Datasheet

Identifying Purposes and Characteristics of Memory
43
or marketability to computer manufacturers. The following list gives you an idea of the vast
array of memory types and subtypes:
DRAM
ßN
Asynchronous DRAM
ßN
FPM DRAM
ßN
EDO DRAM
ßN
BEDO DRAM
ßN
Synchronous DRAM
ßN
SDR SDRAM
ßN
DDR SDRAM
ßN
DDR2 SDRAM
ßN
DDR3 SDRAM
ßN
DRDRAM
ßN
SRAM
ßN
ROM
ßN
Pay particular attention to all synchronous DRAM types. Note that the type of memory
does not dictate the packaging of the memory. Conversely, however, you might notice one
particular memory packaging holding the same type of memory every time you come across
it. Nevertheless, there is no requirement to this end. Lets detail the intricacies of some of
these memory types.
DRAM
DRAM is dynamic random access memory. (This is what most people are talking about
when they mention RAM.) When you expand the memory in a computer, you are add-
ing DRAM chips. You use DRAM to expand the memory in the computer because it’s
a cheaper type of memory. Dynamic RAM chips are cheaper to manufacture than most
other types because they are less complex. Dynamic refers to the memory chips’ need for a
constant update signal (also called a refresh signal) in order to keep the information that is
written there. If this signal is not received every so often, the information will bleed off and
cease to exist. Currently, the most popular implementations of DRAM are based on syn-
chronous DRAM and include SDR SDRAM, DDR, DDR2, DDR3, and DRDRAM. Before
discussing these technologies, let’s take a quick look at the all-but-defunct asynchronous
memory types.
Asynchronous DRAM
Asynchronous DRAM is characterized by its independence from the CPUs external
clock. Asynchronous DRAM chips have codes on them that end in a numerical value that
is related to (often one tenth of the actual value) the access time of the memory. Access
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