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Concept
Reboot
Summary
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 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.
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 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, "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 crashed, it was rebooted. The boot reference carried over to all subsequent types of computers.
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 power-on self-test (POST). 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.
A warm boot is initiated by the BIOS, either as a result of the Control-Alt-Delete key combination or directly through BIOS interrupt INT 19h. It does not perform a POST. Malware may prevent or subvert a warm boot by intercepting the Ctrl + Alt + Delete key combination and prevent it from reaching BIOS.
Official source
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