2.5 System Resources
2.5.2 Interrupt Requests (IRQ's)
Modern computers and operating systems owe their reliability to the organized way in which they handle internal transactions. Various hardware devices, for example, may want to tell the CPU that they have some information available that is ready for transfer. The devices indicate this by making an interrupt request, or IRQ. It is a general rule that IRQs cannot be shared. A device's IRQ will cause the program to stop momentarily as it asks the CPU to service its request. Given that they are so critical to the proper functioning of the system, it is recommended that default device IRQ assignments be followed.

What are cascaded IRQs?
To understand the concept of cascading IRQs, look at how controllers worked with the XT (8088s) and then later, the AT (80286 and later) configurations. The XT BIOS provided for eight IRQ lines, IRQ 0 through 7. The more advanced AT BIOS came with eight more IRQ lines, IRQ 8 through 15. To handle the additional interrupts IRQ 2 was moved to IRQ 9 and the IRQ signal from the second interrupt chip used IRQ 2.

This behavior is described as cascading, which simply means redirecting. In fact, cascading is also commonly referred to as redirecting or vectoring. In this instance, IRQ 9 is said to be cascading, vectoring, or redirecting to IRQ 2. Note that all three words refer to the same process of pointing to somewhere else.

Take the time to understand this concept because it is often confusing to many people. When source information is redirected to a destination, the destination receives cascaded information from the source.