Interrupt handlerFrom Wikipedia, the free encyclopedia

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An interrupt handler, also known as an interrupt service routine (ISR), is a callback subroutine in an

operating system or device driver whose execution is triggered by the reception of an interrupt. Interrupt

handlers have a multitude of functions, which vary based on the reason the interrupt was generated and the

speed at which the interrupt handler completes its task.

An interrupt handler is a low-level counterpart of event handlers. These handlers are initiated by either

hardware interrupts or interrupt instructions in software, and are used for servicing hardware devices and

transitions between protected modes of operation such as system calls.

Contents

1 Overview

2 Interrupt threads

3 Symbian OS

4 See also

Overview

In several operating systems - Linux, Microsoft Windows, and some other operating systems in the past,

interrupt handlers are divided into two parts: the First-Level Interrupt Handler (FLIH) and the

Second-Level Interrupt Handlers (SLIH). FLIHs are also known as hard interrupt handlers or fast

interrupt handlers, and SLIHs are also known as slow/soft interrupt handlers, Deferred Procedure Call.

A FLIH implements at minimum platform-specific interrupt handling similarly to interrupt routines. In

response to an interrupt, there is a context switch, and the code for the interrupt is loaded and executed. The

job of a FLIH is to quickly service the interrupt, or to record platform-specific critical information which is

only available at the time of the interrupt, and schedule the execution of a SLIH for further long-lived

interrupt handling.

FLIHs cause jitter in process execution. FLIHs also mask interrupts. Reducing the jitter is most important for

real-time operating systems, since they must maintain a guarantee that execution of specific code will

complete within an agreed amount of time. To reduce jitter and to reduce the potential for losing data from

masked interrupts, programmers attempt to minimize the execution time of a FLIH, moving as much as

possible to the SLIH. With the speed of modern computers, FLIHs may implement all device and platform-

dependent handling, and use a SLIH for further platform-independent long-lived handling.

FLIHs which service hardware typically mask their associated interrupt (or keep it masked as the case may

be) until they complete their execution. An (unusual) FLIH which unmasks its associated interrupt before it

completes is called a reentrant interrupt handler. Reentrant interrupt handlers might cause a stack overflow

from multiple preemptions by the same interrupt vector, and so they are usually avoided. In a priority

interrupt system, the FLIH also (briefly) masks other interrupts of equal or lesser priority.

A SLIH completes long interrupt processing tasks similarly to a process. SLIHs either have a dedicated kernel

thread for each handler, or are executed by a pool of kernel worker threads. These threads sit on a run queue

in the operating system until processor time is available for them to perform processing for the interrupt.

SLIHs may have a long-lived execution time, and thus are typically scheduled similarly to threads and

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processes.

In Linux, FLIHs are called upper half, and SLIHs are called lower half or bottom half. This is different from

naming used in other Unix-like systems, where both are a part of bottom half.

Interrupt threads

Several operating systems - Solaris, NetBSD, Mac OS X, WinCE and FreeBSD, for example - use different

scheme known as interrupt threads. An interrupt handler provided by the device driver is just a high-priority

thread which runs with interrupts enabled and, more importantly, may block on mutex. This greatly simplifies

locking in the kernel. In addition, an interrupt thread may be preempted by some higher-priority interrupt

thread.

Symbian OS

Due to (amongst other reasons) extended processing in a ISR delays the servicing of other interrupts,

Symbian OS uses Delayed Function Calls (DFCs) to perform processing that would be impossible inside the

ISR [1] (http://developer.symbian.com/main/downloads/papers/HWinterupt/HwInterrupt.pdf) .

See also

Advanced Programmable Interrupt Controller (APIC)

Inter-processor interrupt (IPI)

Interrupt

Interrupt latency

Non-maskable interrupt (NMI)

Programmable Interrupt Controller (PIC)

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Categories: Interrupts | Operating system technology

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