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                  Some Notes about Nachos-Java

			Peter Druschel
         		Rice University

This document focuses on the differences between Nachos-Java and the
original verison of Nachos as distributed by the UC Berkeley, which is
written in C++.  The reader should refer to the UC Berkeley
distribution for full documentation.

In Nachos-Java, the Nachos kernel is written entirely in Java. User
programs continue to be written in C/C++.

In general, we have made every effort to preserve the structure of the
C++ version to simplify the transition and to preserve, as much as
possible, the value of the existing Nachos documentation (e.g., the
roadmap). You will find many of the same classes and methods in the
Java version. However, there are some deviations that were necessary
due to differences between C++ and the Java language.


Here are some of the most important differences:

(1) In Java, there are no functions and all execution must
take place in the body of a class's method. So, you will find that
many functions in the C++ version have become methods of a class.

(2) There are no function or method pointers in Java. Instead, you
will find that objects that implement a certain abstract interface are
used in the Java version. For instance, when creating a new Nachos
thread in the C++ version, you specify a function pointer and argument
to be invoked by the new thread. In the Java version, an object
instance of a class that implements the "Runnable" interface is
specified instead. The new thread invokes the "run()" method of that
object. Any parameters can be specified in the object's instance
variables.

(3) To allow the Java compiler to automatically track dependencies
among source files and recompile all sources that have been modified,
it is necessary to place each class definition into a separate source
file that goes by the Class name, with the ".java" suffix added. (The
only exception is that class definitions can be added to another
class's source file, if those classes are never used or referenced by
classes defined in other source files.)  This has caused some changes
in the names of source files as compared to the C++ version. However,
this change has the added benefit of improved readability.

(4) Unlike C++, Java is a concurrent language that already has support
for threads and synchronization. However, to preserve the Nachos
threads model and its style of synchronization primitives, we have
implemented those facilities on top of native Java threads. VERY
IMPORTANT NOTICE: You must always use Nachos threads and Nachos
synchronization primitives when writing Nachos-Java
code. Instantiating native Java threads and invoking native Java
thread methods on Nachos threads may lead to unexpected results! Also,
you must not use the "wait" and "notify" methods of the Object class,
or the "synchronized" keyword and statement when writing Nachos code;
instead, always use the Nachos-style Lock and Conditions classes to do
your synchronization.


Other differences:

Nachos-Java currently does not support the networking stuff
(Assignment 5).

Nachos-Java reads its executable files directly from the Unix
filesystem. As a result, Nachos-Java continues to read executables
from the Unix filesystem, even when the "Stub" filesystem is replaced
by a "Real" Nachos filesystem.



Sun Java Development Kit (jdk)

Nachos-Java was designed and tested using the Sun Java Development
Kit. We recommend that you use version 1.1.6 of the jdk. The location
of the jdk must be configured in the files code/threads/Makefile and
code/threads/nachos. On Rice's owlnet, the appropriate path is
/usr/site/jdk-1.1.6/bin/.

(Versions of the jdk earlier than 1.1.6 may have problems with memory
leaks and may experience segfaults in the Java runtime during garbage
collection. Later versions (jdk-1.2) provide significantly better
performance, but as of Spring 1999, jdk-1.2 was not yet stable enough
to support Nachos-Java reliably. Running some of the stress tests
caused bogus runtime errors and even segfaults of the Java runtime.)

To compile Nachos, go to nachos-java/code/threads and type "make". All
sources in the code tree will be recompiled as needed.

To run Nachos, go to the threads directory and type "nachos
<arguments>".  The arguments are the same as in the C++ version.

To use the Java debugger, you need to set the CLASSPATH environment
variable appropriately. In csh, for instance, do

setenv CLASSPATH .:../machine:../filesys:../userprog:../threads:../vm

Then, invoke the debugger as "jdb Nachos <arguments>".