How IRQ(interrupt requested line)works

An IRQ or interrupt requested line is a Line to your CPU that other devises like mice can if needed interrupt the CPU. In other words if your CPU(the devise that processes everything on your computer)is working on a Task that is not as important as something like moving your mouse, The IRQ will tell your CPU to stop the unimportant task and do the more important one first which in this case is the mouse. The IRQ will then process the more important task and then after that task is complete will go back to the old task that was not as important.

IRQ are always hardware related. There are some IRQ that are preset by the manufacturer. The system timer which uses IRQ0, Keyboard controller which uses IRQ1, floppy controller which uses IRQ6, real-time clock which uses IRQ8 and the Math Co-processor which uses RQ13 all are set by default and cannot be changed. The rest of the IRQ are often set by the PC user themselves, Users change IRQs ether by setting jumpers on the Hardware itself or by using programs installed on the computer.

IRQ2 cascades to IRQ9 and IRQ9 cascades back to IRQ2.In the older XT BIOS there was only 8 IRQ lines but on the newer AT BIOS they added 8 more lines so that there were more IRQ lines available to send IRQ .IRQ2 gets its interrupt requests from IRQ 8-15 and then IRQ2s instructions are redirected to IRQ9.If you set a devise to use IRQ9 then that interrupt request cascades to IRQ2 so actually IRQ9 just sends the Interrupt request to IRQ2 Just like IRQ2 sends its interrupt requests to IRQ9 when it receive the data from IRQ8-15.Just take into account that if IRQ2 is being used.