Bootable CDs can be configured as "floppy emulation", "hard drive emulation", or "no emulation". Floppy emulation is common for certain OS's (MS-DOS, Windows installer), while no-emulation is common for many other OS's (Linux via ISOLINUX, installer for Windows XP and later)

When a "floppy emulation" CD boots, it goes through two stages:

• Floppy — This isn't the real floppy drive, the data really comes from the CDROM. However, to the software that's running, it DOES look like its data is coming from the floppy drive. The BIOS makes this magic happen.

  The size of this floppy section is limited to ~1-2MB. The boot software's goal at this point is to load the proper CDROM driver, so the operating system can see the files on the CDROM part of the disk. If it doesn't have the proper CD drivers for your hardware (there's only ~1MB of space available, so it can't store EVERY hardware driver), then booting will fail at this point, because it can't access the rest of its files.
  
  

• CD — Once the proper CDROM drivers have been loaded, the operating system can see the rest of the files on the disk (700MB on a CD, 4.7GB on a DVD). Everything is happy at this point, though the operating system does still see the CD data as being split up into two different drives: the ~1MB floppy part, and the much larger CD part.