user manual

AN806: Memory Upgrade Resource Guide Page 2
A user that already has a pair of modules and adds a second pair should take care to
purchase a kit as similar as possible to the original to have the greatest chance of
successful integration. This ensures a high likelihood of success when running a system
with all four DIMM sockets populated.
The number of modules used can also have an effect on users attempting to utilize EPP or
XMP performance profiles. Some BIOS implementations will not allow the selection of these
options when running 4-up. Operation in the 4-up configuration is not a defined part of the
EPP or XMP standard, and users running 4-up should set their memory settings manually.
Overclocking
If you take a look at a current motherboard with overclocking options, you may see some
specifications similar to this:
 4 X 240-pin DIMM sockets supporting a maximum memory capacity of 8GB
 Supports dual channel DDR2 1066/800/667/533 Un-buffered Non-ECC memory
Many users interpret these specifications too liberally. A motherboard that can overclock to
a given speed with 2 modules cannot overclock to that same given speed with 4-up. Again,
this is due to the increased electrical load on the north bridge or memory controller and the
increased heat that accompanies it. So, many users are disappointed when they add
memory and their overclocking capability is reduced. They tend to fault the memory for this
limitation when it is actually a physical limitation of the memory controller.
A good analogy representing visualization of the above specifications is a passenger truck.
Let‟s assume that the maximum hauling capacity of the truck is 8000 pounds representing
8GB of memory. The top speed of the truck is 106.6 mph representing a memory speed of
DDR1066. However, this does not mean that the truck can haul 8000 pounds at 106.6mph.
The physical limitations are similar for a memory controller as 8GB and 1066 are both
tested maximums for this memory controller. When the memory controller speed is
increased for overclocking, it simply cannot manage the same amount of memory at that
overclocked speed for proper stable communication with the CPU.
Operating System
Currently there are several considerations for memory upgrades that relate to the operating
system. The most common is the use of a 32bit operating system versus a 64 bit operating
system
Memory usage in 32-bit operating systems
Since each binary bit in a 32-bit operating system can be used to represent the location of a
byte of memory, a 32-bit operating system has a maximum „memory address size‟ of 2
32