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static RAM

A fast memory technology that requires power to hold its content. Static RAM (SRAM, S-RAM) is used for high-speed registers, caches and relatively small memory banks such as a frame buffer on a graphics card. In contrast, the main memory in a computer is typically dynamic RAM (DRAM). Static RAM chips have access times in the 10 to 30-nanosecond range, while dynamic RAM is usually above 50 ns.

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Static RAM is fast because the six-transistor configuration of its pretzel-like flip-flop circuits keeps current flowing in one direction or the other (0 or 1). The 0 or 1 state can be written and read instantly without waiting for a capacitor to fill up or drain; however, the six transistors take more space than dynamic RAM cells made of one transistor and one capacitor (see dynamic RAM).

Earlier asynchronous static RAM chips performed read and write operations sequentially. Newer synchronous static RAM chips overlap reads and writes.




A Static RAM Cell
When opposite voltages are applied to the column wires, the flip-flop is oriented in one of two directions for a 0 or 1. At that point, the flip-flop (yellow center) becomes a self-perpetuating storage cell as long as a constant voltage is applied.






dynamic RAM

The most common type of computer memory and generally available today in the form of synchronous dynamic RAM chips (see SDRAM). Most all volatile memory is dynamic RAM because it uses only one transistor and one storage capacitor for each bit. It is denser and much less costly than "static RAM," its faster counterpart.

Dynamic RAM Is a Total Loser
Unlike non-volatile firmware chips (flash, ROM, EEPROM, etc.), which hold their contents when the power is turned off, both dynamic RAM (DRAM) and static RAM (SRAM) require constant power.

The capacitors in a dynamic RAM chip are electrical storage tanks that do a poor job of holding a charge. They constantly leak, and the memory chip would lose its content were it not for the refresh circuitry that continuously re-energizes the capacitors approximately 15 times per second.

In 1968, dynamic RAM was patented by IBM, and the first commercial chips came from Intel and Mostek in the early 1970s. See static RAM and memory types.




A Dynamic RAM Cell
DRAM cells are very simple. The combination of voltage on the row and column lines charges a capacitor. The only problem is that the capacitors keep losing their charges, and the bits must be read and re-written to the same state several times each second.










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