GRUB supports the no emulation mode in the El Torito specification1. This means that you can use the whole CD-ROM from GRUB and you don't have to make a floppy or hard disk image file, which can cause compatibility problems.
For booting from a CD-ROM, GRUB uses a special image called cdboot.img, which is concatenated with core.img. The core.img used for this should be built with at least the ‘iso9660’ and ‘biosdisk’ modules. Your bootable CD-ROM will usually also need to include a configuration file grub.cfg and some other GRUB modules.
To make a simple generic GRUB rescue CD, you can use the grub-mkrescue program (see Invoking grub-mkrescue):
$ grub-mkrescue -o grub.iso
You will often need to include other files in your image. To do this, first make a top directory for the bootable image, say, ‘iso’:
$ mkdir iso
Make a directory for GRUB:
$ mkdir -p iso/boot/grub
If desired, make the config file grub.cfg under iso/boot/grub (see Configuration), and copy any files and directories for the disc to the directory iso/.
Finally, make the image:
$ grub-mkrescue -o grub.iso iso
This produces a file named grub.iso, which then can be burned into a CD (or a DVD), or written to a USB mass storage device.
The root device will be set up appropriately on entering your grub.cfg configuration file, so you can refer to file names on the CD without needing to use an explicit device name. This makes it easier to produce rescue images that will work on both optical drives and USB mass storage devices.
 El Torito is a specification for bootable CD using BIOS functions.