SDRAM

Encyclopedia : S : SD : SDR : SDRAM

Several SDRAM ICs on a DIMM package.

SDRAM is synchronous dynamic random access memory, a type of DRAM, or solid state computer memory.

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 signal 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 1996, 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.

See also

• Random access memory (RAM)
• Static RAM (SRAM)
• Non-Volatile RAM (NVRAM)
• Dynamic RAM (DRAM)
• *Fast Page Mode DRAM
• *EDO RAM or Extended Data Out DRAM
• *XDR DRAM
• **DDR SDRAM or Double Data Rate Synchronous DRAM; now being replaced by DDR2
• **RDRAM or Rambus DRAM

External links

• [How RAM works]
• [What is RAM]

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