Wikipedia

Reboot

For other uses, seeReboot (disambiguation).

Incomputing,rebootingis the process by which a runningcomputer systemis restarted, either intentionally or unintentionally. Reboots can be either acold reboot(alternatively known as ahard reboot) in which the power to the system is physically turned off and back on again (causing aninitial bootof the machine); or awarm reboot(orsoft reboot) in which the system restarts while still powered up. The termrestart(as a system command) is used to refer to a reboot when theoperating systemcloses all programs and finalizes all pending input and output operations before initiating a soft reboot.

Terminology

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Etymology

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Early electronic computers (like theIBM 1401) had no operating system and little internal memory. The input was often a stack ofpunch cardsor via aSwitch 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, "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 calledbootingthe computer up. If the computercrashed,it was rebooted. The boot reference carried over to all subsequent types of computers.

Cold versus warm reboot

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"Cold boot" redirects here. For the attack called a "cold boot attack", seeCold boot attack.

ForIBM PC compatiblecomputers, acold bootis a boot process in which the computer starts from a powerless state, in which the system performs a completepower-on self-test(POST).[1][2][3][4]Both the operating system and third-party software can initiate a cold boot; the restart command inWindows 9xinitiates a cold reboot, unless Shift key is held.[1]: 509 

Awarm bootis initiated by theBIOS,either as a result of theControl-Alt-Deletekey combination[1][2][3][4]or directly through BIOS interruptINT19h.[5]It may not perform a complete POST - for example, it may skip the memory test - and may not perform a POST at all.[1][2][4]Malwaremay prevent or subvert a warm boot by intercepting the Ctrl + Alt + Delete key combination and prevent it from reaching BIOS.[6]TheWindows NTfamily of operating systems also does the same and reserves the key combination for its own use.[7][8]

TheLinuxfamily of operating systems supports an alternative to warm boot; the Linux kernel has optional support forkexec,asystem callwhich 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]

Outside the domain of IBM PC compatible computers, the types of boot may not be as clear. According to Sue Loh ofWindows CEBase 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 erasesallforms 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 aWindows Mobile 5.0device, the former is a cold boot and the latter is a clean boot.[9]

Hard reboot

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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 areset,bycycling power,by issuing thehalt-qcommand in mostUnix-likesystems, or by triggering akernel panic.

Hard reboots are used in thecold boot attack.

Restart

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The term "restart" is used by theMicrosoft WindowsandLinuxfamilies 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

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Deliberate

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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,[10]for examplememory leaksor processes thathogresources to the detriment of the overall system, or to terminatemalware.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 amulti-bootsystem without ahypervisor,a reboot is required to switch between installed operating systems.
  • Offensive:As stated earlier, components lose power during a cold reboot; therefore, components such asRAMthat require power lose the data they hold. However, in acold boot attack,special configurations may allow for part of the system state, like a RAM disk, to be preserved through the reboot.[11]

The means of performing a deliberate reboot also vary and may include:

  • Manual, hardware-based:A power switch orreset buttoncan cause the system to reboot.[2]Doing so, however, may cause the loss of all unsaved data.[12]
  • Manual, software-based:Computer software and operating system can trigger a reboot as well; bothMicrosoft Windowsand severalUnix-likeoperating systems can be shut down from the command line[1]: 509 [13][14][15][16][17][18][19][20][21]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.[22]

Power failure

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Unexpected loss of power for any reason (includingpower outage,power supplyfailure or depletion ofbatteryon a mobile device) forces the system user to perform a cold boot once the power is restored. SomeBIOSeshave an option to automatically boot the system after a power failure.[23][24]Anuninterruptible power supply(UPS), backup battery or redundant power supply can prevent such circumstances.

Random reboot

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"Random reboot" is a non-technical term referring to an unintended (and often undesired) reboot following asystem 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 astriple faults.They are generally symptomatic of an error inring 0that is not trapped by anerror handlerin an operating system or a hardware-triggerednon-maskable interrupt.

Systems may be configured to reboot automatically after a power failure, or afatal system errororkernel 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 theWindows NTfamily (fromWindows NT 3.1throughWindows 7) have an option to modify the behavior of the error handler so that a computer immediately restarts rather than displaying aBlue Screen of Death(BSOD) error message. This option is enabled by default in some editions.

Hibernation

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Main article:Hibernation (computing)

The introduction ofadvanced power managementallowed operating systems greater control of hardware power management features. WithAdvanced Configuration and Power Interface(ACPI), newer operating systems are able to manage different power states and thereby sleep and/orhibernate.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

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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 thebsodXScreenSaver"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.[6]

Microsoft App-Vsequencing 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.[25]

Windows deviations and labeling criticism

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Windows 8&10enable (by default) ahibernation-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.[26][27][28]

See also

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References

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  1. ^abcdeJones, James G.; Landes, Craig; Tittel, Ed (2002).A+ Exam Cram 2 (Exam Cram 220-221, Exam Cram 220-222).Que Publishing. pp. 379–380, 509.ISBN9780789728678.
  2. ^abcdCooper, Jim (2002).Using MS-DOS 6.22.Que Publishing. pp. 24, 960, 964.ISBN9780789725738.
  3. ^abTulloch, Mitch; Tulloch, Ingrid (2002).Microsoft Encyclopedia of Networking(2nd ed.). Microsoft Press. p. 172.ISBN0-7356-1378-8.
  4. ^abcSoper, Mark Edward (2004).Absolute Beginner's Guide to A+ Certification.Que Publishing. p. 188.ISBN9780789730626.
  5. ^"Hardware Troubleshooting: Cold Booting Versus Warm Booting".Support.Microsoft. 7 July 2005. Archived fromthe originalon 21 February 2015.
  6. ^abGrimes, Roger (1 June 2001)."2".Malicious Mobile Code: Virus Protection for Windows.O'Reilly Media. p. 50.ISBN9781565926820.
  7. ^"Using CTRL+ALT+DEL key combination to open Windows Security opens Task Manager".Microsoft Support.Microsoft. 15 January 2006.Retrieved19 January2013.
  8. ^Osterman, Larry (24 January 2005)."Why is Control-Alt-Delete the secure attention sequence (SAS)?".Larry Osterman's WebLog.MSDN Blogs.Retrieved15 March2007.
  9. ^Loh, Sue (22 June 2005)."Boot Persistence Terminology".Windows CE Base Team Blog.Microsoft.Retrieved3 March2013.
  10. ^Debogue, Jean."Did you reboot".WeDebugYou.
  11. ^Ken Freeze (28 October 1985),"Jram-3 Rivals Persyst Stretch In RAM Race of Add-On Boards",InfoWorld
  12. ^Shelly, Gary B.; Vermaat, Misty E. (2012).Enhanced Discovering Computers, Complete: Your Interactive Guide to the Digital World, 2013 Edition.Cengage Learning. p. 400.ISBN9781133598312.
  13. ^"shutdown".Microsoft Learn.5 October 2023.
  14. ^"Stop-Computer".Microsoft Learn.
  15. ^shutdown(8)–DarwinandmacOSSystem Manager'sManual
  16. ^shutdown(8)–DragonFly BSDSystem Maintenance and Operation CommandsManual
  17. ^shutdown(8)–FreeBSDSystem Manager'sManual
  18. ^shutdown(8)–LinuxProgrammer'sManual– Administration and Privileged Commands
  19. ^shutdown(8)–NetBSDSystem Manager'sManual
  20. ^shutdown(8)–OpenBSDSystem Manager'sManual
  21. ^shutdown(8)–Solaris 11.4System Administration Commands ReferenceManual
  22. ^Gary A. Donahue, Gary A. (2011).Network Warrior(2nd ed.).Sebastopol:O'Reilly Media.p. 698.ISBN9781449309350.
  23. ^"AsRock 775i65G User Manual"(PDF).p. 32.
  24. ^"Supermicro C7Q67 User's Manual"(PDF).p. 69.[permanent dead link‍]
  25. ^"Microsoft App-V 4.6 SP1 Sequencing Guide".
  26. ^Walter Glenn (3 March 2016)."The Pros and Cons of Windows 10's" Fast Startup "Mode".How-To Geek.
  27. ^Oliver Niehus (14 December 2012)."Windows 8: Fast Boot".MSDN - Oliver's Blog.
  28. ^"Distinguishing Fast Startup from Wake-from-Hibernation".Microsoft Hardware Dev Center.15 December 2021.
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