View source for Reboot

{{Short description|Restarting a computer system or process}}
{{Other uses}}
{{Use dmy dates|date=October 2020}}
In [[computing]], '''rebooting''' is the process by which a running [[computer system]] is restarted, either intentionally or unintentionally. Reboots can be either  a '''cold reboot''' (alternatively known as a '''hard reboot''') in which the power to the system is physically turned off and back on again (causing an [[Booting|initial boot]] of the machine); or a '''warm reboot''' (or '''soft reboot''') in which the system restarts while still powered up. The term '''restart''' (as a system command) is used to refer to a reboot when the [[operating system]] closes all programs and finalizes all pending input and output operations before initiating a soft reboot.

== Terminology ==
=== Etymology ===
Early electronic computers (like the [[IBM 1401]]) had no operating system and little internal memory. The input was often a stack of [[punch cards]] or via a [[Front panel|Switch Register]]. On systems with cards, the computer was initiated by pressing a start button that performed a single command - "read a card". This first card then instructed the machine to read more cards that eventually loaded a user program. This process was likened to an old saying, "[[Bootstrapping|picking yourself up by the bootstraps]]", referring to a horseman who lifts himself off the ground by pulling on the straps of his boots. This set of initiating punch cards was called "bootstrap cards". Thus a cold start was called ''[[booting]]'' the computer up.  If the computer [[Crash (computing)|crashed]], it was rebooted. The boot reference carried over to all subsequent types of computers.

=== Cold versus warm reboot ===
{{Anchor|Cold|Clean|Warm}}
{{Redirect|Cold boot|the attack called a "cold boot attack"|Cold boot attack}}

{{Update|date=December 2023|reason=The described differences between a cold and a warm reboot aren't universally applicable. Also, "BIOS" is not the firmware of a PC, it is just a long-obsolete API for such a firmware, and this API is usually no longer involved in the boot process.|part=|inaccurate=yes}}

For [[IBM PC compatible]] computers, a '''cold boot''' is a boot process in which the computer starts from a powerless state, in which the system performs a complete [[power-on self-test]] (POST).<ref name="Jones, Landes, Tittel (2002)" /><ref name="Cooper (2002)" /><ref name="Tulloch (2002)" /><ref name="Soper (2004)" /> Both the operating system and third-party software can initiate a cold boot; the restart command in [[Windows 9x]] initiates a cold reboot, unless Shift key is held.<ref name="Jones, Landes, Tittel (2002)" />{{rp|509}}

A '''warm boot''' is initiated by the [[BIOS]], either as a result of the [[Control-Alt-Delete]] key combination<ref name="Jones, Landes, Tittel (2002)" /><ref name="Cooper (2002)" /><ref name="Tulloch (2002)" /><ref name="Soper (2004)" /> or directly through BIOS interrupt [[INT (x86 instruction)|INT]] 19h.<ref name="Microsoft (2005)" /> It may not perform a complete POST - for example, it may skip the memory test - and may not perform a POST at all.<ref name="Jones, Landes, Tittel (2002)" /><ref name="Cooper (2002)" /><ref name="Soper (2004)" /> [[Malware]] may prevent or subvert a warm boot by intercepting the Ctrl + Alt + Delete key combination and prevent it from reaching BIOS.<ref name="Grimes (2001)" /> The [[Windows NT]] family of operating systems also does the same and reserves the key combination for its own use.<ref>{{cite web|title=Using CTRL+ALT+DEL key combination to open Windows Security opens Task Manager|url=http://support.microsoft.com/kb/281980|work=Microsoft Support|publisher=Microsoft|access-date=19 January 2013|date=15 January 2006}}</ref><ref>{{cite web |url=http://blogs.msdn.com/larryosterman/archive/2005/01/24/359850.aspx |title=Why is Control-Alt-Delete the secure attention sequence (SAS)? |last=Osterman |first=Larry |work=Larry Osterman's WebLog |publisher=[[MSDN Blogs]] |date=24 January 2005 |access-date=15 March 2007}}</ref>

The [[Linux]] family of operating systems supports an alternative to warm boot; the Linux kernel has optional support for [[kexec]], a [[system call]] which transfers execution to a new kernel and skips hardware or firmware reset. The entire process occurs independently of the system firmware. The kernel being executed does not have to be a Linux kernel.{{Citation needed|date=February 2023}}

Outside the domain of IBM PC compatible computers, the types of boot may not be as clear. According to Sue Loh of [[Windows CE]] Base Team, Windows CE devices support three types of boots: Warm, cold and clean. A warm boot discards program memory. A cold boot additionally discards storage memory (also known as the "object store"), while a clean boot erases ''all'' forms of memory storage from the device. However, since these areas do not exist on all Windows CE devices, users are only concerned with two forms of reboot: one that resets the volatile memory and one that wipes the device clean and restores factory settings. For example, for a [[Windows Mobile 5.0]] device, the former is a cold boot and the latter is a clean boot.<ref>{{cite web|last=Loh|first=Sue|title=Boot Persistence Terminology|url=http://blogs.msdn.com/b/ce_base/archive/2005/06/22/431699.aspx|work=Windows CE Base Team Blog|publisher=Microsoft|access-date=3 March 2013|date=22 June 2005}}</ref>

=== Hard reboot ===
{{Anchor|Hard|Soft|Quick}}
A hard reboot means that the system is not shut down in an orderly manner, skipping file system synchronisation and other activities that would occur on an orderly shutdown. This can be achieved by either applying a [[Reset (computing)|reset]], by [[Power cycling|cycling power]], by issuing the {{code|lang=sh|halt -q}} command in most [[Unix-like]] systems, or by triggering a [[kernel panic]].

Hard reboots are used in the [[cold boot attack]].

=== Restart ===
The term "restart" is used by the [[Microsoft Windows]] and [[Linux]] families of operating systems to denote an operating system-assisted reboot. In a restart, the operating system ensures that all pending I/O operations are gracefully ended before commencing a reboot.

== Causes ==
=== Deliberate ===
[[File:PC-Netzteil (redundant).jpg|thumb|upright|right|Redundant power supplies in a computer system reduce the risk of an unexpected loss of power.]]
Users may deliberately initiate a reboot. Rationale for such action may include:
* '''Troubleshooting:''' Rebooting may be used by users, support staff or system administrators as a technique to work around bugs in software,<ref>{{cite web|title=Did you reboot|last=Debogue|first=Jean|url=http://www.wedebugyou.com/2012/07/did-you-reboot/|publisher=WeDebugYou}}</ref> for example [[memory leak]]s or processes that [[Server hog|hog]] resources to the detriment of the overall system, or to terminate [[malware]]. While this approach does not address the root cause of the issue, resetting a system back to a good, known state may allow it to be used again for some period until the issue next occurs.
* '''Switching operating systems:''' On a [[multi-boot]] system without a [[hypervisor]], a reboot is required to switch between installed operating systems.
* '''Offensive:''' As stated earlier, components lose power during a cold reboot; therefore, components such as [[random-access memory|RAM]] that require power lose the data they hold. However, in a [[cold boot attack]], special configurations may allow for part of the system state, like a RAM disk, to be preserved through the reboot.<ref>{{citation|url=https://books.google.com/books?id=fS8EAAAAMBAJ&pg=PA48|work=InfoWorld|date=28 October 1985|title=Jram-3 Rivals Persyst Stretch In RAM Race of Add-On Boards|author=Ken Freeze}}</ref>

The means of performing a deliberate reboot also vary and may include:
* '''Manual, hardware-based:''' A power switch or [[reset button]] can cause the system to reboot.<ref name="Cooper (2002)" /> Doing so, however, may cause the loss of all unsaved data.<ref name="Shelly, Vermaat (2012)" />
* '''Manual, software-based:''' Computer software and operating system can trigger a reboot as well; both [[Microsoft Windows]] and several [[Unix-like]] operating systems can be shut down from the command line<ref name="Jones, Landes, Tittel (2002)" />{{rp|509}}<ref>{{cite web |url=https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/shutdown |title=shutdown |date=2023-10-05 |website=[[Microsoft Learn]]}}</ref><ref>{{cite web |url=https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.management/stop-computer?view=powershell-7.4 |title=Stop-Computer |website=[[Microsoft Learn]]}}</ref><ref>{{man|8|shutdown|Darwin}}</ref><ref>{{man|8|shutdown|DragonFly BSD}}</ref><ref>{{man|8|shutdown|FreeBSD}}</ref><ref>{{man|8|shutdown|Linux}}</ref><ref>{{man|8|shutdown|NetBSD}}</ref><ref>{{man|8|shutdown|OpenBSD}}</ref><ref>{{man|8|shutdown|Solaris}}</ref> or through the GUI.
* '''Automated:''' Software can be scheduled to run at a certain time and date; therefore, it is possible to schedule a reboot.<ref>{{cite book|last=Gary A. Donahue|first=Gary A.|title=Network Warrior|year=2011|publisher=[[O'Reilly Media]]|location=[[Sebastopol, California|Sebastopol]]|isbn=9781449309350|page=698|edition=2nd}}</ref>

=== Power failure ===
Unexpected loss of power for any reason (including [[power outage]], [[power supply]] failure or depletion of [[Electric battery|battery]] on a mobile device) forces the system user to perform a cold boot once the power is restored. Some [[BIOS]]es have an option to automatically boot the system after a power failure.<ref>{{cite web|url=ftp://europe.asrock.com/manual/775i65G%20R3.0.pdf|title=AsRock 775i65G User Manual|page=32}}</ref><ref>{{cite web|url=ftp://ftp.supermicro.com/CDR-C7_1.02_for_Intel_C7_platform/MANUALS/C7Q67.pdf|title=Supermicro C7Q67 User's Manual|page=69}}{{dead link|date=April 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> An [[uninterruptible power supply]] (UPS), backup battery or redundant power supply can prevent such circumstances.

=== Random reboot{{anchor|Random}} ===
"Random reboot" is a non-technical term referring to an unintended (and often undesired) reboot following a [[Crash (computing)|system crash]], whose root cause may not immediately be evident to the user. Such crashes may occur due to a multitude of software and hardware problems, such as [[triple fault]]s.  They are generally symptomatic of an error in [[Ring (computer security)|ring 0]] that is not trapped by an [[Exception handling|error handler]] in an operating system or a hardware-triggered [[non-maskable interrupt]].

Systems may be configured to reboot automatically after a power failure, or a [[fatal system error]] or [[kernel panic]]. The method by which this is done varies depending on whether the reboot can be handled via software or must be handled at the firmware or hardware level.  Operating systems in the [[Windows NT]] family (from [[Windows NT 3.1]] through [[Windows 7]]) have an option to modify the behavior of the error handler so that a computer immediately restarts rather than displaying a [[Blue Screen of Death]] (BSOD) error message. This option is enabled by default in some editions.

== Hibernation ==
{{Main|Hibernation (computing)}}
The introduction of [[advanced power management]] allowed operating systems greater control of hardware power management features. With [[Advanced Configuration and Power Interface]] (ACPI), newer operating systems are able to manage different power states and thereby sleep and/or [[Hibernation (computing)|hibernate]]. While hibernation also involves turning a system off then subsequently back on again, the operating system does not start from scratch, thereby differentiating this process from rebooting.

== Simulated reboot ==
A reboot may be simulated by software running on an operating system.  For example: the Sysinternals BlueScreen utility, which is used for pranking; or some modes of the {{mono|bsod}} [[XScreenSaver]] "hack", for entertainment (albeit possibly concerning at first glance). Malware may also simulate a reboot, and thereby deceive a computer user for some nefarious purpose.<ref name="Grimes (2001)" />

[[Microsoft App-V]] sequencing tool captures all the file system operations of an installer in order to create a virtualized software package for users. As part of the sequencing process, it will detect when an installer requires a reboot, interrupt the triggered reboot, and instead simulate the required reboot by restarting services and loading/unloading libraries.<ref>{{Cite web|url=http://download.microsoft.com/download/f/7/8/f784a197-73be-48ff-83da-4102c05a6d44/app-v/app-v%204.6%20service%20pack%201%20sequencing%20guide.docx|title=Microsoft App-V 4.6 SP1 Sequencing Guide}}</ref>

== Windows deviations and labeling criticism ==
[[Windows 8]] & [[Windows 10|10]] enable (by default) a [[Hibernation (computing)|hibernation]]-like "Fast Startup" (a.k.a. "Fast Boot") which can cause problems (including confusion) for users accustomed to turning off computers to (cold) reboot them.<ref>{{cite web|url=https://www.howtogeek.com/243901/the-pros-and-cons-of-windows-10s-fast-startup-mode/|title=The Pros and Cons of Windows 10's "Fast Startup" Mode|author=Walter Glenn|date=3 March 2016|website=How-To Geek}}</ref><ref>{{cite web|url=https://blogs.msdn.microsoft.com/olivnie/2012/12/14/windows-8-fast-boot/|title=Windows 8: Fast Boot|author=Oliver Niehus|date=14 December 2012|website=MSDN - Oliver's Blog}}</ref><ref>{{cite web|url=https://docs.microsoft.com/en-us/windows-hardware/drivers/kernel/distinguishing-fast-startup-from-wake-from-hibernation|title=Distinguishing Fast Startup from Wake-from-Hibernation|website=Microsoft Hardware Dev Center|date=15 December 2021 }}</ref>

== See also ==
* [[Booting]]
* [[Cold start (computing)]]
* [[Local reboot]]
* [[Power cycling]]
* [[Reset (computing)]]

== References ==
{{Reflist|2|refs =

<ref name="Jones, Landes, Tittel (2002)">{{Cite book |url=https://books.google.com/books?id=WtE9GN1hhvAC |title=A+ Exam Cram 2 (Exam Cram 220-221, Exam Cram 220-222) |publisher=Que Publishing |year=2002 |first1=James G. |last1=Jones |first2=Craig |last2=Landes |first3=Ed |last3=Tittel |isbn=9780789728678 |pages=379–380, 509}}</ref>

<ref name="Soper (2004)">{{Cite book |url=https://books.google.com/books?id=KHCRkX5d-gAC |title=Absolute Beginner's Guide to A+ Certification |publisher=Que Publishing |year= 2004 |first1=Mark Edward |last1=Soper |isbn=9780789730626 |page=188}}</ref>

<ref name="Cooper (2002)">{{Cite book |url=https://books.google.com/books?id=u7oN-5y7nGsC |title=Using MS-DOS 6.22 |publisher=Que Publishing |year=2002 |first1=Jim |last1=Cooper |isbn=9780789725738 |pages=24, 960, 964}}</ref>

<ref name="Tulloch (2002)">{{Cite book |title=Microsoft Encyclopedia of Networking |edition=2nd |first1=Mitch |last1=Tulloch |first2=Ingrid |last2=Tulloch |publisher=Microsoft Press |isbn=0-7356-1378-8 |year=2002 |page=172}}</ref>

<ref name="Microsoft (2005)">{{Cite web |url=http://support.microsoft.com/kb/102228 |title=Hardware Troubleshooting: Cold Booting Versus Warm Booting |work=Support |publisher=Microsoft |date=7 July 2005 |archive-url=https://web.archive.org/web/20150221095215/http://support.microsoft.com/kb/102228 |archive-date=21 February 2015}}</ref>

<ref name="Grimes (2001)">{{Cite book |chapter-url=https://books.google.com/books?id=GKDtVYJ0wesC&pg=PA50 |title=Malicious Mobile Code: Virus Protection for Windows |publisher=O'Reilly Media |first=Roger |last=Grimes |date=1 June 2001 |chapter=2 |isbn=9781565926820 |page=50}}</ref>

<ref name="Shelly, Vermaat (2012)">{{Cite book |url=https://books.google.com/books?id=GDwIOT1FiwkC |title=Enhanced Discovering Computers, Complete: Your Interactive Guide to the Digital World, 2013 Edition |publisher=Cengage Learning |year=2012 |first1=Gary B. |last1=Shelly |first2=Misty E. |last2=Vermaat |isbn=9781133598312 |page=400}}</ref>

}}

[[Category:Booting]]
[[Category:BIOS]]
[[Category:Computer errors]]
[[Category:Computer jargon]]