RAM stands for Random Access Memory. Actually the main memory or primary storage is referred to as RAM. It consists of memory chips that can be read from and writtern to by the processor. When the computer is powered on, certain operating system files load into RAM from the storage device such as a hard disk. Then operating system remains in RAM as long as the computer has continuous power.

The control unit performs the following functions for main memory.

• Stores the data and instructions (entered from input unit) into the RAM (or main memory).
• Provides the data and instructions from main memory to other parts of the computer.
• Stores the processed results back into the RAM (or main memory).

The RAM plays very important role in data processing. The program and the data must be transferred to RAM before to run the program to process the data. The processor processes the data according to the program instructions by fetching the data and instructions from the RAM. The processor interprets and executes the program instructions while the program is in RAM. During this time the content of RAM may change. RAM can hold multiple programs simultaneously, provided the computer has enough RAM to accommodate all the programs. The program with which you are working usually is displayed on the screen. In Windows environment you can switch from one program to another.

 The storage capcity of RAM or main memory is measured in bytes. A byte is a group of 8-bits. One character takes one byte of memory. The amount of data and program size are also measured in bytes.

The large RAM size increases the accessing speed of the computer. RAM plays an integral part in the internal operation of processor. Microcomputers come with different amounts of RAMs which are from 128MB to 2GB. Advertisements normally list the type of processor, the clock speed of the processor and the amount of RAM in the computer. More RAM means the computer can use powerful program with large size. It also improves the data processing speed of the computer.

Memory Module

RAM chip is smaller in size than the processor chip. A RAM consists of more than one chips. RAM chips usually reside on a memory module which is a small circuit board. Memory slots on the motherboard hold memory modules. There are three types of memory modules. There are:

1. DRAM

DRAM stands for Dynamic Random Access Memory. DRAM is the most common type of RAM used to store data & instructions. In order to maintain data in DRAM chip, the chip is refreshed frequently (hundreds of time a second), otherwise data may be lost. During the refreshing process the CPU has to wait to read & write data in DRAM. It is because DRAM decreases the processing speed of the computer.

Today many DRAM chips exists and most of which are faster than the basic DRAM. The most important are:

SDRAM

SDRAM stands for Synchronous Dynamic RAM. SDRAM are much faster than basic DRAM chips because they are synchronized to the system clock. Often in computer ads, the speed of SDRAM is expressed in megahertz (MHz).

RDRAM

RMRAM stands for Rambus Dynamic RAM. It is faster and more expensive than SDRAM because it uses pipelining technique. It is used in Intel P4. Today most of PCs use SDRAM or RDRAM.

2. SRAM

SRAM stands for Static Random Access Memory. It is faster than DRAM because it does not have to be refreshed frequently and the CPU has not to wait to read & write data. SRAM chips however are more expensive than DRAM chips. Special applications such as cache use SRAM chips.

ROM stands for Read Only Memory. The ROM contains instructions that are permanently stored by the manufacturers when they manufacture the chips. In fact recording data and instructions permanently into this kind of memory is called “burning in the data”. The instructions stored in ROM can only be read but cannot be modified. This is the reason why it is called Read Only Memory.

ROM is a semiconductor chip programmed at the time of its manufacture and is not re-programmable by the user. The programs stored in ROM are called firmware. It is a non-volatile memory. The contents of ROM are not lost when the computer is turned off.

The ROM contains the Basic Input / Output System (BIOS which is a set of instructions that are automatically activated when the computer is turned on. It means that computer uses the instructions of ROM at the time of booting for the following purposes.

• To check different units of computer system.
• To load the operating system into computer memory etc.

Many other devices also contain ROM chips. For example a printer has a ROM chip that contains data or information for fonts. You can say that each electronic device has ROM chip that contains information about that device.

The ROM is further divided into:

1. PROM
2. EPROM
3. EEPROM

1. PROM

PROM stands for Programmable Read Only Memory. PROM is a blank ROM chip on which the user can write his own program instructions and data but only once. However once the program or data is written in PROM chip, it cannot be changed. The programmer uses micro-code instructions to write information in a PROM chip. Once the programmer writes the micro-code on the PROM chip, it functions like a normal ROM chip.

2. EPROM

Once ROM or PROM is programmed, its contents cannot be changed. However there is another type of memory chip called EPROM that overcomes this problem.

EPROM stands for Erasable Programmable Read Only Memory. It is another important form of read only memory. Like PROM, it is initially blank and the user or manufacture can writes his own program or data by using special devices. Unlike PROM the data written in EPROM chip can be erased by using special purpose devices and ultraviolet rays. So program or data written in EPROM chip can be changed and new data can also be added on this form of ROM. When EPROM is in use, its contents can only be read.

3. EEPROM

EEPROM stands for Electrically Erasable Programmable Read Only Memory. This kind of ROM can be re-written by using electrical devices and so data stored on this ROM chip can be easily modified.