Computer memory

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 into the memory. This allows the CPU direct access to the program. Memory is a necessity for any computer.

Because the computer is an electrical device, it 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. The character codes are called ASCII (the American Standard Code for Information Interchange). 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 therefore a group of eight bits, which is called a byte. The computer's memory consists of thousands of bytes. To make it easier, the unit K 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 unit mega can be used. One megabyte equals 1 048 576 bytes.

The CPU calls instructions and data from the computer's memory. Because the same computer performs many tasks the memory is erasable (e.g. audio cassette tapes must be erasable to record different musical selections at different times). 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 (ROM)

The instructions and data are permanently fixed. They can be read and used by the CPU but cannot be changed by the computer user.

Random access memory (RAM)

It is also called volatile memory. The instructions and data are erasable. Therefore they can be defined by the computer user.