of 3.2MB/second, so memory
modules using DDR400 RAM are usually
referred to as PC3200 modules.
Some systems use Rambus DRAM
(RDRAM) modules (RIMMs). Unlike
SDRAM or DDR SDRAM modules, which
perform parallel data transfers over a
64-bit data bus, RIMMs perform serial
data transfers over a 16-bit data bus.
Typical RIMMs contain up to 32 devices
(chips), and data transfer takes place
from chip to chip within the RIMM. Until
recently, the most common RIMM was
the RIMM1600 (also called PC800) 16-bit
RIMM. However, some recent chipsets,
such as those from SiS, support faster
and wider RIMMs. Currently, the most
popular RIMM runs at 600MHz and com-
bines two channels into a single 32-bit
module. It’s referred to as RIMM4800 for
its 4.8MB/second throughput.
RIMMs, unlike standard DIMMs, have
always used heat spreader plates to keep
the memory cool. Although RIMMs have
a much faster clock speed than DIMMs,
their real-world memory performance is
only slightly better in most cases, and
with the traditionally much higher price
of RDRAM memory compared with DDR
SDRAM, systems using RIMMs aren’t all
that popular. (See RAM stats on page 55
for details.)Understanding
dual-channel memory
Dual-channel memory designs are now
widespread, thanks to chipsets from
nVidia (nForce series), Intel (865 and 875
series), VIA (PT880), and the memory
controller built into the AMD Athlon 64
FX and Opteron processors. Dual-channel
doesn’t refer to a different type of mod-
ule, but to how memory is accessed.
Although systems with dual-channel
chipsets can use just one DDR DIMM,
their performance is slower than if you
use two identical DIMMs and enable
dual-channel mode. Why are dual-channel
designs faster? As discussed earlier, the
process of accessing a particular row of
memory takes several clock cycles from
start to finish. A dual-channel design can
access each channel as soon as it is ready
to provide the memory location desired,
which reduces delays.
Although some vendors sell matched
modules that have been tested as a unit,
you don’t need to buy a pair of modules
to upgrade an existing dual-channel sys-
tem that has just one module. You can
buy another module with the same size
and memory timing parameters (prefer-
ably the same brand/model) as your
current module. If you don’t know the
brand and model of the memory in your
PC, use a utility such as SiSoft Sandra(www.sisoftware.co.uk) to determine
the brand and model of memory
installed. You can also switch your
memory setting in your system BIOS
from “by SPD” to “User-defined” to
see what your current module uses
for its default settings.
If your PC supports dual-channel
memory and uses RIMMs, check the
memory socket type to learn whether
you need to add two modules at a
time. A RIMM with 184 pins supports
one channel onboard, so an identi-
cal pair is needed for dual-channel
support. 242-pin RIMMs contain two
channels in a single module, so one
module does the trick for dual-chan-
nel implementations, and two mod-
ules can provide four channels for
high-end chipsets such as the new
SiS R659.RAM Decoder
Here’s how to understand common
abbreviations and terms you’ll see
in memory ads and spec sheets:
2-3-3-7-1T: CAS Latency, RAS
to CAS Delay, RAS Precharge, Active
to Precharge, Command Rate.
16x64: The first number (16)
is the size of each memory chip on
the module (16Mbit). The second
number (64) is the width of the
module in bits. Multiply the size of
chip by the width of the module to
get the size of the module in Mbits.
Then divide by 8 to convert Mbits
to MB. A 64-bit wide module has
no parity or ECC bits, so it does no
error checking or correcting.
32x72: This represents a
256MB module (32x8) with extra bits
for error correction. This module
supports ECC, but the additional bits
are ignored if the chipset doesn’t
support ECC, or if ECC is disabled in
the BIOS setup program.
Single-banked: RAM mod-
ules that have just one bank.
TSOP: Thin small outline pack-
age refers to the packaging used in
most RAM modules today.
CSP: Chip scale package refers
to packaging that uses a ball grid
array instead of pins to connect the
chip to the board.èFigure 3: It’s definitely the least sexy spec to look at, but these numbers
(often found on the packaging or RAM module itself) express exactly what
your memory is capable of. Check out our table on page 52 to find out how
each value affects your memory speed.