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Input Devices
by John Anthony


Input devices let you enter information into a PC.

The keyboard, mouse and touch-screen display are examples of input devices.

Input devices get information into the computer by interrupting the CPU from it's current program, in much the same way that you are interrupted in your daily life.

Handling an interrupt at home
If you are home preparing dinner or watching TV and the door bell rings you have to stop what you are doing, go to the door, see who it is, see what they want, take any appropriate action, say good bye and then go back and pick up where you left off, right?

Handling an interrupt on a PC
When you press a key on the keyboard the same thing happens. An interrupt handler (software routine) says 'Hey CPU, stop what you're doing, go over to the keyboard port, read the value of the key the operator just pressed and store this value in memory location X.'

The CPU stops, reads the key, stores the key value in location X, and takes any further appropriate action depending on which key was pressed. Often an interrupt will cause a PC to run a very complex, time consuming routine to complete the interrupt service routine. Once the interrupt has been completely serviced the PC goes back to doing what it was doing before the interrupt.

The difference is that you probably get interrupted infrequently compared to a PC, unless you have a really busy job, like a customer service center representative, where it could be a few times every minute. A CPU can be interrupted hundreds, even thousands of times per second depending on the environment it is running in.

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