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In a
computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
, 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. Hardware interrupts are used to handle events such as receiving data from a
modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more c ...
or
network card A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network. Ear ...
, key presses, or mouse movements. Interrupt lines are often identified by an index with the format of ''IRQ'' followed by a number. For example, on the
Intel 8259 The Intel 8259 is a Programmable Interrupt Controller (PIC) designed for the Intel 8085 and Intel 8086 microprocessors. The initial part was 8259, a later A suffix version was upward compatible and usable with the 8086 or 8088 processor. The 82 ...
family of programmable interrupt controllers (PICs) there are eight interrupt inputs commonly referred to as ''IRQ0'' through ''IRQ7''. In
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was int ...
based
computer system A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These progra ...
s 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'' (although historically as the number of hardware devices increased, the total possible number of interrupts was increased by means of cascading requests, by making one of the IRQ numbers cascade to another set or sets of numbered IRQs, handled by one or more subsequent controllers). Newer
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was int ...
systems integrate an
Advanced Programmable Interrupt Controller In computing, Intel's Advanced Programmable Interrupt Controller (APIC) is a family of interrupt controllers. As its name suggests, the APIC is more advanced than Intel's 8259 Programmable Interrupt Controller (PIC), particularly enabling the co ...
(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. During the early years of personal computing, IRQ management was often of user concern. With the introduction of plug and play devices this has been alleviated through automatic configuration.


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 In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the b ...
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 also virtually eliminated manual configuration.


x86 IRQs

Early PCs using the Intel 8086/8088 processors only had a single PIC, and are therefore limited to eight interrupts. This was expanded to two PICs with the introduction of the 286 based PCs. Typically, on systems using the
Intel 8259 The Intel 8259 is a Programmable Interrupt Controller (PIC) designed for the Intel 8085 and Intel 8086 microprocessors. The initial part was 8259, a later A suffix version was upward compatible and usable with the 8086 or 8088 processor. The 82 ...
PIC, 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 APIC with IOAPIC systems, 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 The Intel 8259 is a Programmable Interrupt Controller (PIC) designed for the Intel 8085 and Intel 8086 microprocessors. The initial part was 8259, a later A suffix version was upward compatible and usable with the 8086 or 8088 processor. The 82 ...
interrupt controllers, PCI interrupt lines were routed to the 16 IRQs using a PIR (PCI interrupt routing) integrated into the BIOS. On APIC with MSI systems, typically there are 224 interrupts available. The easiest way of viewing this information on
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for se ...
is to use
Device Manager Device Manager is a component of the Microsoft Windows operating system. It allows users to view and control the hardware attached to the computer. When a piece of hardware is not working, the offending hardware is highlighted for the user to d ...
or System Information (msinfo32.exe). On
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whic ...
, 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 Keyboard may refer to: Text input * Keyboard, part of a typewriter * Computer keyboard ** Keyboard layout, the software control of computer keyboards and their mapping ** Keyboard technology, computer keyboard hardware and firmware Music * Mus ...
on
PS/2 port The PS/2 port is a 6-pin mini-DIN connector used for connecting keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 ...
(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 In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel ...
controller for
serial port In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel ...
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 In computing, a parallel port is a type of interface found on early computers ( personal and otherwise) for connecting peripherals. The name refers to the way the data is sent; parallel ports send multiple bits of data at once (paralle ...
3 or sound card * IRQ 6
floppy disk controller A floppy-disk controller (FDC) has evolved from a discrete set of components on one or more circuit boards to a special-purpose integrated circuit (IC or "chip") or a component thereof. An FDC directs and controls reading from and writing to a ...
* IRQ 7 parallel port 1 (shared with parallel port 2, if present). 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

* IRQ 8 real-time clock (RTC) * IRQ 9
Advanced Configuration and Power Interface Advanced Configuration and Power Interface (ACPI) is an open standard that operating systems can use to discover and configure computer hardware components, to perform power management (e.g. putting unused hardware components to sleep), auto co ...
(ACPI) system control interrupt on Intel chipsets. Other chipset manufacturers might use another interrupt for this purpose, or make it available for the use of peripherals (any devices configured to use IRQ 2 will actually be using IRQ 9) * IRQ 10 The Interrupt is left open for the use of peripherals (open interrupt/available, SCSI or NIC) * IRQ 11 The Interrupt is left open for the use of peripherals (open interrupt/available, SCSI or NIC) * IRQ 12
mouse A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...
on
PS/2 port The PS/2 port is a 6-pin mini-DIN connector used for connecting keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 ...
* IRQ 13 CPU
co-processor A coprocessor is a computer processor used to supplement the functions of the primary processor (the CPU). Operations performed by the coprocessor may be floating-point arithmetic, graphics, signal processing, string processing, cryptography or ...
or integrated floating point unit or
inter-processor interrupt An inter-processor interrupt (IPI), also known as a ''shoulder tap'', is a special type of interrupt by which one processor may interrupt another processor in a multiprocessor system if the interrupting processor requires action from the other pro ...
(use depends on OS) * IRQ 14 primary ATA channel (ATA interface usually serves
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...
s and
CD drive In computing, an optical disc drive is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only ...
s) * IRQ 15 secondary ATA channel


Conflicts

In early IBM-compatible
personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...
s, 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 A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
. For example, if a
modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more c ...
expansion card In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus s ...
is added into a system and assigned to IRQ4, which is traditionally assigned to the
serial port In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel ...
 1, it will likely cause an IRQ conflict. Initially, IRQ 7 was a common choice for the use of a sound card, but later IRQ 5 was used when it was found that IRQ 7 would interfere with the printer port (LPT1). The
serial port In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel ...
s are frequently disabled to free an IRQ line for another device. IRQ 2/9 is the traditional interrupt line for an MPU-401 MIDI port, but this conflicts with the ACPI system control interrupt (SCI is hardwired to IRQ9 on Intel chipsets); this means ISA MPU-401 cards with a hardwired IRQ 2/9, and MPU-401 device drivers with a hardcoded IRQ 2/9, cannot be used in interrupt-driven mode on a system with ACPI enabled. In some rare conditions, two ISA devices could share the same IRQ as long as they were not used simultaneously. To solve this problem, the later PCI bus allows for IRQ sharing.
PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common m ...
does not have physical interrupt lines, and uses Message Signaled Interrupts (MSI) to the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also ...
s if available.


See also

*
Advanced Programmable Interrupt Controller In computing, Intel's Advanced Programmable Interrupt Controller (APIC) is a family of interrupt controllers. As its name suggests, the APIC is more advanced than Intel's 8259 Programmable Interrupt Controller (PIC), particularly enabling the co ...
(APIC) * Programmable Interrupt Controller (PIC) *
Intel 8259 The Intel 8259 is a Programmable Interrupt Controller (PIC) designed for the Intel 8085 and Intel 8086 microprocessors. The initial part was 8259, a later A suffix version was upward compatible and usable with the 8086 or 8088 processor. The 82 ...
* Interrupt handler * Input/output base address * Plug and play * Polling *
Interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...


References


Further reading

* Gilluwe, Frank van. ''The Undocumented PC, Second Edition'', Addison-Wesley Developers Press, 1997. *

* Solari, Edward. ''PCI & PCI-X Hardware and Software Architecture & Design, Sixth Edition'', Research Tech Inc., 2004.


External links


Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A: System Programming Guide, Part 1, chapter 6
more information on Intel 64 and IA-32 interrupt handling

{{DEFAULTSORT:Interrupt Request Interrupts