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What makes bootable media bootable?

As far as I believe, if you 'drag and drop' an ISO image onto a device such as a usb or cd it won't boot as It's just random files, the BIOS doesn't know It's bootable.

What makes a device bootable to the BIOS; how do you flag a device bootable? What files are essential to a bootable device, ie .img files?

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marked as duplicate by Tom Wijsman, Wuffers, techie007, slhck, Nifle Aug 6 '11 at 10:36

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As far as CD's go, look at the El Torito Standard. Basically, it all works out to a "magic number" that the BIOS reads off of a device which is specifically reserved for bootable devices. –  new123456 Aug 4 '11 at 13:11
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Actually there are some details involved with the process. To boot-up a device, it must be formated with a partition that begins with a specific code on the first sectors, these partition area are called MBR.

A Master Boot Record (MBR) is the bootsector of a hard disk. That is, it is what the BIOS loads and runs, when it boots a hard disk. The MBR is the very first sector of the hard disk; it contains an MBR Bootstrap program (described below), and a Partition Table. Devices that emulate a hard disk during system initialization must also contain an MBR, because they must also have Partition Tables -- even if they are not bootable. The BIOS will only boot an MBR from a device if that device is in the "boot sequence" stored in CMOS, and if the MBR on the device is formatted correctly. On the other hand, if a device is not in the boot sequence (but has a "drive number"), it is still possible for a Real Mode program (such as another MBR or bootloader) to load and boot that device's MBR directly.

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That is a bit complex issue. You could start by reading something about master boot record: http://en.wikipedia.org/wiki/Master_boot_record

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An ISO file is an Image, should not be interpreted or burned as Data, but burned as an Image, this lays out the data contained in the image on the disc properly with boot sector, if it is a bootable ISO image. Not all ISO images are bootable (no boot sector).

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