SDR SDRAM
SDR SDRAM is
Single Data Rate
synchronous dynamic random access memory, a type of computer memory. The
term is used to contrast with Double Data Rate SDRAM, or DDR SDRAM, but
since single data rate SDRAM was the only sort available when SDRAM was
introduced, is simply called "SDRAM", rather than "SDR SDRAM".
While other forms of DRAM have an
asynchronous interface, meaning that it reacts immediately to changes in
its control inputs, SDRAM has a synchronous interface, meaning that it
waits for a clock pulse before responding to its control inputs- it synchronizes
with the computer's system bus, and thus with the processor. The clock is
used to drive an internal finite state machine that can pipeline incoming
commands. This allows the chip to have a more complex pattern of operation
than plain DRAM.
Pipelining means that the chip can
accept a new command before it has finished processing the previous one. In
a pipelined write, the write command can be immediately followed by another
command without waiting for the data to be written to the memory array. In
a pipelined read, the requested data appears a fixed number of clock pulses
after the read command. It is not necessary to wait for the data to appear
before sending the next command. This delay is called the latency, and is
an important parameter to be considered when purchasing SDRAM for your
computer.
SDRAM for PCs is rated for its
potential maximum clock rate. Speeds included PC66, PC100, and PC133,
representing 66, 100 and 133 megahertz. There were additional unofficial
speeds for enthusiast SDRAM parts, such as PC150, for 150 MHz SDRAM comes
in 168-pin DIMMs for desktops, or 144-pin SODIMMs mainly used in laptops,
while DDR SDRAM comes in 184-pin DIMMs, and DDR2 uses a 240-pin
configuration.
SDRAM was introduced in 1997, and by
2000 had replaced all other types of DRAM in modern computers, because of
its greater speed, and much lower latency.
SDRAM is also available in registered
memory varieties, for systems that need greater scalability.
Currently, 168-pin SDRAM type is not
used in new PC systems, and PCs come with DDR or DDR2 SDRAM, with DDR2
quickly phasing out DDR.
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