Reboot and Causes of Reboot
In computing, rebooting is the process by which a running computer system is restarted, either intentionally or unintentionally. Reboots can be either "cold" (alternatively known as "hard") where the power to the system is physically turned off and back on again, causing an initial boot of the machine, or warm (alternatively known as soft) where the system restarts without the need to interrupt the power. The term restart 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.
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, for example memory leaks or processes that 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 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.
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. Doing so, however, may cause the loss of all unsaved data.
- Manual, software-based: Computer software and operating system can trigger a reboot as well; more specifically, Microsoft Windows operating systems are outfitted with a restart command that closes open programs and eliminates data loss due to reboot.
- Automated: Software can be scheduled to run at a certain time and date; therefore, it is possible to schedule a reboot.
Unexpected loss of power for any reason (including power outage, power supply failure or depletion of battery on a mobile device) forces the system user to perform a cold boot once the power is restored. Some BIOSes have an option to automatically boot the system after a power failure. An uninterruptible 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 a 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 faults. They are generally symptomatic of an error in ring 0 that is not trapped by an 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.
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