RANDOM ACCESS MEMORY

Random Access Memory or RAM semiconductor memories are the most widely used form of electronic memory found in microprocessors. Two basic types of RAM have been evolved in these systems. Dynamic RAM is noted for high capacity, moderate speeds, and low-power consumption. A memory cell of this type of circuit is achieved by charge-storage capacitors with drive transistors. The presence or absence of charge in a capacitor is interpreted by the RAM’ sense line as a logical 1 or 0). Because of a natural tendency for a charge to discharge itself into a lower energy-state configuration, dynamic RAM requires periodic charge refreshing in order to maintain data storage. Traditionally, the charge refreshing function of a RAM means that this type of system needs additional circuitry in order to perform its memory function.

Static RAM is an alternative to dynamic RAM. Static RAM is designed to store 1s and)s using traditional flip-flop gate configurations. This type of memory is faster and requires no refresh circuitry for continuous operation. An operator simply addresses the static RAM and, after a very brief delay, obtains the stored information from a specific location. Static RAM is easier to use and to design circuitry around, but the complexity of a memory cell occupies a great deal more space than its dynamic counterpart. Static memory cells are also classified as volatile memory. With the energy source removed or turned off momentarily, a RAM will lose its memory.

Many RAMs employ a single metal-oxide semiconductor (MOS) transistor for each memory location.

Binary information is stored in the transistor as a charge on a small capacitor. No charge across the transistor gate-channel electrodes indicates a 0 state, and a

charge appearing across the two electrodes represents a 1. When a row select line is activated, it energizes the gate of each transistor in the entire row. When a column line is selected, it energizes the source-drain electrodes of each transistor in the column. Simultaneous activation of a row and column energizes a specific transistor memory cell.

A charge placed on a discrete MOS transistor of a memory cell may be restored periodically in order to overcome leakage. Charge regeneration is generally achieved by a special transistor amplifier. In practice, charge regeneration occurs

every few milliseconds on a continuous basis. To write a word into the memory, a specific address is first selected according to the data supplied by the address bus.

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