Hardware Reference
In-Depth Information
JEDEC helps to prevent that type of scenario for items such as memory by getting all the
memorymanufacturerstoworktogethercreatingsharedindustrystandardscoveringmemory
chips and modules. JEDEC-approved standards for memory could then be freely shared by
all the member companies, and no one single company would have control over a given
standard or any of the companies producing compliant components. FPM, SDRAM, DDR,
DDR2,andDDR3SDRAMareexamplesofJEDECmemorystandardsusedinPCs,whereas
EDO and RDRAM are proprietary examples. You can find out more about JEDEC standards
for memory and other semiconductor technology at
www.jedec.org
.
Because of variations on speeds (timing), voltage, and other issues, purchasing memory
matching the correct industry-standard type doesn't guarantee that it will work in a given
system.Alwaysbesurethememoryyoupurchaseworkswithyoursystemorthatyoucan
getarefundorreplacementifitdoesn't.Eventhoughindustrystandardsdoexist,allowing
modulesfrommanysourcestofitagivensystem,Inormallyrecommendthatyoulookfor
memory modules that the system or memory manufacturer has approved for the system.
Often you can find a list of approved modules or suppliers in the system documentation
or on the system or memory module manufacturer's website.
Speed and Performance
The speed and performance issues with memory are confusing to some people because of
allthedifferentwaystoexpressthespeedsofmemoryandprocessors.Memoryspeedwas
originally expressed in nanoseconds (ns), whereas the speeds of newer forms of memory
are usually expressed in megahertz (MHz) and megabytes per second (MBps) instead.
Processor speed was originally expressed in megahertz (MHz), whereas most current pro-
cessor speeds are expressed in gigahertz (GHz). Although all these different speed units
might seem confusing, it is relatively simple to translate from one to the other.
A
nanosecond
is defined as one billionth of a second—a short piece of time indeed. To
put some perspective on just how small a nanosecond really is, consider that the speed of
light is 186,282 miles (299,792 kilometers) per second in a vacuum. In one billionth of
a second (one nanosecond), a beam of light travels a mere 11.80 inches or 29.98 centi-
meters—slightly less than the length of a typical ruler!
Memory speeds have often been expressed in terms of their cycle times (or how long
it takes for one cycle), whereas processor speeds have almost always been expressed in
terms of their cycle speeds (number of cycles per second). Cycle time and cycle speed are
actually just different ways of saying the same thing; that is, you can quote chip speeds in
cycles per second, or seconds per cycle, and mean the same thing.
As an analogy, you could express the speed of a vehicle using the same relative terms.
In the United States vehicle speeds are normally expressed in miles per hour. If you were
