The Keyboard is a common input device. It is practically attached to every computer system. Its primary purpose is to allow interaction during procession between the operator and the processor. Usually a keyboard is divided into Function keys, Alphanumeric keys and Numeric keypad. A standard keyboard has 84 keys, however, now days there are keyboard having 104 or even more keys.

1] Pressing a key causes a change in the amount of current flowing though a circuit associated specifically with that key.
2] A microprocessor built into the keyboard, such as the intel 8048, constantly scan circuits leading to the keys. It detects the increase and decrease in current from the key that has been pressed. By detecting both an increase and a decrease in current, the processor can tell when a key has been pressed and when it's been released. Each key has a unique set of codes, even if, to the users, the keys seem identical. The processor can, for example, distinguish between the left and the right shift keys. To distinguish between a real signals and an aberrant current fluctuation, the scan is repeated hundreds of times each second. Only signals for two or more scans are acted upon by the processor.
3]Depending on which key's circuits carries a signal to the microprocessor, the processor generates a  number, called a scan code. There are two scan codes for each key , one for when the keys is depressed and the other for when it's released. Then the processor sends an interrupt signal over the keyboard cable to tell the processor that a scan code is waiting for it. An interrupt tells the processor to drop whatever alse it is doing and to divert its attention to the service requested by the interrupt.
4] The BIOS (basic input/output system ) reads the scan code from the keyboard port and sends a signal to the keyboard it can delete the scan code from its buffer.
5] Depending on the status indicated by the bytes, the BIOS translates the approriate scan code into an ASC II code used by the PC, that stands for a character, or  into a special code for a character, or into a special code for a function key or a cursor movement key. Uppercase and lowercase characters have different ASC II codes. Applications can choose to interpret any keystrike to display a character or as command. The BIOS places the ASC II or special key code into its own memory buffer, where it is  retrieved by the operating system or application software as soon as any current operation is finished.

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