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What actually happens when a user reboots his or her system with an OS disk (or other bootable disk) in the CD/DVD drive?

For example, when I put in my OS installation CD and start the computer up, it says "Press any key to continue from CD or DVD...". How this is done on the actual CD/DVD? Because this indicates that when each time CPU is rebooted it first checks the CD drive...

Now my question is, how does a CD/DVD put these contents on it to indicate that it is a normal disk versus a bootable/OS disk? How does the computer identify such discs on boot?

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3 Answers 3

up vote 11 down vote accepted

The El Torito CD-ROM standard is an extension to the ISO-9660 spec that allows booting from CD. Some OS installation cds, Windows is one example, use these extensions to make installing an operating system easier for the user. Your computer is probably configured to check the CD-ROM drive each time the system boots for a bootable CD. Many CD burning applications have options to burn CDs using the El Torito specification.

In the case of a Windows installation disc, it may require you to hit a key to boot from the cd, otherwise it will try to boot from your hard drive instead. You can change the boot behavior of your system by going into the BIOS. It is possible to make a change to your system in your computer's BIOS that will render it unbootable. Most likely this can be repaired easily, but may cause less-knowledgable users a little trouble to recover.

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thanks for the answer! –  Ant's Jul 6 '11 at 1:26

Crash Course

When the computer is turned on, the BIOS (which is "just" another program, which is the first one run) is read into memory and executed by the CPU. (I have no idea how this is done, sorry.) It then performs any preparations or tests needed, according to the settings which are set up.

Those settings (which are incorrectly sometimes called "the BIOS") are what you can control when you boot your computer, by pressing e.g. Del or F10. They specify which devices the BIOS allows booting from.

Upon reading the settings, the BIOS tries, in order, to read the first sector of each device (called the Master Boot Record on most hard disks, or just "sector 0"), checks to see that it's bootable (it must end in the hexadecimal number 0xAA55), and if so, it just copies the sector to memory and transfers the control to the first byte of the data. After that, the MBR is on its own, and needs to load whatever that is needed.

Often times, for hard disks, this means that the MBR must parse the partition table and then read the first sector of the active partition to which it must boot, and then transfer control to that sector (called the boot sector). The boot sector then reads any files needed from the partition and calls the boot loader, which loads the operating system.

Of course, this last piece is not required. The device's boot sector can just do whatever it wants, and in the case of CDs or DVDs, other standards are in place that govern what is happening, e.g. El Torito. Different devices behave differently after they get control from the BIOS, so it's hard to say what happens after that without knowing about a specific type of device.

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thnaks for the answer, and there is no need to ask sorry in first para ^_O –  Ant's Jul 6 '11 at 1:26

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