CertiGuide to A+ (A+ 4 Real)  9  Chapter 8: Motherboards

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The central processing unit is a busy device. It has to process data for every device and application on the computer. How do devices manage to get the attention of the CPU when it needs to have its output processed? If it cannot get the CPU’s attention then it does not function. Imagine moving your mouse only to discover the cursor does not move on the monitor (other than when your Windows 98 machine is locked up) or typing text using your keyboard only to discover that no characters are being printed to the monitor. Devices and processes need a method of getting the CPU’s attention. This is accomplished typically in two ways: Polling and Interrupting.

• Polling is the process of the CPU periodically querying each device and process running on the computer asking if they need attention. This is particularly inefficient and not the usual method of communication between a device and the CPU. Interrupting is the more common method used.
  
  
• Interrupting is the method of a device or process getting the attention of the CPU. This was originally accomplished through Interrupt Controllers.

The original IBM PC and XT were designed with a grand total of eight interrupt channels. The interrupt count started with zero, leading up to seven. When the IBM AT was released with the 80286 CPU, the expansion bus jumped to 16-bits. At the same time, a second interrupt controller was added.

Rather than refuse compatibility with the PC and XT, the designers created a cascade so both interrupt controllers could talk to the CPU on the single control line.

The cascade point the designers selected was interrupt number 2 and sent it over to the second interrupt controller. They choose to land at interrupt number 9. If on the rare chance you have an expansion device that needs interrupt number two, it will be pushed over to number nine. Note that interrupt number 9 is usable, if you have, any devices that need interrupt number two; the 'faking out' process consumes interrupt number nine.

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