Interrupt request

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In computing, an interrupt request (or IRQ) is a phrase used to refer to either the act of interrupting the bus lines used to signal an interrupt, or the interrupt input lines on a Programmable Interrupt Controller (PIC). Interrupt lines are often identified by an index with the format of IRQ followed by a number. For example, on the Intel 8259 family of PICs there are eight interrupt inputs commonly referred to as IRQ0 through IRQ7. In x86 based computer systems that use two of these PICs, the combined set of lines are referred to as IRQ0 through IRQ15. Technically these lines are named IR0 through IR7, and the lines on the ISA bus to which they were historically attached are named IRQ0 through IRQ15.

Newer x86 systems integrate an APIC that conforms to the Intel APIC Architecture. These APICs support a programming interface for up to 255 physical hardware IRQ lines per APIC, with a typical system implementing support for only around 24 total hardware lines.

1 Overview
2 x86 IRQs
3 See also
4 References

[edit] Overview

When working with computer hardware, installing and removing devices, the system relies on interrupt requests. There are default settings that are configured in the system BIOS and recognized by the operating system. These default settings can be altered by advanced users. Modern Plug and Play technology has not only reduced the need for concern for these settings, but has virtually eliminated manual configuration.

[edit] x86 IRQs

See Intel 8259 for a common list of hardware IRQ lines in x86 systems. The system uses 16 default IRQ's. The easiest way of viewing this information on Windows is to use Device Manager or System Information:

IRQ 0 - System timer. Reserved for the system. Cannot be changed by a user.
IRQ 1 - Keyboard. Reserved for the system. Cannot be altered even if no keyboard is present or needed.
IRQ 2 - Second IRQ controller. See below for explanation.
IRQ 3 - COM 2(Default) COM 4(User)
IRQ 4 - COM 1(Default) COM 3(User)
IRQ 5 - Sound card (Sound Blaster Pro or later) or LPT2(User)
IRQ 6 - Floppy disk controller
IRQ 7 - LPT1(Parallel port) or sound card (8-bit Sound Blaster and compatibles)
IRQ 8 - Real time clock
IRQ 9 - ACPI SCI or ISA MPU-401
IRQ 10 - Free / Open interrupt / Available / SCSI
IRQ 11 - Free / Open interrupt / Available / SCSI
IRQ 12 - PS/2 connector Mouse / If no PS/2 connector mouse is used, this can be used for other peripherals
IRQ 13 - Math co-processor. Cannot be changed
IRQ 14 - Primary IDE. If no Primary IDE this can be changed
IRQ 15 - Secondary IDE

IRQs 0 to 7 are managed by one Intel 8259 PIC, and IRQs 8 to 15 by a second Intel 8259 PIC. The first PIC, the master, is the only one that directly signals the CPU. The second PIC, the slave, instead signals to the master on its IR2 line, and the master passes the signal on. There are therefore only 15 interrupt request lines available for hardware.

[edit] See also

Computer science Portal

[edit] References

Gilluwe, Frank van. The Undocumented PC, Second Edition, Addison-Wesley Developers Press, 1997. ISBN 0-201-47950-8
Shanley, Tom. ISA System Architecture, Third Edition, Addison-Wesley Publishing Company, 1995. ISBN 0-201-40996-8
Solari, Edward. PCI & PCI-X Hardware and Software Architecture & Design, Sixth Edition, Research Tech Inc., 2004. ISBN 0-9760865-0-6