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A live CD (also live DVD, live disc, or live operating system) is a complete bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than loading from a hard disk drive.
A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS Setup function by pressing a special key when the computer is first turned on (such as Delete, F1, F2, F10 or F12), and then changing the boot order. [6]
Restarting a computer also is called rebooting, which can be "hard", e.g. after electrical power to the CPU is switched from off to on, or "soft", where the power is not cut. On some systems, a soft boot may optionally clear RAM to zero. Both hard and soft booting can be initiated by hardware such as a button press or by a software command.
This can be a harddisk, floppy, CD/DVD, network connection, USB-device, etc. depending on the BIOS. In the case of a floppy the BIOS interprets its boot sector (first sector) as code, for NTLDR this could be a NTLDR boot sector looking for the ntldr file on the floppy.
Windows Recovery Environment can also be installed to a hard drive partition by OEMs, [28] and customized with additional tools such as a separate system recovery tool for restoring the computer back to its original state. [29] As of Windows Vista SP1, users can create their own bootable CD containing the recovery environment.
Once all the boot and system drivers have been loaded, the kernel starts the session manager (smss.exe), which begins the login process. After the user has successfully logged into the machine, winlogon applies User and Computer Group Policy setting and runs startup programs declared in the Windows Registry and in "Startup" folders. [5]
This also allows them to be run from non-writable media such as a CD-ROM. [5] Since each instance of BartPE is a new installation, the BartPE "boot" disk needs original Windows Setup files in order to operate. The Bart PE Builder application interprets and condenses files from a Windows setup CD to create the BartPE installation.
On the original IBM PC and XT, if no bootable disk was found, the BIOS would try to start ROM BASIC with the interrupt call to interrupt 18h. Since few programs used BASIC in ROM, clone PC makers left it out; then a computer that failed to boot from a disk would display "No ROM BASIC" and halt (in response to interrupt 18h).