Developing your custom kernel with qemu

I'm preparing a presentation that is hopefully going to be shown at an internal event at Sogeti. We're discussing the internals of Operating Systems. For this presentation, the intention is to compile a pre-created, very simple and basic kernel, which doesn't do much but print "Hello World!".

Well, if you're interested in doing kernel development yourself, there are plenty of resources around to help you get started. The not-so-easy thing is getting your development environment in order, so that you can actually run and debug the kernel. Here's a set of commands that help you do that. I'm using qemu as a VM emulator, gcc and nasm for development and Ubuntu as a host development system.

1. qemu-img create myos.img 32M
2. losetup /dev/loop1 ./myos.img
3. fdisk -u -C65 -S63 -H16 /dev/loop1
4. ( create a primary bootable partition across the entire disk)
5. losetup -d /dev/loop1
6. losetup -o32256 /dev/loop1 ./myos.img
7. mkfs -t ext3 /dev/loop1
8. mount -t ext3 /dev/loop1 /mnt/image
9. df -Th
10. losetup /dev/loop2 ./myos.img
11. ln -s /dev/loop2 /dev/loop
12. cd /mnt/image
13. mkdir boot
14. mkdir boot/grub
15. cd boot/grub
16. cp /boot/grub/stage1 ./
17. cp /boot/grub/stage2 ./
18. cp /boot/grub/e2fs_stage1_5 ./
19. grub
20. grub>device (hd0) /dev/loop
21. grub>root (hd0,0)
22. grub>setup (hd0)
23. grub>quit
24. vi /mnt/image/boot/grub/menu.lst
25. menu.lst:
    timeout 0
    default 0
    title minimal­kernel
    root (hd0,0)
    kernel (hd0,0)/minimal­kernel
26. rm /dev/loop
27. losetup -d /dev/loop0
28. losetup -d /dev/loop1
29. mbchk kernel.bin
30. cp kernel.bin /mnt/image/minimal-kernel
31. umount /mnt/image

And then run it with:

qemu -m 100 -hda myos.img -boot c -no-kqemu

There you go! Of course, once the image is created and sorted, for development you only need to repeat step 30. You can remount the image in a build script:

mount -o loop,offset=32256 ./myos.img /mnt/image

Here are more resources:

• http://www.osdever.net
• http://www.jamesmolloy.co.uk/tutorial_html/
• http://www.osdever.net/bkerndev/index.php (Bran's kernel development tutorial)
• Intel Architecture Software Developer’s Manual (search Google)
  
• http://oreilly.com/catalog/linuxdrive3/book/index.csp (Linux device drivers, 3rd edition. FREE as in beer!)
• http://tldp.org/LDP/tlk/tlk.html
• http://www.ibm.com/developerworks/linux/library/l-linuxboot