Rebooting (computing)
Encyclopedia
In computing
, rebooting is the process by which a running computer system is restarted.
Reboots can be either "hard", whereby the power to the system is physically turned off, or "soft" where the system restarts without the need to interrupt the power or trigger a reset line.
s, after a hard reboot the filesystem may be in an "unclean" state, and an automatic scan of on-disk filesystem structures will be done before normal operation can begin.) A hard reboot may be caused by power failure, by accident, deliberately as a last resort to forcibly cause the system to go into a known state from a system freeze
or critical error
. It can also be used by intruders to access cryptographic keys from RAM, in which case it is called a cold boot attack
. The cold boot attack relies on the data remanence
property of DRAM
and SRAM
to retrieve memory contents which remain readable in the seconds to minutes after power has been removed.
However, other types of reset
can also disrupt the operating system in the same manner as power loss, for example
or (directly) triggering a reset line. It usually, though not always, refers to an orderly shutdown
and restarting of the machine.
The Control-Alt-Delete
key combination on the original PC from Sphere 1
was designed to allow a soft reboot reducing wear on the hardware. This kind of reboot will not usually reset the hard disks, so that they have time to update the write cache to permanent storage. Hard disks will also keep their configuration (like C/H/S
adjustments, HPA
, DCO
, internal passwords...) over these reboots. Special configurations may allow for part of the system state, like a RAM disk, to be preserved through the reboot.
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 independent of the system firmware. The kernel being executed does not have to be a Linux kernel.
s.
Operating systems in the Windows NT
family (from Windows NT 3.1
through Windows 7) have an option to skip their Blue Screen of Death
(BSOD) and immediately restart the computer in the event of a fatal error, thus users can be led to believe that a Windows computer suffers from random reboots. (Blue Screens of Death in the Windows NT family offer no option of pressing any key and seeing if the computer continues functioning, as was the case for Windows 9x
.)
or kernel panic
. The method by which this is done varies depending whether the reboot can be handled in software, or must be handled at the firmware or hardware level.
In Windows, when an error occurs in the boot process, a Blue Screen of Death
or a Black Screen of Death
may occur. On Unix and Unix-like operating systems, such as Linux and Mac OS X, a fatal error in the boot process may cause a kernel panic.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, rebooting is the process by which a running computer system is restarted.
Reboots can be either "hard", whereby the power to the system is physically turned off, or "soft" where the system restarts without the need to interrupt the power or trigger a reset line.
Hard reboot
A hard reboot (also known as a cold reboot, cold boot or cold start) is when power to a computer is abruptly turned off and then turned back on. This starts the computer without first performing any shutdown procedure (With many operating systems, especially those using disk cacheCache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
s, after a hard reboot the filesystem may be in an "unclean" state, and an automatic scan of on-disk filesystem structures will be done before normal operation can begin.) A hard reboot may be caused by power failure, by accident, deliberately as a last resort to forcibly cause the system to go into a known state from a system freeze
Hang (computing)
In computing, a hang or freeze occurs when either a single computer program, or the whole system ceases to respond to inputs. In the most commonly encountered scenario, a workstation with a graphical user interface, all windows belonging to the frozen program become static, and though the mouse...
or critical error
Fatal error
In computing, a fatal error or fatal exception error is an error that causes a program to abort and may therefore return the user to the operating system. When this happens, data that the program was processing may be lost. A fatal error is usually distinguished from a fatal system error...
. It can also be used by intruders to access cryptographic keys from RAM, in which case it is called a cold boot attack
Cold boot attack
In cryptography, a cold boot attack is a type of side channel attack in which an attacker with physical access to a computer is able to retrieve encryption keys from a running operating system after using a cold reboot to restart the machine from a completely "off" state...
. The cold boot attack relies on the data remanence
Data remanence
Data remanence is the residual representation of data that remains even after attempts have been made to remove or erase the data. This residue may result from data being left intact by a nominal file deletion operation, by reformatting of storage media that does not remove data previously written...
property of DRAM
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
and SRAM
Static random access memory
Static random-access memory is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM , it does not need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit...
to retrieve memory contents which remain readable in the seconds to minutes after power has been removed.
However, other types of reset
Reset (Computing)
In a computer or data transmission system, to reset means to clear any pending errors or events and bring a system to normal condition or initial state usually in a controlled manner. It is usually done in response to an error condition when it is impossible or undesirable for a processing activity...
can also disrupt the operating system in the same manner as power loss, for example
- Reset and System Reset buttons
- Initial Program Load (IPL) without prior OS shutdown
- POR on IBM mainframes
Soft reboot
A soft reboot (also known as a warm reboot) is restarting a computer under software control, without removing powerElectric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
or (directly) triggering a reset line. It usually, though not always, refers to an orderly shutdown
Shutdown (computing)
To shut down or power off a computer is to remove power from a computer's main components in a controlled way. After a computer is shut down, main components such as CPUs, RAM modules and hard disk drives are powered down, although some internal components, such as an internal clock, may retain...
and restarting of the machine.
The Control-Alt-Delete
Control-Alt-Delete
Control-Alt-Delete is a computer keyboard command on IBM PC compatible systems that can be used to reboot the computer, and summon the task manager or Windows Security in more recent versions of the Microsoft Windows operating system...
key combination on the original PC from Sphere 1
Sphere 1
The Sphere I was a personal computer completed in 1975 by Michael Donald Wise of Sphere Corporation, of Bountiful, Utah. The Sphere I featured a Motorola 6800 CPU, onboard ROM, Monitor, 4 KB of RAM, and a keyboard with a numeric keypad. Sphere I. The Sphere I was among the earliest microcomputers...
was designed to allow a soft reboot reducing wear on the hardware. This kind of reboot will not usually reset the hard disks, so that they have time to update the write cache to permanent storage. Hard disks will also keep their configuration (like C/H/S
Cylinder-head-sector
Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...
adjustments, HPA
Host Protected Area
The host protected area, sometimes referred to as hidden protected area, is an area of a hard drive that is not normally visible to an operating system .- History :HPA was first introduced in the ATA-4 standard cxv .-How it works:...
, DCO
Device configuration overlay
Device configuration overlay is a hidden area on many of today’s hard disk drives . Usually when information is stored in either the DCO or host protected area , it is not accessible by the BIOS, OS, or the user. However, certain tools can be used to modify the HPA or DCO...
, internal passwords...) over these reboots. Special configurations may allow for part of the system state, like a RAM disk, to be preserved through the reboot.
The Linux kernel has optional support for kexec
Kexec
In computing, kexec is a mechanism of the Linux kernel that allows "live" booting of a new kernel "over" the currently running kernel. kexec skips the bootloader stage and directly loads the new kernel into memory, where it starts executing immediately...
, a system call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
which transfers execution to a new kernel and skips hardware or firmware reset. The entire process occurs independent of the system firmware. The kernel being executed does not have to be a Linux kernel.
Random reboot
Random reboot is a non-technical term referring to an unintended (and often undesired) reboot for which the cause is not immediately evident to the user. Such reboots may occur due to a multitude of software and hardware problems, such as triple faultTriple fault
A triple fault is a special kind of exception generated by the CPU when an exception occurs while the CPU is trying to invoke the double fault exception handler, which itself handles exceptions occurring while trying to invoke a regular exception handler....
s.
Operating systems in the Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
family (from Windows NT 3.1
Windows NT 3.1
Windows NT 3.1 is the first release of Microsoft's Windows NT line of server and business desktop operating systems, and was released to manufacturing on 27 July 1993. The version number was chosen to match the one of Windows 3.1, the then-latest operating environment from Microsoft, on account of...
through Windows 7) have an option to skip their Blue Screen of Death
Blue Screen of Death
To forse a BSOD Open regedit.exe,Then search: HKLM\SYSTEM\CurrentControlSet\services\i8042prt\ParametersThen make a new DWORD called "CrashOnCtrlScroll" And set the value to 1....
(BSOD) and immediately restart the computer in the event of a fatal error, thus users can be led to believe that a Windows computer suffers from random reboots. (Blue Screens of Death in the Windows NT family offer no option of pressing any key and seeing if the computer continues functioning, as was the case for Windows 9x
Windows 9x
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...
.)
Rebooting automatically
Systems may reboot automatically at a scheduled time, after a power failure, or a fatal system errorFatal System Error
Fatal System Error is a nonfiction work written by Joseph Menn that exposes a story of espionage that penetrates the network of international mobsters and hackers who use the Internet to extort money from businesses, steal from tens of millions of consumers, and attack government networks.Its main...
or kernel panic
Kernel panic
A kernel panic is an action taken by an operating system upon detecting an internal fatal error from which it cannot safely recover. The term is largely specific to Unix and Unix-like systems; for Microsoft Windows operating systems the equivalent term is "Bug check" .The kernel routines that...
. The method by which this is done varies depending whether the reboot can be handled in software, or must be handled at the firmware or hardware level.
In Windows, when an error occurs in the boot process, a Blue Screen of Death
Blue Screen of Death
To forse a BSOD Open regedit.exe,Then search: HKLM\SYSTEM\CurrentControlSet\services\i8042prt\ParametersThen make a new DWORD called "CrashOnCtrlScroll" And set the value to 1....
or a Black Screen of Death
Black Screen of Death
The Black Screen of Death is generally a colloquialism used for the black error screen displayed by some operating systems after encountering a critical system error which can cause the system to shut down to prevent damage.-Windows 3.x:...
may occur. On Unix and Unix-like operating systems, such as Linux and Mac OS X, a fatal error in the boot process may cause a kernel panic.