Hardware Reference
In-Depth Information
Note
A bank is the smallest amount of memory needed to form a single row of memory addressable
by the processor. It is the minimum amount of physical memory that the processor reads or
writes at one time and usually corresponds to the data bus width of the processor. If a
processor has a 64-bit data bus, a bank of memory also is 64 bits wide. If the memory runs
dual- 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. Systems might
have specific design limitations for the maximum capacity of module they can 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 controller signals to
pass directly to the memory chips on the module with no interference. This is not only the cheapest
design, but also the fastest and most efficient. The only drawback 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 onboard might be restricted on some systems in certain combinations.
Systems designed to accept extremely large amounts of RAM (such as servers) often require
registered modules. A registered module uses an architecture that has register chips on the module
that act as an interface between the actual RAM chips and the chipset. The registers temporarily hold
data passing to and from the memory chips and enable many more RAM chips to be driven or
otherwise placed on the module than the chipset could normally support. This allows for motherboard
designs that can support many modules and enables each module to have a larger number of chips. In
general, registered modules are required by server or workstation motherboards designed to support
more than four sockets. One anomaly is the initial version of the AMD Athlon 64 FX processor,
which also uses registered memory because its Socket 940 design was based on the AMD Opteron
workstation and server processor. Subsequent Socket 939, AM2, and Socket F versions of the Athlon
FX no longer require registered memory.
To provide the space needed for the buffer chips, a registered DIMM is often taller than a standard
DIMM.
Figure 6.13
compares a typical registered DIMM to a typical unbuffered DIMM.
