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### 4.2 Making a GRUB bootable CD-ROM

GRUB supports the no emulation mode in the El Torito specification6. 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 Writing your own configuration file), 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.

Next: The map between BIOS drives and OS devices, Previous: Installing GRUB using grub-install, Up: Installation [Contents][Index]