Microsoft have decided to put all the source code up for MS-DOS up on Github for all to see.
It has versions 1.25 and 2.0 up there are available should you be interested in how it all works, or just even to create your own version of DOS.
So go have a look and see what versions you can create, or add your great additions!.
I am always interested in your thoughts so if you have any comments or feedback then please feel free to add any comments, or you can mail me here.