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.
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