xv6 is a simple unix-like teaching operating system developed at MIT. xv6 is re-implementation of Dennis Ritchie's and Ken Thompson's Unix Version 6 (v6). xv6 is implemented in ANSI C for an x86 based multiprocessors.
Xv6 is simple enough to teach operating system walking through it's code in a semester. xv6 has implemented most of modern operating system core functionalists which makes easier to start look through in operating system code for a newbie.
xv6 is around 8000 lines code yet still contains the important concepts and organization of Unix.
Understanding code Conceptually
xv6 is mainly written is ANSI c & some part in assembly. shell & perl are also used for making some files.
Basically understanding code of xv6 includes these sections
The dissection of code can be found in Struture of xv6 sourcecode