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In order for a CD to be bootable, it must contain two files: BOOTCAT. BIN and BOOTIMG.BIN. BOOTCAT.BIN is a catalog file, and BOOTIMG.BIN is an image file an image of a bootable floppy disk. That’s why you need an existing bootable floppy in order to make a bootable CD.
When you browse the contents of a bootable CD in Windows or at a command prompt, you won’t see any of the files that you would find on a bootable floppy; that’s because they are all stored within BOOTIMG.BIN.
When you boot from the bootable CD, everything in BOOTIMG.BIN and BOOTCAT.BIN shows up as being on the A: drive, while everything else on the CD shows up on the regular CD drive letter. It tricks the PC into thinking that there is actually a floppy in the A: drive.