Hardware Reference
In-Depth Information
Memorymodulesofeachtypeandcapacityareavailableinvariousspeedratings.Consult
your motherboard documentation for the correct memory speed and type for your system.
If a system requires a specific speed memory module, you can almost always substitute
faster speeds if the one specified is not available. Generally, no problems occur in mixing
module speeds, as long as you use modules equal to or faster than what the system re-
quires. Because there's little price difference between the various speed versions, I often
buy faster modules than are necessary for a particular application, especially if they are
the same price as slower modules. This might make them more usable in a future system
that could require the faster speed.
Because SDRAM and newer modules have an onboard SPD ROM that reports their
speed and timing parameters to the system, most systems run the memory controller and
memory bus at the speed matching the slowest module installed.
Note
A
bank
is the smallest amount of memory needed to form a single row of memory address-
ablebytheprocessor.Itistheminimumamountofphysicalmemorythattheprocessorreads
or writes at one time and usually corresponds to the data bus width of the processor. If a pro-
cessor has a 64-bit data bus,a bank ofmemory also is 64bits wide. Ifthe memory runsdual-
or tri-channel, a virtual bank is formed that is two or three times the absolute data bus width
of the processor.
You can't always replace a module with a higher-capacity unit and expect it to work. Sys-
temsmighthavespecificdesignlimitationsforthemaximumcapacityofmoduletheycan
take. A larger-capacity module works only if the motherboard is designed to accept it in
the first place. Consult your system documentation to determine the correct capacity and
speed to use.
Registered Modules
SDRAM through DDR3 modules are available in unbuffered and registered versions.
Most PC motherboards are designed to use unbuffered modules, which allow the memory
controllersignalstopassdirectlytothememorychipsonthemodulewithnointerference.
Thisisnotonlythecheapestdesign,butalsothefastestandmostefficient.Theonlydraw-
back is that the motherboard designer must place limits on how many modules (meaning
module sockets) can be installed on the board, and it could limit how many chips can be
on a module. So-called double-sided modules that really have multiple banks of chips on-
board might be restricted on some systems in certain combinations.
Systems designed to accept extremely large amounts of RAM (such as servers) often re-
quire registered modules. A registered module uses an architecture that has register chips
