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The computer memory is a temporary storage area. It holds the data and instructions that the Central Processing Unit (CPU) needs. Before a program can be run, the program is loaded from some storage medium into the memory. This allows the CPU direct access to the program. Memory is a need for any computer.
A computer is usually an electrical device, which understands only electricity on and electricity off. This is expressed by using two symbols – 0 and 1 – which are called binary digits or bits. Numbers and text characters are represented as codes, which are made up of combinations of 0s and 1s. Simple character codes are called ASCII (the American Standard Code for Information Interchange), and Unicode. In ASCII, eight bits – any combination of 0s and 1s – form one character or symbol. For example, the letter A is denoted by the code 01000001. The basic working unit of the computer's memory is a group of eight bits, which is called a byte.
The computer's memory consists of many millions of bytes. To make it easier, the unit K (for kilobytes) can be used to express memory capacity. One K equals 1,024. For example, 64K bytes of memory is the same as 65,536 (1,024 × 64 = 65,536) bytes. For larger memory capacities, the units mega and giga can be used. One megabyte of computer memory usually means 1024 kilobytes, which is 1,048,576 bytes, whereas one gigabyte means 1024 megabytes, which is 1,073,741,824 bytes.
The CPU calls instructions and data from the computer's memory. Because the same computer performs different tasks at different times, the memory is erasable—much like audio cassette. But there are some programs and instructions which the computer needs. It does not matter what function you are performing. These programs often are permanently recorded in the memory. So they cannot be destroyed. As a result, the computer's memory usually consists of two parts:
Read only memory
Read only memory (ROM) is the permanent memory which is used to store important control programs and systems software to perform a variety of functions, such as booting up or starting up programs. ROM is non-volatile. That means the contents are not lost when the power is switched off. Its contents are written at the time of manufacture, but in modern (2012) computers may be changed using special software.
Random access memory
Random access memory (RAM) is used as the working memory of a computer system. It stores input data, intermediate results, programs, and other information temporarily. It can be read and written. It is usually volatile, that is all data will be lost when the power is turned off. In most cases it is loaded again from hard disk.