The physical equipment that makes up a computer system is called hardware. The two most important pieces of hardware are the primary memory and the processor. The primary memory, sometimes called the main memory, stores information and programs in the computer. The processor in a personal computer is a microprocessor. It carries out programs and transforms information. Adding or subtracting numbers, arranging text, and producing pictures and sounds are all ways the processor transforms data. A processor works very fast. It can carry out more than 5 million logical operations in a single second. Equipment other than the processor and primary memory is called peripheral hardware, and the individual devices are sometimes called peripherals. Peripheral hardware includes input devices, output devices, secondary memories, and communication devices. Input devices are used for entering data and programs into the computer. A keyboard for typing words and numbers--and thus entering them into the computer--is one of the most common input devices. A mouse can also be used to give commands to a computer. When this handheld box is moved on a flat surface, it causes a pointer to point at a specific instruction or other data displayed on a computer screen. Clicking a button on the mouse causes the instruction to be carried out or the data to be selected for use elsewhere. Other input devices include a joystick for moving figures about on a screen and a graphic tablet consisting of a pad and a "wired" pen for producing illustrations. Output devices let a person get information from the computer. They include a monitor (television screen) for showing text and pictures, a printer for producing data on paper, a plotter for making drawings, and a speaker for producing sounds. A secondary memory, also called an auxiliary memory or mass storage, is used for storing data and programs for long periods of time. Secondary memories are generally bigger and less expensive--but slower--than the main memory, which is built into the computer itself. The two chief kinds of secondary memory are magnetic disks and magnetic tapes. The disks are much faster than the tapes. Some disks, called floppy disks or diskettes, are made of flexible material and can be removed from the disk drives that operate them. Diskettes can store about 1 million characters (letters or numbers). Other disks, called hard disks, hold tens of millions of characters and generally are not removable. They are often installed in the same case as the processor. Hard disks are more expensive than floppy disks, but they are faster and more convenient. All the computer's programs and other data can be kept on a hard disk so that they can all be used without having to change disks. Communication devices connect computers to one another. These devices include modems, which connect a computer to a telephone. Modems enable a computer to transmit data to other computers via telephone lines or other communications networks, and to receive data from distant computers. Communication devices called local area networks connect computers in the same building directly to one another. They provide much faster communication than do modems.