ive never decompiled any code before, but from what i read, you decompile to machine code aka assembly. Personally, I woudl think it would be a tough task to go through all that code.
Ther is Free DOS that is Open Source DOS. That has a DOS code. Umm one question. What is dos written in? I was told that It was written in BASIC, and I am pretty sure that it is true.