General Information
===================
The cross compilers are installed in all systems in 
Software Systems Laboratory.

The cross compilers are needed if you are installing
Nachos in your home computers.

On a Red-Hat machine, they can be installed as follows
	rpm -ivh <file-name>.rpm

On Debian machines, you can use the .deb files by issuing
	dpkg -i <file-name>.deb

Introduction to Nachos
======================
Nachos runs on a simlated machine which understands MIPS.

Since we have a simulator, one might thing that the kernel is also running on
the simulator. But it is not so. In a linux system the Nachos kernel (that the
student writes) as well as the simulator is a user level process.
The simulator reads in the user test program executable as the data and
interprets them, simulating their execution on a real MIPS machine.
The user programs are written in a subset of C. 

Nachos executable contains both the simulator and the kernel. The simulated
machine's physical memory and registers are the data structures in Nachos
program. Thus the Nachos and the simulator runs side-by-side. 

Whenever an interrupt or a system call occurs in the user program, the 
simulator handles the control over to Nachos kernel.

Unlike the ordinary OS, user programs (executing in Nachos Kernel) will
execute as multiple independent threads, each having a separate stack. Thread
switching happens via a call to low-level assembly code called SWITCH written
in threads/switch.s

Working with Nachos
===================
To compile Nachos 
1. Go to code/
2. Type make

You can make the executables for the files in code/test by doing "make" in 
that directory.

Nachos executables will be created in the directories network/  threads/   vm/ 
filesys/ userprog/. Based on the stage in which you are currently working, 
cd to the directory and execute nachos.

Read threads/main.cc to understand the command line options supported by Nachos.
Read the Makefile.* in the Nachos directory to see how the source is compiled.

Compiling
=========
The Nachos code directory includes several subdirectories with source code 
for different pieces of the Nachos system. The subdirectories include 
Makefiles that allow you to automatically build the right
components for specific assignments using the make command.

Most relevant directories will be threads/ and userprog/ 

If you type make in one of these directories, it will execute a sequence of
commands to compile and link Nachos, yielding an executable program called 
nachos in that directory. 

All of your testing will be done by running these nachos executables built 
from your modified Nachos code. You should study the Makefiles to understand 
how dependency identification and recompilation work.

The dependency information determines which .cc files are rebuilt when a given
.h file changes. The dependency lines in each subdirectory’s Makefile (e.g.,
nachos/threads/Makefile) are created automatically using the make depend 
facility. 

For example, if you type cd threads; make depend, 
this will regenerate the dependency information in the threads/Makefile . It 
is extremely important to keep your dependency information up to date.

Debugging
=========
1. Use printf
2. Standard debuggers like gdb
3. Use DEBUG primitive provided by Nachos.

Searching
=========
1. Doxygen pages for nachos 3.4 is available at Athena. Search facility is 
   available there.
2. One can also use grep to search for strings in a directory tree.
   Eg: 
   $  pwd
   /home/guest/nachos-3.4
   $  grep -nr DEBUG code/

   ... shows all files in the directory code/ that contains DEBUG along with 
   their line numbers. This is very useful sometimes.

Known Issues
============
1. Compilation can show weired errors if you are running make from an NTFS/FAT
   filesystem. Better compile the source from a ext2/3/4 partition.

Some useful links
=================
http://people.cs.uchicago.edu/~odonnell/OData/Courses/CS230/NACHOS/
http://www.cs.duke.edu/~chase/cps110-archive/nachos-guide/
http://www.cs.washington.edu/homes/tom/cs162sp95/
http://cseweb.ucsd.edu/classes/fa00/cse120/