Overview  
Devices such as the keyboard and disk controller are connected to the
CPU via Interrupt ReQuest lines (IRQs).  When these devices need
servicing, they signal to the CPU which immediately begins executing
code at a particular interrupt vector.

  Standard IRQs  

INT 08H IRQ 0 Timer       Interrupts every 55ms (18.2 times per second)

INT 09H IRQ 1 Keyboard    Interrupts on each press and release of a key
and periodically while the key is held down.

INT 0aH IRQ 2 cascade     Connected to the secondary 8259a interrupt
controller on ATs; may be used by EGA for
vertical retrace.  See EGA I/O Ports.

INT 0bH IRQ 3 COM 2/4     Interrupts when COM2 or COM4 has something to
say.  See Serial Ports.

INT 0cH IRQ 4 COM 1/3     Interrupts when COM1 or COM3 has something to
say.  See Serial Ports.

INT 0dH IRQ 5 LPT 2       AT: Interrupts when the printer on LPT2 is ready
for another character (only if printer port 27aH
is set up for this).  See Printer Ports.

PC/XT: This was used for the hard disk.  See

INT 0eH IRQ 6 diskette    Interrupts after each action of the floppy
diskette controller (read, write, seek, etc).

INT 0fH IRQ 7 LPT 1       Interrupts when LPT1 is ready for another
character (only when controller port 37aH is set
up for this).  See Printer Ports.

INT 70H IRQ 8 RT Clock    Interrupts 1024 times per second (on AT-class or
better computers).  This is generated by the
same hardware that handles the non-volatile data
in the CMOS Memory.

INT 71H IRQ 9 redir IRQ2  On AT-class computers, IRQ 2 is used as the
connection to a secondary 8259a interrupt
controller.  Actual occurrences of IRQ 2 are
redirected to IRQ 9.  This IRQ may be generated
by EGA/VGA.  See EGA I/O Ports.

INT 72H IRQ 10 (reserved) There is no default device for this IRQ.
INT 73H IRQ 11 (reserved) There is no default device for this IRQ.
INT 74H IRQ 12 (reserved) There is no default device for this IRQ.

INT 75H IRQ 13 math chip  The 80x87 math coprocessor generates this
interrupt on exceptions (errors).

INT 76H IRQ 14 hard disk  Interrupts when the hard disk controller
finishes an operation.  See AT Hard Disk Ports.

INT 77H IRQ 15 (reserved) There is no default device for this IRQ.

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IRQs: Hardware Interrupts