Interrupts: The technology explained

Put simply, 'Interrupts' are used by I/O devices to operate more efficiently.

Back in the days before Plug and play, every card you put in your computer had a set of jumpers.  These jumpers (or DIP switches) were used to configure the card.  Common items such as Video, Sound, Keyboard, and COM ports and printers had standard settings.  However if you tried to add something like a SCSI card or additional COM ports you would run into conflicts.  The computer has a limited number of IRQ's (Interrupts) available.  The lower the interrupt number the higher the priority it has on the system.  If to interrupts occur at the same time; the one with the lower number goes first.  Therefore, when any device needs to communicate with the processor it will 'Interrupt' and do its business.  To use another analogy, Interrupts are like butting in line at the grocery store.  You will get serviced sooner. Then the processor will continue until it is interrupted again.  This occurs very quickly.  Without using interrupts, the processor would continually have to check all devices for activity even if there was none.  The Interrupt makes things work more efficient because the CPU will only speak to the device if the device Interrupts.  Today we have Plug and Play and rarely have to configure anything manually.  The device manufacturers and Microsoft have made it easy for us.  In fact, very few devices even have jumpers or switches anymore!

Below is a list of the most common bus interupt (IRQ) assignments for PC/XTbus and PC/AT (ISA, MCA & EISA) bus based systems. They may be helpful in trouble-shooting and/or eliminating potential conflicts when adding devices
to your computer.

Bus Interupt (IRQ) Assignments For PC/AT (ISA, MCA & EISA) Buses:

IRQ Type Of Bus Slot Standard Function
---- ---------------- -----------------
0 None System Timer
1 None Keyboard Controller
2 None Second IRQ Controller
3 8-Bit COM Port 2 (or COM 4, if installed/enabled)
4 8-Bit COM Port 1 (or COM 3, if installed/enabled)
5 8-Bit Parallel Port (LPT 2, if installed/enabled)
6 8-Bit Floppy Disk Controller
7 8-Bit Parallel Port (LPT 1)
8 None Real-Time Clock
9 8-Bit Available (Redirected IRQ 2)
10 16-Bit Available
11 16-Bit Available
12 16-Bit PS/2 Mouse Port (if installed/enabled)
13 None Math Coprocessor
14 16-Bit Hard Disk Controller
15 16-Bit Available

Bus Interupt (IRQ) Assignments For PC/XT Buses:

IRQ Type Of Bus Slot Standard Function
---- ---------------- -----------------
0 None System Timer
1 None Keyboard Controller
2 8-Bit Available (early Tandy 1000s used as HD controller)
3 8-Bit COM Port 2 (or COM 4, if installed/enabled)
4 8-Bit COM Port 1 (or COM 3, if installed/enabled)
5 8-Bit Hard Disk Controller (early Tandy 1000s used IRQ 2)
6 8-Bit Floppy Disk Controller
7 8-Bit Parallel Port (LPT 1)
 

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