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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Resources | Interrupts (IRQs) | IRQ Details By Number ]


IRQ Number: 2

16-Bit Priority: n/a

Bus Line: No

Typical Default Use: Cascade for IRQs 8 to 15.

Other Common Uses: Not generally used. Can be used by modems, very old (EGA) video cards, as an alternative IRQ for COM3 (third serial port) or COM4 (fourth serial port). Rerouted to IRQ9 and appears to software as IRQ9.

Description: This is the interrupt number that is used to cascade the second interrupt controller to the first, allowing the use of extra IRQs 8 to 15. This use as a linkage between the two interrupt controllers means that IRQ2 is no longer available for normal use. For compatibility with older cards that used IRQ2 on the original PC or XT machines (which had only one controller and a normal IRQ2 line), the motherboard of modern PCs reroutes IRQ2 to IRQ9. Hence IRQ2 can still be used but appears to the system as IRQ9. The most common cards that do this are old EGA video cards, and newer cards making IRQ2 available with the knowledge that it will be routed to IRQ9.

Conflicts: This interrupt is normally not used on most systems, mostly because the whole IRQ2/IRQ9 thing confuses a lot of people so they tend to avoid it. Conflicts on this line generally come from trying to use a device on IRQ2 and another on IRQ9 at the same time. Some modems and serial port cards allow IRQ2 to be used as an alternative for the two standard lines used for modems and serial ports (IRQ3 and IRQ4) in order to avoid conflicts in those two heavily-contested areas. This is generally a good configuration decision since unused IRQs from 3 to 7 are harder to find than unused IRQs from 10 to 15. If you want to use IRQ2, move any device using IRQ9 to another line like 10 or 11.

Next: IRQ3

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