GRUB
GRUB 2 is the GNU Project's next-generation bootloader. It has a more complete feature set than GRUB 0.97 (commonly referred to as "GRUB Legacy"). Still, all things considered, GRUB Legacy is more mature and most of the available documentation is for GRUB Legacy (hence the brief page).
History
GRUB 2 started its life as the PUPA (note the pun) research project and was rewritten from the ground up. Since then GRUB 2 (actually at time of update 1.97) has grown more stable and even hobby operating systems are starting to make use of the new bootloader instead of GRUB Legacy.
Features
- Basic scripting support
- GUI (better bootsplash support, custom colors, custom themes, ...)
- Memory management
- Cleaner design
- Better portability
- Internationalization
- Rescue mode
Upgrading from GRUB Legacy
WARNING: These steps have not been tested very well yet. Use at your own risk!
Since GRUB 2 is very different from GRUB Legacy, the directions for getting your kernel up and running are different. GRUB 2 differs from GRUB Legacy in that to implement all but the most basic functionality, the user must load so-called "modules": little bits of code that add components (e.g. a different file system or a VGA font). This section gives you an overview of the process you need to go through when you want to have GRUB 2 load your kernel. It's actually rather simple to create a GRUB2 image (assuming you have GRUB2 either built or installed):
ISO instructions
There have been a lot of tries to make Grub2 work good with ISOs, but mostly failed. The only combination of commands that seems to work is the following.
First create a directory tree called "iso", where you put your kernel (and any other needed files) somewhere. Then in the boot/grub subdirectory create the grub.cfg file which is your configuration.
Now run:
grub-mkrescue -o bootable.iso iso
Be sure that your grub.cfg is syntactically correct. A common mistake is to put the menuentry brace on newline. It must be like:
menuentry "Place your os name here" {
}
Floppy instructions
mkdir tmp
grub-mkimage -p /boot -o tmp/core.img multiboot sh fat # This should work.. I hope :D
Explanation
Let's go through those grub-mkimage options:
| -p | By default, GRUB 2 looks in /boot/grub for its configuration file. -p changes this. |
| -o | Like so many other GNU tools, grub-mkimage uses -o to set the output file. By default, it's stdout. |
| multiboot | This module is required to load multiboot-compliant kernels. |
| biosdisk | This module is required for GRUB 2 to be able to boot from a LiveCD. |
| iso9660/fat | Allows GRUB 2 to look on the image for different files. |
| sh | This module allows GRUB to parse the configuration file. |
GRUB 2, like GRUB Legacy, needs a configuration file to find your kernel. In GRUB Legacy it's called menu.lst, but in GRUB2, it's called grub.cfg. The syntax for the configuration file is also a bit different.
Here's a sample configuration file (NOTE: This file should be placed into the /boot/grub folder of your disk image, and be named grub.cfg):
set timeout=15
set default=0 # Set the default menu entry
menuentry "OS Name" {
multiboot /boot/kernel-file # The multiboot command replaces the kernel command
boot
}
That's basically it. Copy these files to a disk image, pop it in an emulator, and you're done!
Double check that you put the brace on the same line of "menuentry". It can't be on a new line. This is not C.
USB instructions
Fewer and fewer systems have a floppy disc controller these days, but USB ports are found on all. Modern BIOSes can boot from a USB device, usually by pressing some special key during startup.
Putting GRUB 2 on a bootable USB storage device is a nice way to experiment with your OS on different computers. Here's how you set this up (using Linux):
1. Create a FAT32-formatted USB disk, without partitions:
sudo mkfs.vfat -F 32 -n YourLabel -I /dev/sdX
(where sdX is your USB device)
The "-I" option is needed because we are targeting a partition-less device
2. Remove your USB device, and plug it back in. The auto-mounter on your OS should detect it now.
3. Invoke grub-install (on some systems this command is called grub2-install, located under /usr/sbin or /usr/local/sbin)
sudo grub-install --root-directory=/media/YourLabel --no-floppy --recheck --force /dev/sdX
It is important to do this as root (or sudo), else the generated device.map listing available boot devices can be empty. /media/YourLabel is the mount point under Fedora 16, it may be different for other distributions.
4. Create a grub.cfg for your kernel, and copy it to your new bootable USB disk
Multiboot
Some versions of GRUB 2 like to put multiboot modules in relatively high physical memory addresses, in contrast to GRUB-legacy which loaded them into low memory. Be careful when making your kernel work with GRUB 2 that it is not making any assumptions about where the multiboot modules will appear.
When your kernel gets control, the machine state is defined as follows: Multiboot machine state. Your code should have minimal dependency on this initial state; for example, define your own GDT instead of relying on the GDT setup by GRUB.