Types of Main Memories

The memory unit-communicates directly with processor is called the main memory. Main memory is sometimes called semiconductor memory, since storage cells are made out of semi-conducting material. These memories are available in integrated circuit form called memory ICs or silicon chips of memory. Main memory holds the run time or execution time program instruction and data that are essential to execute or carry out the specified word or task. Main memories are classified into 2 types. They are:

1) Random Access Memory (RAM)
2) Read Only Memory (ROM)

Random Access Memory:

RAM is also called as read/write memory since it permits both memory read and memory write operation. Therefore programs and data can be stored into the RAM. Similarly the contents of RAM can be erased soon after the completion of current program execution. Results of computation can be written on to it. It is just like a scratch pad where information can be stored and erased. RAM is a volatile memory i.e., the stored data and programs will be erased, immediately when the power supply is switched off. It is a temporary memory and holds the information as long as there is electric power but looses stored data when power supply is cut off.

Read Only Memory:

ROM, as the name itself indicates is a memory that performs the read operation only. It will not permit writing operation. Hence the contents of ROM cannot be altered. The binary information stored in ROM is made permanent during the hard ware productions of the ROM unit and cannot be modified or erased by writing different data words into it.

ROM is a non-volatile memory i.e, contents ROM will not disappear when the power is switched off. The binary data stored in ROM during its hardware production is fused or mosted such that it always permanent irrespective of presence or absence of the supplied electric energy.

Therefore ROM memory chips are used to store permanently the system programs, operating system programs, controls words etc., that are essential for the correct working computer.

Whereas program to be executed and data to be processed are brought into RAM as and when necessary; computer reads program instructions and data from RAM during execution and stored the computed results back on to RAM.

Therefore RAM & ROM being main memories constitute the essential storage unit of a computer without which the user’s program cannot be executed especially in stored program computers.

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