Datasheet

Identifying Purposes and Characteristics of Memory
53
As mentioned earlier, DRDRAM is based on a 16-bit channel. However, dual-channel
implementation is required with DRDRAM; it’s not an option. The dual-channel archi-
tecture can be implemented utilizing two separate 16-bit RIMMs (leading to the generally
held view that RIMMs must always be installed in pairs) or the newer 32-bit single-module
design (not doing much to dispel the “pair” view, despite the facts). Typically, mother-
boards with the 16-bit single- or dual-channel implementation provide four RIMM slots
that must be filled in pairs, while the 32-bit versions provide two RIMM slots that can be
lled one at a time. A 32-bit RIMM essentially has two 16-bit modules built in (possibly
contributing to the persistence of the “pair” view) and requires only a single motherboard
slot, albeit a physically different slot. So you must be sure of the module your motherboard
accepts before upgrading.
Unique to the use of RIMM modules, a computer must have every RIMM slot occu-
pied. Even one vacant slot will cause the computer not to boot. Any slot not populated
with live memory requires an inexpensive blank of sorts called a continuity RIMM, or
C-RIMM, for its role of keeping electrical continuity in the DRDRAM channel until
the signal can terminate on the motherboard. Think of it like a fusible link in a string
of holiday lights. It seems to do nothing, but no light works without it. However, 32-bit
modules terminate themselves and do not rely on the motherboard circuitry for termi-
nation, so vacant 32-bit slots require a module known as a continuity and termination
RIMM (CT-RIMM).
SODIMM
Notebook computers and other computers that require much smaller components don’t use
standard RAM packages, such as the SIMM or the DIMM. Instead, they call for a much
smaller memory form factor, such as a small outline DIMM. SODIMMs are available in
many physical implementations, including the older 32-bit (72- and 100-pin) configura-
tion and newer 64-bit (144-pin SDR SDRAM, 200-pin DDR/DDR2, and 204-pin DDR3)
configurations.
All 64-bit modules have a single keying notch. The 144-pin module’s notch is slightly off-
center. Note that although the 200-pin SODIMMs for DDR and DDR2 have slightly different
keying, it’s not so different that you don’t need to pay close attention to differentiate the two.
They are not, however, interchangeable. Figure 1.34 shows an example of a 144-pin, 64-bit
module. Figure 1.35 is a photo of a 200-pin DDR2 SODIMM.
MicroDIMM
A newer, and smaller, RAM form factor is the MicroDIMM. The MicroDIMM is an
extremely small RAM form factor. In fact, it is over 50 percent smaller than a SODIMM,
only 45.5 millimeters (about 1.75 inches) long and 30 millimeters (about 1.2 inches—a bit
bigger than a quarter) wide. It was designed for the ultralight and portable subnotebook
style of computer. These modules have 144 pins or 172 pins and are similar to a DIMM
in that they use a 64-bit data bus. Often employed in laptop computers, SODIMMs and
MicroDIMMs are mentioned in Chapter 4 as well.
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