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<title>So You Wanna Create Your Own x86 Operating System? LG #85</title>
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<A HREF="http://www.linuxgazette.com/">
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<IMG ALT="LINUX GAZETTE" SRC="../gx/2002/lglogo_200x41.png"
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WIDTH="200" HEIGHT="41" border="0"></A>
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<SMALL>...<I>making Linux just a little more fun!</I></SMALL>
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<BIG><BIG><STRONG><FONT COLOR="maroon">So You Wanna Create Your Own x86 Operating System?</FONT></STRONG></BIG></BIG>
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<BR>
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<STRONG>By <A HREF="../authors/mahoney.html">Patrick Mahoney</A></STRONG>
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<h2>1. Introduction</h2>
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One of the great difficulties a hobbyist programmer faces when trying
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to start the development of his own OS is finding out where to
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start. Many books describe in-depth theoretical OS concepts, yet noone
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seems to take a hobyist programmer by the hand and bring him face to
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face with these concepts. This is precisely what this article aims at
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doing.
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<p>
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Several articles related to this topic appeared in the last few issues
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of the Linux Gazette. I plan to approach it in a much less programming
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oriented manner, only presenting to the reader the tools and tips he
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will need to begin the development of his own OS. Once done with this
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article, the interested reader should be all set to start browsing the
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resources available to him and start designing and coding.
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<p>
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You might not be aware of it, but operating system development doesn't
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start at the beginning. (!!) Writing a solid bootloader is a whole
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project in itself, and I would not advise one to begin an OS
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development project by writing a bootloader. Many reliable ones are
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available for free (Grub, lilo, ppcboot, etc...). If you plan on
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writing your own, I suggest you delay this task to a later stage of
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the project. In this article, I will be using GNU Grub, the Grand
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Unified Bootloader.
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<h2>2. Description of the development environment</h2>
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To ease the pain that OS development will bring to you, you will need
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to set up an adapted development environment which meets a certain
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number of requirements:
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<p>
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<ul>
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<li>You should get to rapidly test your newly compiled kernel
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<li>You should never have to reboot your development machine
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<li>You should not need to use floppies as a storage medium for your OS
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</ul>
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<p>
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This article will present one of many possible environments which
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meets these requirements. It will consist of a development machine and
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a testbed machine that both lie on a common network.
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<h3>2.1 The development machine</h3>
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Obviously, this machine will need to be equipped with a good set of
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programming tools: assembly and C compilers, a linker and a 'make'
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utility are musts.
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<p>
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A tool I found more useful than I initially thought it would be is an
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emulator. Such a tool will help debug your kernel and will allow you
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to rapidly test your newly added line of code. Don't be fooled,
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though. An emulator never replaces a good ol' testbed machine.
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<p>
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Next, you need a TFTP server. This tool will allow your testbed machine's
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tftp enabled bootloader to acquire a kernel from the development
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machine via the network connection.
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<h3>2.2 The testbed machine</h3>
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All this machine needs is a network card and a TFTP enabled bootloader
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that supports it.
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<h2>3. Setup of the development environment</h2>
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<h3>3.1 The development machine</h3>
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The chosen programming tools are:
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<ul>
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<li>gcc 2.95.4
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<li>ld 2.13.90.0.10
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</ul>
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<p>
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Bochs version 1.4.1 is the chosen x86 emulator. Special care should be
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taken to compile it with debugger mode enabled. These commands should do
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the job:
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<pre>
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$ ./configure --enable-x86-debugger
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$ make
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</pre>
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In order to properly use Bochs, you need to create a disk image. This
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image needs to have both a bootloader and a filesystem. This can
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be done using the <a href="misc/mahoney/mkbimage">mkbimage</a>
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script. If you're too lazy to do it yourself, grab <a
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href="misc/mahoney/c.img.gz">this</a> gzipped 10MB disk image and add
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<pre>
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diskc: file=c.img, cyl=24, heads=16, spt=63
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</pre>
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to your .bochrc file.
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<p>
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As for the TFTP server, I chose to use atftpd. It's an easy to use
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linux-based TFTP server implementation.
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<h3>3.2 The testbed machine</h3>
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The chosen bootloader is GNU Grub version 0.92. Special care should be
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taken to enable Grub's tftp client to talk to your network card. My
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testbed machine has a cheap NE2000 ISA clone. Following carefully the
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netboot/README.netboot instructions, I used these commands:
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<pre>
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$ ./configure --enable-ne --enable-ne-scan=0x220
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$ make
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</pre>
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Note that a PnP PCI card would be easier to configure. Now, you can
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either install the Grub images on the testbed machine's MBR or on a
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floppy which your testbed machine will boot from. I prefer the latter,
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since my testbed machine is also used for other purposes, and
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therefore, I'd rather not play with its HD.
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<pre>
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$ cat ./stage1/stage1 ./stage2/stage2 > /dev/fd0
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</pre>
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Now just insert your floppy in your testbed machine to see if your
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network card gets recognized. You can either configure it by hand or
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use a dhcp server, if any.
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<pre>
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grub> dhcp
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Probing... [NE*000]
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NE2000 base 0x220, addr 00:C0:A8:4E:5A:76
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Address: 192.168.22.14
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Netmask: 255.255.255.0
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Server: 192.168.22.1
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Gateway: 192.168.22.1
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</pre>
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Note that you won't have to configure these parameters by hand each
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time you boot. See the GNU Grub documentation and the 'grub-install'
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script for details.
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<p>
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That's it! You're all set to test your setup!
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<h2>4. Testing your development environment setup...</h2>
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As I mentioned earlier, I will leave the core OS programming stuff to
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the experts out there. So in order to test your setup, we will use the
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example kernel from the GNU Grub sources located in the /docs directory.
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<p>
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The kernel is built from three source files: boot.S, kernel.c and
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multiboot.h. You can build the kernel by doing:
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<pre>
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$ gcc -I. -c ./boot.S
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$ gcc -I. -c ./kernel.c
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$ ld ./kernel.o ./boot.o -o kernel -Ttext 100000
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</pre>
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Here's a quick and incomplete explanation. Multiboot is a standard
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that defines a way for the bootloader to pass information to the
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kernel it tries to load. boot.S accepts this information, sets up a
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stack, and calls 'cmain'. This function sets up the vga display,
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reads the information passed to him, prints some stuff and
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leaves. Then, boot.S gets the control back, prints the string
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'Halted.', and enters an infinite loop. Pretty simple stuff, right?
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The reader is invited to dig into the code to get more details.
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<h3>4.1 ...with Bochs</h3>
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The plan is to mount your disk image via a loopback device, copy your
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kernel on the filsystem of the image, unmount it, and fire off
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Bochs. Of course, you have to add an offset to the start of the
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filesystem. But you knew that, right?
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<pre>
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# /sbin/losetup -o 32256 /dev/loop1 ./c.img
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# /bin/mount -t ext2 /dev/loop1 /mnt/osdev/
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# cp <path-to-grub>/docs/kernel /mnt/osdev
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# umount /mnt/osdev/
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# /sbin/losetup /dev/loop1 -d
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$ bochs
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</pre>
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Of course, that can be automated by your Makefile. Once in Grub,
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simply do:
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<pre>
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grub> kernel (hd0,0)/kernel
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grub> boot
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</pre>
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<a href="misc/mahoney/bochs_screenshot.jpg">
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<img alt="Bochs screenshot" src="misc/mahoney/bochs_screenshot.jpg" width="400" height="220"></a>
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<BLOCKQUOTE><EM>(Click the image for the full size.)</EM></BLOCKQUOTE>
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<h3>4.2 ...with your testbed machine</h3>
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First, setup your TFTP server so that the client can retrieve your
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kernel:
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<pre>
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# /usr/sbin/atftpd --daemon /home/bono/src/grub-0.92/docs
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</pre>
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Fire off your testbed machine. Configure your network connection as
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shown above. Next, specify your devel machine's ip address as the TFTP
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server address and the location of the kernel image. Note that this
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option can be set by the dhcp server. Finally, start the boot
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process.
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<pre>
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(...)
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grub> tftpserver 192.168.22.36
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Address: 192.168.22.14
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Netmask: 255.255.255.0
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Server: 192.168.22.36
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Gateway: 192.168.22.1
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grub> kernel (nd)/kernel
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[Multiboot-elf, <0x100000:0x807:0x0>, <0x101808:0x0:0x4018>,
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shtab=0x106190, entry=0x100568]
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grub> boot
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</pre>
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A screen similar to that of Bochs should appear on your testbed
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machine's display.
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<h2>5. Where to go from here</h2>
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Well you're pretty much set to start the development of your
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OS. Lots of good documentation resides on the web. Browse, post,
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ask, think. Monolithic or micro kernel? Segmentation or paging?
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<p>
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If your debugging needs come to outgrow both the emulator and your
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kernel's printk's, one setup you could add to your OS is a serial
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debugger. This can range from some bytes thrown on the serial port, to
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a gdb-compatible remote-debugging extension. This information could be
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retrieved and processed by your development machine through a
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null-modem serial cable. It's a handy common practice in OS
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development.
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<h2>6. Resources</h2>
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<ul>
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<dt><li><a href="http://cwx.prenhall.com/bookbind/pubbooks/tanenbaum/">Tanenbaum' os dev book</a>
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<dd>The bible of operating system development
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<dt><li><a href="http://groups.google.ca/groups?q=alt.os.development">alt.os.development</a>
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<dd>There, you'll find the solution to many of your problems!
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<dt><li>Freenode IRC's #osdev (irc.debian.org)
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<dd> Friendly folks who never go to bed!
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<dt><li><a href="http://osdev.berlios.de/">A few osdev tutorials</a> including Tim Robinson's.
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<dd> Tim's been there!
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<dt><li><a href="http://www.nondot.org/sabre/os/articles">The Operating System Resource Center</a>
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<dt><li><a href="http://inferno.cs.univ-paris8.fr/~am/tutorial/os/tutorial00.html">BosoKernel</a>
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<dd>Nicely done x86 beginner's tutorial. (French)
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<dt><li><a
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href="ftp://download.intel.com/design/PentiumII/manuals/24319202.pdf">
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Intel Architecture Software Developer's Manual Volume 3: System
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Programming</a>
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<dd>Don't leave home without it.
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</dl>
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</ul>
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<h2>7. Thanks</h2>
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Many thanks to all those who have accepted to patiently answer
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my never-ending questions on #osdev: pavloskii, geist, oink,
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byrdkernel, air.
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<!-- *** BEGIN copyright *** -->
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<hr>
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<CENTER><SMALL><STRONG>
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Copyright © 2002, Patrick Mahoney.
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Copying license <A HREF="../copying.html">http://www.linuxgazette.com/copying.html</A><BR>
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Published in Issue 85 of <i>Linux Gazette</i>, December 2002
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</STRONG></SMALL></CENTER>
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