Hello Mr. Kiran V. Sutar,

The following web site Interfacing the AT Keyboard by Adam Chapweske has all of the answers you are looking for. It tell in detail what happens when you press a key and when you release a key. He also goes into many different keys telling how they are handled. http://panda.cs.ndsu.nodak.edu/~achapwes/PICmicro/keyboard/keyboard_old.html
Regards,
Charles Bannister

how to handle the LED keys
LEDs:
Beleive it or not, your keyboard does not decide when the NUM LOCK, CAPS LOCK, and SCROLL LOCK light-emitting diodes are to be turned on/off. Rather, it's up to your computer to tell your keyboard to turn on/off these LEDs. So when you press the "Caps Lock" key, your keyboard just sends the make key for "Caps Lock" to the computer--it does not, at this time, turn on the Caps Lock LED. In order to turn on the Caps Lock LED, your computer must send a command to your keyboard to turn it on. In other words, the LEDs on your keyboard and their cooresponding keys are completely independent of each other. Just because the "Caps Lock" key is pressed does not mean the CAPS LOCK LED will be lit--it depends on whether or not your computer wants to turn that LED on (and it usually does.) One way to see if your computer is locked up or just slow is by pressing the "Caps Lock" key and seeing if the cooresponding LED changes state. If it does, your computer is still able to operate on data; if it does not, your computer is locked up tighter than a drum.