The following IRQs used by ISA devices are considered to be fixed assignments.
Fixed ISA Interrupts
| Hardware IRQ | Default assignment |
| IRQ 0 | System timer |
| IRQ 1 | Keyboard |
| IRQ 2 | Second programmable interrupt controller (PIC) cascade |
| IRQ 3 | COM 2 |
| IRQ 4 | COM 1 |
| IRQ 5 | Sometimes LPT 2 — not considered fixed |
| IRQ 6 | Standard FDC |
| IRQ 7 | LPT 1 |
| IRQ 8 | Real-time clock / CMOS |
| IRQ 9 | — |
| IRQ 10 | Sometimes COM 4 — not considered fixed |
| IRQ 11 | Sometimes COM 3 — not considered fixed |
| IRQ 12 | PC/2-style mouse |
| IRQ 13 | Coprocessor |
| IRQ 14 | Primary IDE controller |
| IRQ 15 | Secondary IDE controller |