MAIN & RANDOM ACCESS MEMORY (RAM)

MAIN MEMORY

Main memory is the computer's primary storage. It is an extenstion of the Central Processing Unit (CPU) and is directly accessible to it. Its function is controlled by the Control Unit (CPU), which sends READ or WRITE signal for the appropriate memory location. Main memory accepts data and instructions from the input unit, exchanges data with the supplies instructions to other parts of the Central Processing Unit (CPU). This memory unit also stores those instructions and data, which are currently being used by the CPU or those instructions and data, awaiting immediate execution. Its capacity must be sufficient to retain both the usable amount of data and instructions needed for processing it. Some operations require more instructions or data than can be held is main memory at one time. In such cases, main memory is supplemented with auxiliary storage.

The capacity of a storage device is expressed as number of bytes, characters or bits. One byte is a group of eight bits that forms one character. Thus if the memory of a computer is 32000 bytes, it means that it can sote 32000 characters. Semiconductor storage (intergrated circuits) and magnetic cores are the most widely used forms of main storage.

Besides reduced size increased speed, semiconductor storage offers the advantage of non-destructive read. There are various storage types available, which include RAM, ROM, PROM, EROM, and EAROM. The most common kind of semiconductor storage is RAM (Random Access Memory) and ROM (Rad Only Memory).

RANDOM ACCESS MEMORY (RAM)

RAM is a memory in which each element of information has its own address (location) and from which any element can be retrieved easily and quickly by using that address. It is a temporary memory of a computer used to store the data and the instructions of the user when the computer is on. It is also called as Read-And-Write Memory (Ram) since the computer can store (write) data at any selected location (address) and at any subsequent time, retrieve (read) the data. The contents are lost when the power to the computer is shut off. Thus RAM is active only when computer is on and also referred as a volatile memory because the data stored in it disappears whenever he computer is switched off. However the RAM has the advantage of much better access time, higher transfer rate and simple interfacing. Generally RAM is meausured in K bytes or M bytes. Large RAM sizes provides larger amount of information which a computer can hold for processing and hence increases the processing speed.

Unlike magnetic secondary storage devices, such as tape as disk, RAM (primary storage) has no moving parts. RAM, a read-and write- memory, enables data to be both read and written to memory. Data can be accessed from RAM without any mechanical movement at electronic speed, close to the spee of light. Most of today's computers use CMOS (Complementary Metal-Oxide Semiconductor) technology for RAM. A CMOS memory chip less than one fourth the size of a postage stamp can store more than 4,000,000 bits, or more than 400,000 characters of data. Physically, memory chips are installed on single in-line memory modules or SIMMs. SIMMs are circuit boards with links directly to the processor.

The most common RAM technologies are dynamic RAM (DRAM) and static RAM (SRAM). DRAM is used in more computer than SRAM. However, SRAM is faster than DRAM because it does not have to be refreshed hundreds of time a second like DRAM. It is also much more expensive and requires more space and power than does DRAM.

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