RAM

The read and write memory (R/W memory)of a computer is called RAM.The users can write information into RAM  and read information from it.It is accessible to users.The user enters his program and data into RAM.It possesses random access property.In a random access memory any memory location can be accessed in a random manner without going through any other memory location.The accessed in a random manner without going through any other memory location.The access time is same for each memory location. RAM is a voltatile memory.The information written into it is retained in it as long as the power supply is on.As soon as the power supply goes off (or interrupted)its stored information is lost.The programmer has to reload his program and the data into the RAM when  the power supply is resumed.

RAM is not the correct name of R/W memory because both RAM and ROM possess random access property.But R/W memory has been called RAM since long and in practice it has become an established name.Thus R/W memory is commonly called RAM.

There are two important types of RAMs: static RAM and dynamic RAM. Static RAMs retain stored information only as long as the power supply is on.But a dynamic RAM loses its stored information in a very short time( a few milliseconds) even through the power supply is on.In a dynamic RAM a binary state is stored on the gate to source stray capacitor of a transistor of the MOS memory circuit.The presence of charge on the stray capacitor represents 1,and the absence of charge on the stray capacitor represents 0.The charge on these capacitors leak away as a results of normal leakage in a few milliseconds.Therefore,dynamic RAMs have to be refreshed periodically,generally every 2  milliseconds.The dynamic RAMs are cheaper and have high packing density and moderate speed. They consume less power.They are used where large capacity of memory is needed.Static RAMs are costlier and consume more power.They do not need refreshing circuitry.They have higher speed than dynamic RAMs.Static RAM dynamic RAM are also written as SRAM and DRAM respectively.