Editing Operating system (section)

=====Software interrupt=====
A software interrupt is a message to a [[Process (computing)|process]] that an event has occurred.<ref name="lpi-ch20-p388_quote1"/> This contrasts with a ''hardware interrupt'' — which is a message to the [[central processing unit]] (CPU) that an event has occurred.<ref name="osc-ch2-p30">{{cite book
  | last = Silberschatz
  | first = Abraham
  | title = Operating System Concepts, Fourth Edition
  | publisher = Addison-Wesley
  | year = 1994
  | page = 30
  | isbn = 978-0-201-50480-4
  | quote = Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus.
}}</ref> Software interrupts are similar to hardware interrupts — there is a change away from the currently running process.<ref name="lpi-ch20-p388_quote2">{{cite book
  | last = Kerrisk
  | first = Michael
  | title = The Linux Programming Interface
  | publisher = No Starch Press
  | year = 2010
  | page = 388
  | isbn = 978-1-59327-220-3
  | quote = Signals are analogous to hardware interrupts in that they interrupt the normal flow of execution of a program; in most cases, it is not possible to predict exactly when a signal will arrive.
}}</ref> Similarly, both hardware and software interrupts execute an [[Interrupt handler|interrupt service routine]].

Software interrupts may be normally occurring events. It is expected that a [[Preemption (computing)#Time slice|time slice]] will occur, so the kernel will have to perform a [[context switch]].<ref name="lpi-ch20-p388_quote3">{{cite book
  | last = Kerrisk
  | first = Michael
  | title = The Linux Programming Interface
  | publisher = No Starch Press
  | year = 2010
  | page = 388
  | isbn = 978-1-59327-220-3
  | quote = Among the types of events that cause the kernel to generate a signal for a process are the following: A software event occurred. For example, ... the process's CPU time limit was exceeded[.]
}}</ref> A [[computer program]] may set a timer to go off after a few seconds in case too much data causes an algorithm to take too long.<ref name="lpi-ch20-p388">{{cite book
  | last = Kerrisk
  | first = Michael
  | title = The Linux Programming Interface
  | publisher = No Starch Press
  | year = 2010
  | page = 388
  | isbn = 978-1-59327-220-3
}}</ref>

Software interrupts may be error conditions, such as a malformed [[machine code|machine instruction]].<ref name="lpi-ch20-p388"/> However, the most common error conditions are [[division by zero]] and [[segmentation fault|accessing an invalid memory address]].<ref name="lpi-ch20-p388"/>

[[User (computing)|Users]] can send messages to the kernel to modify the behavior of a currently running process.<ref name="lpi-ch20-p388"/> For example, in the [[Command-line interface|command-line environment]], pressing the ''interrupt character'' (usually [[Control-C]]) might terminate the currently running process.<ref name="lpi-ch20-p388"/>

To generate ''software interrupts'' for [[x86]] CPUs, the [[INT (x86 instruction)|INT]] [[assembly language]] instruction is available.<ref name="intel-developer">{{cite web
 |url=https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
 |access-date=2022-05-05
 |title=Intel® 64 and IA-32 Architectures Software Developer's Manual
 |volume=2
 |date=September 2016
 |publisher=[[Intel Corporation]]
 |page=610
 |archive-date=23 March 2022
 |archive-url=https://web.archive.org/web/20220323231921/https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
 |url-status=live
 }}</ref> The syntax is <code>INT X</code>, where <code>X</code> is the offset number (in [[hexadecimal]] format) to the [[interrupt vector table]].