JamVM 1.5.1
===========

Welcome to the twenty-fourth release of JamVM!

JamVM is a Java Virtual Machine which conforms to the JVM
specification version 2 (blue book).  In comparison to most other
VM's (free and commercial) it is extremely small, with a stripped
executable on PowerPC of only ~200K, and Intel 180K.  However, unlike
other small VMs (e.g. KVM) it is designed to support the full
specification, and includes support for object finalisation, 
soft/weak/phantom references, the Java Native Interface (JNI) and
the Reflection API.

Supported Architectures
=======================

JamVM currently only includes an interpreter (keeps it small).  This
means it is easy to port to new architectures as most of the code
is written in C.  So far, JamVM has been built and tested on :

Linux
-----

- PowerPC: for many years my main platform, so this is well tested.
           Built and tested on G3 and G4 systems.

- i386: built and tested on i586, i686 (Pentium III), Pentium 4,
        Athlon and Core 2 Duo.  At least an i486 is needed because
        JamVM uses the cmpxchg instruction, which was first introduced
        on the i486.

- ARM: Originally ported and tested on the iPAQ running Linux (3950,
       with xscale PXA250 processor, though it should work on StrongARM).
       Later development on Neo1973 (ARM920T core).  Little and Big Endian
       machines are supported, soft/kernel FP emulation, and OABI or
       EABI.

- AMD64: built and tested on AMD Athlon 64, Pentium 4 and Core 2 Duo
         (using EM64T).

- MIPS: built and tested on mipsel using the O32 ABI.  The system
        was an embedded wireless router.

Mac OS X/Darwin
---------------

- PowerPC: built and tested on Mac OS X 10.3 (Panther) and 10.4
           (Tiger) on G3, G4 and G5 systems.  It should also work
           on 10.2, but you may need to install Fink.  Note, gcc-3.3
           gives better performance than gcc-4.0.

- PowerPC64 (a.k.a G5):
           to build a 64-bit executable CFLAGS must include
           the flag -arch ppc64.  If no option is given a 32-bit
           executable is built by default.

- i386: JamVM has been built and tested on the Apple MacBook Pro
        (Intel Core Duo and Core 2 Duo).

- ARM: iPhone (jailbroken)

BSD Variants
------------

OpenBSD  : i386, powerpc, ARM, AMD64 (x86_64).
FreeBSD  : i386, powerpc, ARM, AMD64 (x86_64).
KFreeBSD : i386


Class Libraries
===============

JamVM is designed to use the GNU classpath Java class library (see
http://www.gnu.org/software/classpath).  A number of classes are
reference classes which must be modified for a particular VM.  These are
provided and built along with JamVM (see INSTALL).  JamVM 1.5.1 works with
Classpath from version 0.92 to 0.97 (the latest development snapshot).
It should also work with the latest version direct from CVS (but check
the Classpath mailing lists for possible patches).  Later snapshots of
Classpath may also work, if no modifications have been made to the VM
interface.

Note, JamVM 1.5.1 will not work with versions of Classpath earlier than
0.92, nor will it work with the class library from Sun or IBM's Virtual
Machines, or OpenJDK.

Building and Installing
=======================

Please see the INSTALL file for full instructions.

Running
=======

You don't need to setup any environment variables to run JamVM.  By default,
JamVM installs in /usr/local/jamvm/bin (see INSTALL for how to change it).
Add /usr/local/jamvm/bin to your path, and you should be able to simply run
`jamvm'.

CLASSPATH, -classpath, -bootclasspath and LD_LIBRARY_PATH variables
-------------------------------------------------------------------

The location of user classes can be specified on the JamVM command line using
-classpath (or -cp) or via the CLASSPATH environment variable.  If neither is
specified, the classpath defaults to '.', i.e. the current directory.  The
-classpath option takes priority -- if this is specified, CLASSPATH is ignored.

The system classes (GNU Classpath and JamVM's VM classes) are loaded via the
system class loader.  By default it searches the GNU Classpath installation
directory and JamVM's installation location.  This can be overridden using the
-Xbootclasspath command line option, or the BOOTCLASSPATH variable.

By default JamVM searches the GNU Classpath installation directory and the
current directory ('.') for native libraries.  Extra directories can be
specified using the LD_LIBRARY_PATH environment variable.

Testing
=======

Currently JamVM has been used to run fairly major command-line programs
including Javac, SPECJvm 98 benchmark suite, jBYTEmark and Mauve.  It has
also been successfully used with Jetty (a Java-based web server and servlet
container, Tomcat, JEdit and Eclipse.

FEEDBACK
========

I welcome feedback of any kind, e.g. bug reports, suggestions, etc.  I'm always
interested to hear how and if people are using JamVM so even if you don't have
a problem drop me an email (email address at the bottom).

JamVM "Features"
================

For those interested in the design of virtual machines, JamVM includes a number
of optimisations to improve speed and reduce foot-print.  A list, in no
particular order, is given below.

- Uses native threading (posix threads).  Full thread implementation
  including Thread.interrupt()

- Object references are direct pointers (i.e. no handles)

- Supports class loaders

- Efficient thin locks for fast locking in uncontended cases (the
  majority of locking) without using spin-locking

- Two word object header to minimise heap overhead (lock word and
  class pointer)

- Execution engine supports many levels of optimisation (see
  configure --help) from basic switched interpreter to inline-threaded
  interpreter with stack-caching (equivalent performance to a simple
  JIT).

- Stop-the-world mark and sweep garbage collector

- Thread suspension uses signals to reduce supend latency and improve
  performance (no suspension checks during normal execution)

- Full object finalisation support within the garbage collector
  (with finaliser thread)

- Full GC support for Soft, Weak and Phantom References.  References
  are enqueued using a seperate thread (the reference handler)

- Full GC support for class and class-loader unloading (including
  associated shared libraries)

- GC utilitises a mark/sweep collector with compaction

- Garbage collector can run synchronously or asynchronously within its
  own thread

- String constants within class files are stored in hash table to
  minimise class data overhead (string constants shared between all
  classes)

- Supports JNI and dynamic loading for use with standard libraries

- Uses its own lightweight native interface for internal native methods
  without overhead of JNI 

- JamVM is written in C, with a small amount of platform dependent
  assembler, and is easily portable to other architectures.


That's it!

Robert Lougher <rob@lougher.org.uk>
28th October 2007.