Incomputing,rebooting is the process by which a runningcomputer system is 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 boot of 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 system closes all programs and finalizes all pending input and output operations before initiating a soft reboot.
Early electronic computers (like theIBM 1401) had no operating system and little internal memory. The input was often a stack ofpunch cards or 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 calledbooting the computer up. If the computercrashed, it was rebooted. The boot reference carried over to all subsequent types of computers.
 | This article'sfactual accuracy may be compromised due to out-of-date information. The reason given is: 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.. Please help update this article to reflect recent events or newly available information.(December 2023) |
ForIBM PC compatible computers, acold boot is 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 9x initiates a cold reboot, unless Shift key is held.[1]: 509
Awarm boot is initiated by theBIOS, either as a result of theControl-Alt-Delete key combination[1][2][3][4] or directly through BIOS interruptINT 19h.[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]Malware may prevent or subvert a warm boot by intercepting the Ctrl + Alt + Delete key combination and prevent it from reaching BIOS.[6] TheWindows NT family of operating systems also does the same and reserves the key combination for its own use.[7][8]
Operating systems based onLinux support an alternative to warm boot; the Linux kernel has optional support forkexec, asystem 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]
Outside the domain of IBM PC compatible computers, the types of boot may not be as clear. According to Sue Loh ofWindows CE Base Team, Windows CE devices support three types of boots: Warm, cold and clean.[9] A warm boot discards program memory. A cold boot additionally discards storage memory (also known as the "object store"), while a clean boot erasesall 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 aWindows Mobile 5.0 device, the former is a cold boot and the latter is a clean boot.[9]
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-q command in mostUnix-like systems, or by triggering akernel panic.
Hard reboots are used in thecold boot attack.
The term "restart" is used by theMicrosoft Windows andLinux 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.
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Users may deliberately initiate a reboot. Rationale for such action may include:
The means of performing a deliberate reboot also vary and may include:
Unexpected loss of power for any reason (includingpower outage,power supply failure or depletion ofbattery on a mobile device) forces the system user to perform a cold boot once the power is restored. SomeBIOSes have 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" 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 0 that is not trapped by anerror handler in an operating system or a hardware-triggerednon-maskable interrupt.
Systems may be configured to reboot automatically after a power failure, or afatal system error orkernel 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 NT family (fromWindows NT 3.1 throughWindows 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.
The introduction ofadvanced power management allowed 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.
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-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.[25]
Windows 8 &10 enable (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]
shutdown(8) – Darwin andmacOS System Manager'sManualshutdown(8) – DragonFly BSD System Maintenance and Operation CommandsManualshutdown(8) – FreeBSD System Manager'sManualshutdown(8) – Linux Programmer'sManual – Administration and Privileged Commandsshutdown(8) – NetBSD System Manager'sManualshutdown(8) – OpenBSD System Manager'sManualshutdown(8) – Solaris 11.4 System Administration Commands ReferenceManual