The circuitry in a computer which controls the manipulation of data is called the central processing unit, or CPU. In the machines of the mid-twentieth century, CPUs were large units comprised of perhaps several racks of electronic circuitry that reflected the significance of the unit. However, technology has shrunk these devices drastically. The CPUs found in today’s desktop computers and notebooks are packaged as small flat squares (approximately two inches by two inches) whose connecting pins plug into a socket mounted on the machine’s main circuit board (called the motherboard). In smartphones, mini-notebooks, and other Mobile Internet Devices (MID), CPU’s are around half the size of a postage stamp. Due to their small size, these processors are called microprocessors.
• CPU Basics
A CPU consists of three parts: the arithmetic/logic unit, which contains the circuitry that performs operations on data (such as addition and subtraction); the control unit, which contains the circuitry for coordinating the machine’s activities; and the register unit, which contains data storage cells (similar to main memory cells), called registers, that are used for temporary storage of information within the CPU. Some of the registers within the register unit are considered general-purpose registers whereas others are special-purpose registers.
General-purpose registers serve as temporary holding places for data being manipulated by the CPU. These registers hold the inputs to the arithmetic/logic unit’s circuitry and provide storage space for results produced by that unit. To perform an operation on data stored in main memory, the control unit transfers the data from memory into the general-purpose registers, informs the arithmetic/logic unit which registers hold the data, activates the appropriate circuitry within the arithmetic/logic unit, and tells the arithmetic/logic unit which register should receive the result. For the purpose of...
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