| B | Computer Memory |
To process information electronically, data are stored in a computer in the form of binary digits, or bits, each having two possible representations (0 or 1). If a second bit is added to a single bit of information, the number of representations is doubled, resulting in four possible combinations: 00, 01, 10, or 11. A third bit added to this two-bit representation again doubles the number of combinations, resulting in eight possibilities: 000, 001, 010, 011, 100, 101, 110, or 111. Each time a bit is added, the number of possible patterns is doubled. Eight bits is called a byte; a byte has 256 possible combinations of 0s and 1s.
A byte is a useful quantity in which to store information because it provides enough possible patterns to represent the entire alphabet, in lower and upper cases, as well as numeric digits, punctuation marks, and several character-sized graphics symbols, including non-English characters such as p. A byte also can be interpreted as a pattern that represents a number between 0 and 255. A kilobyte—1,024 bytes—can store about 1,000 characters; a megabyte can store about 1 million characters; a gigabyte can store about 1 billion characters; and a terabyte can store about 1 trillion characters. Computer programmers usually decide how a given byte should be interpreted—that is, as a single character, a character within a string of text, a single number, or part of a larger number. Numbers can represent anything from chemical bonds to dollar figures to colors to sounds.
The physical memory of a computer is either random access memory (RAM), which can be read or changed by the user or computer, or read-only memory (ROM), which can be read by the computer but not altered in any way. One way to store memory is within the circuitry of the computer, usually in tiny computer chips that hold millions of bytes of information. The memory within these computer chips is RAM. Memory also can be stored outside the circuitry of the computer on external storage devices, such as magnetic floppy disks, which can store about 2 megabytes of information; solid-state memory drives, called flash drives, which can store several gigabytes of information; hard drives, which can store gigabytes of information; compact discs (CDs), which can store up to 680 megabytes of information; and digital versatile discs (DVDs), which can store 8.5 gigabytes of information. A single CD can store nearly as much information as several hundred floppy disks, and some DVDs can hold more than 12 times as much data as a CD. Blu-ray, which is a newer version of DVD technology, can store up to 50 gigabytes.
| C | The Bus |
The bus enables the components in a computer, such as the CPU and the memory circuits, to communicate as program instructions are being carried out. The bus is usually a flat cable with numerous parallel wires. Each wire can carry one bit, so the bus can transmit many bits along the cable at the same time. For example, a 16-bit bus, with 16 parallel wires, allows the simultaneous transmission of 16 bits (2 bytes) of information from one component to another. Early computer designs utilized a single or very few buses. Modern designs typically use many buses.
| D | Input Devices |
Input devices, such as a keyboard or mouse, permit the computer user to communicate with the computer. Other input devices include a joystick, a rodlike device often used by people who play computer games; a scanner, which converts images such as photographs into digital images that the computer can manipulate; a touch panel, which senses the placement of a user’s finger and can be used to execute commands or access files; and a microphone, used to input sounds such as the human voice which can activate computer commands in conjunction with voice recognition software. “Tablet” computers allow users to interact with their screens using a penlike device or their fingertips.
| E | The Central Processing Unit |
Information from an input device or from the computer’s memory is communicated via the bus to the central processing unit (CPU), which is the part of the computer that translates commands and runs programs. The CPU is a microprocessor chip—that is, a single piece of silicon containing millions of tiny, microscopically wired electrical components. Information is stored in a CPU memory location called a register. Registers can be thought of as the CPU’s tiny scratchpad, temporarily storing instructions or data. When a program is running, one special register called the program counter keeps track of which program instruction comes next by maintaining the memory location of the next program instruction to be executed. The CPU’s control unit coordinates and times the CPU’s functions, and it uses the program counter to locate and retrieve the next instruction from memory.
In a typical sequence, the CPU locates the next instruction in the appropriate memory device. The instruction then travels along the bus from the computer’s memory to the CPU, where it is stored in a special instruction register. Meanwhile, the program counter changes—usually increasing a small amount—so that it contains the location of the instruction that will be executed next. The current instruction is analyzed by a decoder, which determines what the instruction will do. Any data the instruction needs are retrieved via the bus and placed in the CPU’s registers. The CPU executes the instruction, and the results are stored in another register or copied to specific memory locations via a bus. This entire sequence of steps is called an instruction cycle. Frequently, several instructions may be in process simultaneously, each at a different stage in its instruction cycle. This is called pipeline processing.
| F | Output Devices |
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