Central Processing Unit (CPU)

Abbreviation of central processing unit, and pronounced as separate letters. The CPU is the brains of the computer. Sometimes referred to simply as the processor or central processor, the CPU is where most calculations take place. In terms of computing power, the CPU is the most important element of a computer system.

On large machines, CPUs require one or more printed circuit boards. On personal computers and small workstations, the CPU is housed in a single chip called a microprocessor.

Two typical components of a CPU are:

The arithmetic logic unit (ALU), which performs arithmetic and logical operations.

The control unit, which extracts instructions from memory and decodes and executes them, calling on the ALU when necessary.

It is also called clock rate, the speed at which a microprocessor executes instructions. Every computer contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components. The CPU requires a fixed number of clock ticks (or clock cycles) to execute each instruction. The faster the clock, the more instructions the CPU can execute per second.

Clock speeds are expressed in megahertz (MHz) or gigahertz (GHz).

The internal architecture of a CPU has as much to do with a CPU?s performance as the clock speed, so two CPUs with the same clock speed will not necessarily perform equally. Whereas an Intel 80286 microprocessor requires 20 cycles to multiply two numbers, an Intel 80486 or later processor can perform the same calculation in a single clock tick. (Note that clock tick here refers to the system?s clock, which runs at 66 MHz for all PCs.) These newer processors, therefore, would be 20 times faster than the older processors even if their clock speeds were the same. In addition, some microprocessors are superscalar, which means that they can execute more than one instruction per clock cycle.

Like CPUs, expansion buses also have clock speeds. Ideally, the CPU clock speed and the bus clock speed should be the same so that neither component slows down the other. In practice, the bus clock speed is often slower than the CPU clock speed, which creates a bottleneck. This is why new local buses, such as AGP, have been developed.

There are overall two main types of storage:

★ Primary Storage ( memory ): This type of storage is primarily used to store information temporarily. The CPU does not directly access the secondary memory, instead it always relies on the primary memory.

★ Secondary storage: This type of storage is primarily used to store data or long periods. This type of memory is stored in external devices such as hard drives etc.

★ Any data that resides on a disk or input device cannot be accessed by the CPU directly, it should always reside in the main memory. The control unit accesses the data from the disk memory and places it in the main memory.

★ The data is present in the primary memory until the program requiring it is executed or is terminated.

★ Registers can be considered to be special purpose memory which reside within the CPU.

★ The registers are high speed temporary memory used to help the CPU get access to data and instructions quickly and effectively. There are various type of registers such as Instruction register, Status register and data register.

★ The instructions register is responsible for the storage of the of the instructions that are being currently executed.

★ The status register is used to keep a tab of the status operations of the arithmetic logical unit.

★ The data register is used to store information or data that is to be processed, it is also used to store information or results that have been derived from a process.