The following IRQs are used by ISA devices and 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 floppy disk controller (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 Integrated Device Electronics (IDE) controller
IRQ 15
Secondary IDE controller