8259 PIC - OSDev Wiki
Jun 30, 2021 · Thus, in an AT, IRQ line 2 is used to signal the second chip. Because of this, IRQ 2 is not available for use by hardware devices, which got wired to IRQ 9 on the slave PIC instead. The real mode BIOS used to set up an interrupt handler for IRQ 9 that redirects to the IRQ 2 handler. This way DOS drivers who used IRQ 2 continued to work.
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