More on OS development...

I'm just playing around with the toy OS and learning quite a lot about how a computer works and what the kernel for a computer really has to do. I am not even working on device I/O or advanced features at all yet. Just memory management.

Memory management is subject to the following considerations:

• Where to put the kernel code and data (you do not want to overwrite this)
• Where to put the GDT and IDT
• Where to put the memory "page directory" (full 4GB directory == 8 MB)
• The ratio between kernel space vs. user space (linux = 1:3)
  
• How to implement an optimal "malloc" routine
• Page swapping (but not required, a kernel may also fault)
• Optimization of the above requirements, like is done in Linux > 2.4
• Memory page sharing through copy-on-write for example (by default the process shares the same code + data, but on any attempt to write to the page, the kernel makes a copy of that page)

So... great... lots of considerations. I am just at the part of where to put everything. My decisions haven't been ideal yet, because of boot-up limitations mostly. But slowly I should be able to work towards page allocation for a process, assigning it to a process and then subsequently the ability to run a certain process (albeit hardcoded in the kernel itself for the time being).

As said before, this is not going to be anything big, but will greatly assist in understanding other OS's a lot better. Maybe even I could use this knowledge to jump-start another project for a limited device and make the machine do something really specific in an easy way.

G>

How to start writing your own OS

Writing a fully fledged Operating System is not a task that can be finished by a small group of people, but it is still possible to write one that gets near to "DOS" kind of functionality, all by yourself. What you put in... you decide! It probably only runs on your hardware, or it doesn't make use of specialized features of hardwares, but you should ask yourself what kind of things you want to have for that particular OS and code away!

The tutorials here show you the start of writing one. If you get into the downloads area and look at one of those tutorials, you can even get a template OS to start from. Notice that Linux 0.01 is also part of the list and is a good read (and a chuckle!) to work from.

The first thing to do when writing an OS, I think, is "choose your tools". If you don't get them right from the start, you'll spend significant amounts of time rewriting it later. I am using gcc + nasm, since these are widely available and free. Then, start by building a hard disk image using the info from the site here.

You probably do not want to get started with a boot loader (it's really complicated!) and with the boot loaders available now, why should you? I did the following on Ubuntu Linux to create a hard drive image to start from. You go up to the point where they start to copy the guest OS, because that guest OS... is basically your real OS that will run inside qemu (more about qemu later).

http://wiki.xensource.com/xenwiki/InstallGuestImage

In the steps where "grub" gets installed... I had to manually copy a menu.lst into the "/boot/grub" folder. It didn't get created automatically. Maybe I should have "chroot-ed" instead into the "/mnt" and then try it again. If you take care of this image, you only will need to do it once.

Anyway, I copied in a menu.lst that looks like:

default 0
timeout 3
hiddenmenu

groot=(hd0,0)

title
root (hd0,0)
kernel /kernel.bin
boot

Of course you will need to replace the "kernel.bin" every time you recompile. I have this in my Makefile as such:

losetup -o 32256 /dev/loop1 ./test/hd.img
mount /dev/loop1 /mnt/temp
sudo cp kernel.bin /mnt/temp/kernel.bin
sudo umount /mnt/temp
losetup -d /dev/loop1
losetup -d /dev/loop0

And then you can test the OS through Qemu (better than VMWare, since it has a shorter time for startup and it is free):

qemu -hda ./test/hd.img -boot c -std-vga -localtime

Qemu is installable through apt, but the kernel module needs to be downloaded from the site. No worries... plenty of documentation there. On my Ubuntu Edgy I downloaded, untarred, ./configure-d and used make; make install and it all worked.

The cool thing nowadays is that you can downloaded various OS's from here and there and merge the concepts and see what happens. Also, an OS is not necessarily multi-user or multi-tasking, nor does it necessarily need to page out memory to disk and back again (you can simply page-fault and panic).

Unless you have a specific need or idea to follow, I wouldn't even start to believe it would be the start of the next "better" OS... But it's good to write one "OS" in your life to better understand how they all work.