Hardware Reference
In-Depth Information
Figure 6.13. A typical registered DIMM is taller than a typical unbuffered DIMM to provide
room for buffer chips.
Tip
If you are installing registered DIMMs in a slimline case, clearance between the top of the
DIMM and the case might be a problem. Some vendors sell low-profile registered DIMMs that
are about the same height as unbuffered DIMMs. Use this type of DIMM if your system does
not have enough head room for standard registered DIMMs. Some vendors sell only this type
of DIMM for particular systems.
The important thing to note is that you can use only the type of module your motherboard (or chipset)
is designed to support. For most, that is standard unbuffered modules or, in some cases, registered
modules.
SDR DIMM Details
SDR DIMMs use a completely different type of presence detect than SIMMs, called
serial presence
detect
(SPD). It consists of a small EEPROM or flash memory chip on the DIMM that contains
specially formatted data indicating the DIMM's features. This serial data can be read via the serial
data pins on the DIMM, and it enables the motherboard to autoconfigure to the exact type of DIMM
installed.
DIMMs can come in several varieties, including unbuffered and buffered as well as 3.3V and 5V.
Buffered DIMMs have additional buffer chips on them to interface to the motherboard. Unfortunately,
these buffer chips slow down the DIMM and are not effective at higher speeds. For this reason, most
PC systems (those that do not use registered DIMMs) use unbuffered DIMMs. The voltage is simple
—DIMM designs for PCs are almost universally 3.3V. If you install a 5V DIMM in a 3.3V socket, it