xv6 README for CS 537

From the xv6 readme:
   xv6 is a re-implementation of Dennis Ritchie's and Ken Thompson's Unix
   Version 6 (v6).  xv6 loosely follows the structure and style of v6,
   but is implemented for a modern x86-based multiprocessor using ANSI C.

For more information on xv6, see the website:
   http://pdos.csail.mit.edu/6.828/xv6/

To obtain the xv6 source code, copy the tarball and then extract it:
   cp ~cs537-2/ta/xv6/xv6.tar.gz .
   tar -xzf xv6.tar.gz
   cd xv6

Please use the source code provided here. This version has been modified
to run correctly on the mumble lab and to operate with the grading scripts.

To build the xv6 kernel, bootloader, and user programs run:
   make

In order to easily run xv6, we will use the emulator QEMU (qemu.org).
QEMU will emulate an x86 processor and devices like the display and 
disks. QEMU also provides support for debugging via gdb. QEMU is 
available on the mumble lab computers (but not on other lab computers).

The xv6 makefile has target setup for running in QEMU. To start xv6 in
QEMU, just run:
   make qemu

This will create disk images, start QEMU, and boot xv6. The QEMU window
is the virtual terminal for the emualted computer. Serial communication
is displayed in the terminal where you ran "make qemu". 

Once xv6 boots, it starts the shell "sh". You can then run user commands,
such as "ls".

To stop the emulation, just close the QEMU window (xv6 does not have a 
shutdown command)

To debug xv6 using gdb, run:
   make qemu-gdb

Then open another terminal and navigate to your xv6 directory. Now run:
   gdb

At this point you can add breakpoints in gdb. Some examples:
   break main.c:main
   break bootasm.S:start
   break trap
   break alltraps

To start or continue running the emulation, in gdb run:
   continue
or just
   c

To pause the emulation, press Ctrl-C from within gdb. Once the emulation
is paused, you can set more breakpoints, step through the code, or examine
memory (break, step, print, x). See the gdb manual for a full set of
commands and help (http://sourceware.org/gdb/current/onlinedocs/gdb/).

Note that make is smart about re-compiling. If you update source files, 
simply run make again and the correct commands will be run to re-build 
the object files and executables. If you add new source files, you may
need to update the Makefile to include those files. If you modify the 
Makefile itself (or any part of the build configuration), you should do 
a clean build to ensure that all files are compiled using the new rules.
To do this run:
   make clean
   make

For more information on make, see the manual 
(http://www.gnu.org/software/make/manual/make.html).