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Interrupt request

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In a computer, an interrupt request (or IRQ) is a hardware signal sent to the processor that temporarily stops a running program and allows a special program, an interrupt handler, to run instead. Interrupts are used to handle such events as data receipt from a modem or network, or a key press or mouse movement. The interrupt request level (IRQL) is the priority of an interrupt request.

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 Advanced Programmable Interrupt Controller (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.

Overview

When working with personal 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.

x86 IRQs

Typically, on systems using the Intel 8259, 16 IRQs are used. 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 IRQ 2 line, and the master passes the signal on to the CPU. There are therefore only 15 interrupt request lines available for hardware.

On newer systems using the Intel APIC Architecture, typically there are 24 IRQs available, and the extra 8 IRQs are used to route PCI interrupts, avoiding conflict between dynamically configured PCI interrupts and statically configured ISA interrupts. On early APIC systems with only 16 IRQs or with only Intel 8259 interrupt controllers, PCI interrupt lines were routed to the 16 IRQs using a PIR integrated into the southbridge.

The easiest way of viewing this information on Microsoft Windows is to use Device Manager or System Information (msinfo32.exe). On Linux, IRQ mappings can be viewed by executing cat /proc/interrupts or using the procinfo utility.

Master PIC

  • IRQ 0 – system timer (cannot be changed)
  • IRQ 1 – keyboard controller (cannot be changed)
  • IRQ 2 – cascaded signals from IRQs 8–15 (any devices configured to use IRQ 2 will actually be using IRQ 9)
  • IRQ 3 – serial port controller for serial port 2 (shared with serial port 4, if present)
  • IRQ 4 – serial port controller for serial port 1 (shared with serial port 3, if present)
  • IRQ 5 – parallel port 2 and 3  or  sound card
  • IRQ 6 – floppy disk controller
  • IRQ 7 – parallel port 1. It is used for printers or for any parallel port if a printer is not present. It can also be potentially be shared with a secondary sound card with careful management of the port.

Slave PIC

Conflicts

In IBM-compatible personal computers, an IRQ conflict is a once common hardware error, received when two devices were trying to use the same interrupt request (or IRQ) to signal an interrupt to the Programmable Interrupt Controller (PIC).

The PIC expects interrupt requests from only one device per line, thus more than one device sending IRQ signals along the same line will generally cause an IRQ conflict that can freeze a computer.

In some rare conditions, two devices can share the same IRQ as long as they are not used simultaneously.

If one adds a modem expansion card to a system, and assign it to IRQ4, which is traditionally assigned to the serial port 1, it will likely cause an IRQ conflict.

However, the PCI specification specifically allows for IRQ sharing, alleviating this issue.

See also

References

  1. ^ Oshins, Jake (December 30, 2001). "RE: ACPI Machines and IRQ 9 [was: Communicating with the NT developers]". Retrieved April 17, 2014.
  • 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
  • IRQ interrupt request
  • Expansion Bus Configuration

More information on the Intel 8259 PIC and its IRQ lines can be found in the IA-32 Intel Architecture Software Developer’s Manual, Volume 3A: System Programming Guide, Part 1, freely available on the Intel website.

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