Yes..Bootable CDs have some programs in the first sector of the Disc.
When you choose 1st boot device as CD Drive, BIOS passes the control to the program residing in 1st sector of the CD, that is , the program in the 1st sector loads into RAM and gets executed. This results in the loading of OS like DOS etc.
Bootable CD: A bootable CD-Rom is a CD from which you boot the PC. A PC can boot the CD if the PCs BIOS supports El Torito Bootable CD-ROM Format Specification.
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