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Static Random Access Memory

Static Random Access Memory (SRAM) is a type of semiconductor memory.

The word "static" indicates that the memory retains its contents as long as power is applied, unlike dynamic RAM (DRAM) that needs to be periodically refreshed.

Each bit in an SRAM is built from a pair of transistors that form a latch[?], and it is this structure that allows the value of the bit to be read much faster than in a DRAM.

Random access indicates that any location in the memory can be accessed, i.e. written or read, at the same speed regardless of the previous location that was accessed.

SRAM should not be confused with SDRAM, which stands for synchronous DRAM and is entirely different from SRAM, or with pseudostatic RAM (PSRAM), which is DRAM disguised as SRAM.

Table of contents

Types of SRAM

By Transistor Type

  • bipolar (not much used now: consumes a lot of power but is very fast)
  • CMOS (the commonest type)

By Function

  • asynchronous
  • synchronous

Applications

Fast SRAM is faster than DRAM and is used where speed is the most important requirement, as in the cache of a CPU and in digital signal processing circuits. Slow, low-capacity SRAMs are used where low power consumption and low cost are the most important requirements, as in battery-powered backup RAM. SRAM is less dense than DRAM (fewer bits per unit area) and is therefore not suitable for high-capacity, low-cost-per-megabyte applications such as PC extended memory[?].

The power consumption of SRAM varies widely depending on its speed. Fast SRAM is much more power-hungry than DRAM, and some ICs can consume power of the order of a watt at full speed. Slow SRAM, such as the battery-powered "CMOS" RAM on PC motherboards, can have a very low power consumption, in the region of a microwatt.



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