An act of interrupting is referred to as an interrupt request ("IRQ").
Interrupts may be implemented in hardware as a distinct system with control lines, or they may be integrated into the memory subsystem.
If implemented in hardware, an interrupt controller circuit such as the IBM PC's Programmable Interrupt Controller (PIC) may be connected between the interrupting device and to the processor's interrupt pin to multiplex several sources of interrupt onto the one or two CPU lines typically available.
If implemented as part of the memory controller, interrupts are mapped into the system's memory address space.
Processors typically have an internal interrupt mask which allows software to ignore all external hardware interrupts while it is set. This mask may offer faster access than accessing an interrupt mask register (IMR) in a PIC, or disabling interrupts in the device itself. In some cases, such as the x86 architecture, disabling and enabling interrupts on the processor itself acts as a memory barrier, in which case it may actually be slower.
An interrupt that leaves the machine in a well-defined state is called a precise interrupt. Such an interrupt has four properties:
An interrupt that does not meet these requirements is called an imprecise interrupt.
The phenomenon where the overall system performance is severely hindered by excessive amounts of processing time spent handling interrupts is called an interrupt storm.
Typically, the processor samples the interrupt input at predefined times during each bus cycle such as state T2 for the Z80 microprocessor. If the interrupt isn't active when the processor samples it, the CPU doesn't see it. One possible use for this type of interrupt is to minimize spurious signals from a noisy interrupt line: a spurious pulse will often be so short that it is not noticed.
Multiple devices may share a level-triggered interrupt line if they are designed to. The interrupt line must have a pull-down or pull-up resistor so that when not actively driven it settles to its inactive state. Devices actively assert the line to indicate an outstanding interrupt, but let the line float (do not actively drive it) when not signalling an interrupt. The line is then in its asserted state when any (one or more than one) of the sharing devices is signalling an outstanding interrupt.
This class of interrupts is favored by some because of a convenient behavior when the line is shared. Upon detecting assertion of the interrupt line, the CPU must search through the devices sharing it until one requiring service is detected. After servicing this device, the CPU may recheck the interrupt line status to determine whether any other devices also need service. If the line is now deserted, the CPU avoids checking the remaining devices on the line. Since some devices interrupt more frequently than others, and other device interrupts are particularly expensive, a careful ordering of device checks is employed to increase efficiency.
There are also serious problems with sharing level-triggered interrupts. As long as any device on the line has an outstanding request for service the line remains asserted, so it is not possible to detect a change in the status of any other device. Deferring servicing a low-priority device is not an option, because this would prevent detection of service requests from higher-priority devices. If there is a device on the line that the CPU does not know how to service, then any interrupt from that device permanently blocks all interrupts from the other devices.
The original PCI standard mandated shareable level-triggered interrupts. The rationale for this was the efficiency gain discussed above. (Newer versions of PCI allow, and PCI Express requires, the use of message-signalled interrupts.)
Multiple devices may share an edge-triggered interrupt line if they are designed to. The interrupt line must have a pull-down or pull-up resistor so that when not actively driven it settles to one particular state. Devices signal an interrupt by briefly driving the line to its non-default state, and let the line float (do not actively drive it) when not signalling an interrupt. This type of connection is also referred to as open collector. The line then carries all the pulses generated by all the devices. However, interrupt pulses from different devices may merge if they occur close in time. To avoid losing interrupts the CPU must trigger on the trailing edge of the pulse (e.g., the rising edge if the line is pulled up and driven low). After detecting an interrupt the CPU must check all the devices for service requirements.
Edge-triggered interrupts do not suffer the problems that level-triggered interrupts have with sharing. Service of a low-priority device can be postponed arbitrarily, and interrupts will continue to be received from the high-priority devices that are being serviced. If there is a device that the CPU does not know how to service, it may cause a spurious interrupt, or even periodic spurious interrupts, but it does not interfere with the interrupt signalling of the other devices. However, it is fairly easy for an edge triggered interrupt to be missed - for example if interrupts have to be masked for a period - and unless there is some type of hardware latch that records the event it is impossible to recover. Such problems caused many "lockups" in early computer hardware because the processor didn't know it was expected to do something. More modern hardware often has one or more interrupt status registers that latch the interrupt requests; well written edge-driven interrupt software often checks such registers to ensure events are not missed.
The elderly ISA bus uses edge-triggered interrupts, but does not mandate that devices be able to share them. The parallel port also uses edge-triggered interrupts. Many older devices assume that they have exclusive use of their interrupt line, making it electrically unsafe to share them. However, ISA motherboards include pull-up resistors on the IRQ lines, so well-behaved devices share ISA interrupts just fine.
A common use of a hybrid interrupt is for the NMI (non-maskable interrupt) input. Because NMIs generally signal major – or even catastrophic – system events, a good implementation of this signal tries to ensure that the interrupt is valid by verifying that it remains active for a period of time. This 2-step approach helps to eliminate false interrupts from affecting the system.
Message-signalled interrupts behave very much like edge-triggered interrupts, in that the interrupt is a momentary signal rather than a continuous condition. Interrupt-handling software treats the two in much the same manner. Typically, multiple pending message-signalled interrupts with the same message (the same virtual interrupt line) are allowed to merge, just as closely-spaced edge-triggered interrupts can merge.
Message-signalled interrupt vectors can be shared, to the extent that the underlying communication medium can be shared. No additional effort is required.
Because the identity of the interrupt is indicated by a pattern of data bits, not requiring a separate physical conductor, many more distinct interrupts can be efficiently handled. This reduces the need for sharing. Interrupt messages can also be passed over a serial bus, not requiring any additional lines.
PCI Express, a serial computer bus, uses message-signalled interrupts exclusively.
Some devices with a badly-designed programming interface provide no way to determine whether they have requested service. They may lock up or otherwise misbehave if serviced when they do not want it. Such devices cannot tolerate spurious interrupts, and so also cannot tolerate sharing an interrupt line. ISA cards, due to often cheap design and construction, are notorious for this problem. Such devices are becoming much rarer, as hardware logic becomes cheaper and new system architectures mandate shareable interrupts.
A classic system timer interrupt interrupts periodically from a counter or the power-line. The interrupt handler counts the interrupts to keep time. The timer interrupt may also be used by the OS's task scheduler to reschedule the priorities of running processes. Counters are popular, but some older computers used the power line frequency instead, because power companies in most Western countries control the power-line frequency with a very accurate atomic clock.
A disk interrupt signals the completion of a data transfer from or to the disk peripheral. A process waiting to read or write a file starts up again.
A power-off interrupt predicts or requests a loss of power. It allows the computer equipment to perform an orderly shutdown.
Interrupts are also used in typeahead features for buffering events like keystrokes.
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