Julien Lecomte’s Blog

Back in university, I took a few classes dealing with operating system design. These classes were extremely theoretical and, in some ways, helped me throughout my curriculum and my career, serving as a solid base I could then build upon to gain new knowledge. However, after spending a few years working with high level languages in sandboxed environments, you tend to forget how things work at the lower level, and that sometimes leads to less than optimal higher level code. As a consequence, a few months ago, I decided it was time for me to brush up on my core CS skills. However, I needed a tangible goal. And then I thought: why not write an operating system? OK, not a full blown operating system of course, but the seed of a very basic one (calling Simplix an operating system is a bit of a stretch since it cannot be used for anything actually useful) One that other people could look at and actually understand (Even MINIX, which was designed to be easy to understand by students, is not that easy to grasp without spending a lot of time hunched over the code) Here are the high level characteristics of Simplix:

• Target architecture: PC with a single Intel 386 or better CPU
• Monolithic, interruptible, non preemptible kernel
• Hardware interrupt handling using the Intel 8259 PIC
• Software interrupt handling
• Basic management of physical memory
• Peripherals: keyboard, video screen
• Support for kernel threads and user space processes
• Support for virtual memory using segmentation
• Support for system calls

Here is a screenshot showing Simplix running inside the Bochs emulator:

Over the next few months, I will be posting several articles on this blog in an attempt to explain how Simplix works, so please stay tuned! In the meantime, you can already take a look at the complete up to date source code and even download it. Also, if you are interested in the topic of operating system development, I warmly recommend reading the bible of system programming: Operating Systems: Design and Implementation (Second Edition) by Andrew S. Tanenbaum and Albert S. Woodhull. I got mine used on Amazon.com for $8… Cheers!

In this article, I try to synthesize several (incomplete or inaccurate) articles I’ve found on the Internet to guide you through the process of creating a disk image you may use with the Bochs emulator. The steps described below are intended for a GNU/Linux system and some of them require super user privileges. Also, you need to have GRUB installed on your system.

1. Create a disk image. Here, I create a file named disk.img, containing 10080 blocks (each block being 512 bytes, this will create a file that’s about 5MB)

$ dd if=/dev/zero of=disk.img count=10080

2. Use FDISK to create a partition table on the image file:

$ fdisk disk.img

   x     -> Extra functionality
   c 10  -> 10 cylinders
   h 16  -> 16 heads
   s 63  -> 63 sectors per track
   r     -> Return to main menu
   n     -> Create a new partition
   p     -> Primary
   1     -> Partition #1
   1     -> First cylinder
   10    -> Last cylinder
   a     -> Set bootable flag
   1     -> Partition number
   w     -> Write partition to disk

Note that you have to tell fdisk about the geometry of your disk, and that geometry has to match the size of the disk you created in step 1 (10 cylinders * 16 heads * 63 sectors per track = 10080 blocks). For an introduction on disk geometry, cylinders, heads and sectors, read this Wikipedia article.

Let’s take a quick look at the Partition Table in the Master Boot Record (See this Wikipedia article for a description of the structure of the partition table)

$ hexdump disk.img

The 16 bytes at offset 446 (0×1BE) are:

0180 0001 0f83 093f 003f 0000 2721 0000

Keep in mind that these values are stored using little-endian convention (see this Wikipedia article to find out more about the meaning of endianness in computer science). Also see this article to find out how the CHS values are computed.

3. Setup the loopback device. In order to do this, you need to calculate the offset (in bytes) of the first sector of your single partition. Use the following command:

$ fdisk -l -u disk.img

  Device Boot      Start         End      Blocks   Id  System
disk.img1   *          63       10079        5008+  83  Linux

This tells us that our single partition starts at the 63rd block. Hence our offset is 63 * 512 = 32256.

Finally, type:

$ losetup -o 32256 /dev/loop0 disk.img

4. Format the disk (EXT2FS)

$ mkfs.ext2 /dev/loop0

5. Mount the disk:

$ mount -o loop /dev/loop0 /mnt

6. Now, let’s install GRUB. Start by copying the necessary GRUB files:

$ mkdir -p /mnt/boot/grub
$ cp /boot/grub/stage1 /boot/grub/stage2 /mnt/boot/grub/
$ vi /mnt/boot/grub/grub.conf

   title=MyKernel
   root (hd0,0)
   kernel /mykernel

7. Unmount the device:

$ umount /mnt

8. Detach the loopback device:

$ losetup -d /dev/loop0

9. Finish up the GRUB installation:

$ grub --device-map=/dev/null

   device (hd0) disk.img
   geometry (hd0) 10 16 63
   root (hd0,0)
   setup (hd0)
   quit

10. Setup your .bochsrc file in the same directory as your disk image:

megs: 32
romimage: file=/usr/local/share/bochs/BIOS-bochs-latest, address=0xf0000
vgaromimage: file=/usr/local/share/bochs/VGABIOS-elpin-2.40
ata0-master: type=disk, path="disk.img", mode=flat, cylinders=10, heads=16, spt=63
cpu: count=1, ips=15000000
mouse: enabled=0
log: out.bochs
boot: disk

That’s it. Start Bochs and the GRUB interface should appear: