Components of a Computer

Major Components of a Computer

Control unit

The control unit is the circuitry that controls the flow of data through the processor, and coordinates the activities of the other units within it. In a way, it is the "brain within the brain", as it controls what happens inside the processor, which in turn controls the rest of the PC.

 Arithmetic logic unit (ALU)

Simple processes are performed by the Arithmetic Logic Unit (ALU for short). The ALU is made up of devices called gates that receive one or more inputs and, based upon what function they are designed to perform, output a result. The ALU in our example performs one of seven functions: NOT, Left Shift, Right Shift, Add, Subtract, AND, and OR.

The ALU takes two inputs, loaded from registers on the chip. For the commands Not through Right Shift, only the first input is used. The second is completely ignored. The ALU can also let its first input pass through unchanged. The function executed is determined by three additional input bits, specifying the number of the function. Depending on the result of the function, the ALU outputs two bits, called N and Z. N is a one when the result is negative (i.e. the left-moat bit is a one), and Z is a one when the result is zero. These bits are helpful for determining what the computer should do next. Then the answer is routed back to a register to be stored for future use.

Memory

The term “memory” identifies data storage that comes in the form of chips, and the word “storage” is used for memory that exists on tapes or disks. Moreover, the term memory is usually used as a shorthand for “physical memory”, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk.

Every computer comes with a certain amount of physical memory, usually referred to as “main memory” or “RAM”. You can think of main memory as an array of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (or characters) of information.

There are several different types of memory:

• RAM (random-access memory): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
• ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.
• PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.
• EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.
• EEPROM (electrically erasable programmable read-only memory): An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.

Input/output (I/O)

Before a computer can process your data, you need some method to input the data into the machine. The device you use will depend on what form this data takes (be it text, sound, artwork, etc.).Similarly, after the computer has processed your data, you often need to produce output of the results. This output could be a display on the computer screen, hardcopy on printed pages, or even the audio playback of music you composed on the computer.The terms “input” and “output” are used both as verbs to describe the process of entering or displaying the data, and as nouns referring to the data itself entered into or displayed by the computer.