MS-DOS 6.0 source code is released by Microsoft.

U can get it from www.microsoft.com

Look at this
http://www.reactos.com/

Search Around for "Bonifide OS Dev" they have a mass of sources and technical data that will help you get started :)

May be hxxp://oslib.sourceforge.net/ help you

Charm OS. I think it is a very good start point:

http://gryaznov.net/projects/list.php?p=chaos

I think linux source is best choice.

What type of OS do you want to write? What is the traget system? Intel CPU? MCU? Real Time OS?
It all depends on your goal. You need to clearly explain the details of your OS project.

Next step..
 

If you have the boot loader ready you can write a simple command interpreter.

Look OSKit project:
h**p://www.cs.utah.edu/flux/oskit/
h**p://www.nongnu.org/oskit/

I've seen MenuetOS also... I can't seem to get it to work, since my graphics are too advanced for it. I've tried everything, but it looks simple enough. Take a look at that, maybe it'll help you find a way to do certain things.

eCos
 

The embedded configurable operating system (ECOS)

http://ecos.sourceware.org

is worth a look.

heeh i don't know how advanced is your gfx but for me it works OK, ihave Geforce Ti4800SE ;o))

May be linux is a good choice. I try to read the sourcecode about linux,but it is very difficult for me. :rolleyes:

http://linux.ustc.edu.cn/~chyang/books/ulk2nd.pdf
http://www.nondot.org/sabre/os/articles

Crazy idea!
 

You would go crazy if you try to read the source of Linux, or BSD. It will take you couple months just to understand the low level code. Why not try to start with this books "linux kernel internal", then move up from there. I also have a book "Writing 32-bit OS" from Sams which have all the source in there too. PM me if you want the source.