Hardware Reference
In-Depth Information
takes time and might be difficult, particularly after you have a problem with a system.
Caution
If your system supports dual- or tri-channel memory, be sure you use the correct memory
sockets to enable multichannel operation. Check the documentation to ensure that you use the
correct sockets. Most multichannel systems still run in single-channel mode if the memory is
not installed in a way that permits full multichannel operation, but performance is lower than if
the memory were installed properly. Some systems provide dual-channel support if an odd
number of modules is installed, as long as the total capacity of two modules installed in one
channel equals the size of the single module in the other channel and all modules are the same
speed and latency. Again, check your documentation for details.
Memory Module Speed
When you replace a failed memory module or install a new module as an upgrade, you typically must
install a module of the same type and speed as the others in the system. You can substitute a module
with a different (faster) speed, but only if the replacement module's speed is equal to or faster than
that of the other modules in the system.
Some people have had problems when “mixing” modules of different speeds. With the variety of
motherboards, chipsets, and memory types, few ironclad rules exist. When in doubt as to which speed
module to install in your system, consult the motherboard documentation for more information.
Substituting faster memory of the same type doesn't result in improved performance if the system still
operates the memory at the same speed. Systems that use DIMMs or RIMMs can read the speed and
timing features of the module from a special SPD ROM installed on the module and then set chipset
(memory controller) timing accordingly. In these systems, you might see an increase in performance
by installing faster modules, to the limit of what the chipset supports.
To place more emphasis on timing and reliability, there are Intel and JEDEC standards governing
memory types that require certain levels of performance. These standards certify that memory
modules perform within Intel's timing and performance guidelines.
The same common symptoms result when the system memory has failed or is simply not fast enough
for the system's timing. The usual symptoms are frequent parity check errors or a system that does not
operate. The POST might report errors, too. If you're unsure of which chips to buy for your system,
contact the system manufacturer or a reputable chip supplier.
See “
Parity Checking
,
”
p.
359
.
Parity and ECC
Part of the nature of memory is that it inevitably fails. These failures are usually classified as two
basic types: hard fails and soft errors.
The best understood are hard fails, in which the chip is working and then, because of some flaw,
physical damage, or other event, becomes damaged and experiences a permanent failure. Fixing this
type of failure normally requires replacing some part of the memory hardware, such as the chip,
SIMM, or DIMM. Hard error rates are known as HERs.
