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DisVM Project

The DisVM project is a re-implementation of the Dis virtual machine specification that defined the user space of the Inferno OS. The Inferno OS was designed to re-hash the ideas in the Plan 9 from Bell Labs operating system but implemented using a platform-agnostic language. The Dis virtual machine was intrinsic to the Inferno OS and ran the byte-code compiled Limbo language. This meant that only the Inferno OS need be ported to a platform and all Limbo compiled applications would 'just-work'. If this sounds familiar, it should. Inferno OS was developed in the early 90's and project Oak - what would become Java - was being developed by Sun Microsystems. The major difference between the two approaches was that Oak's virtual machine would be ported to various platforms and work with the host OS, whereas the Dis virtual machine was tied to Inferno OS which meant there was yet another layer between the host and the Dis virtual machine. This decision had advantages, but also had the large disadvantage of requiring an entire separate OS to run Limbo code.

This project's intent is to take the Java approach and provide an implementation of the Dis virtual machine that is separate from any OS, including the Inferno OS. The project is written in C++ - using only the standard library where possible - and is intended to support compilation on any platform that has a conforming C++ 11 compiler. At present various modules written in Limbo (even the official Limbo compiler) have been verified to run successfully using this implementation. Some precompiled byte-code modules (.dis extension) have been provided in the Downloads section.

Current Implementation Notes:

Only parts of the built-in Sys and Math modules have been written
The disvm-exec project does not support the original Command entry module since the official Draw module is not currently supported. Instead the entry point is defined by limbo/Exec.m.

Components

This section contains implementation details on DisVM components.

Garbage Collection - `src/vm/garbage_collector.cpp`

The Dis virtual machine specification described a hybrid garbage collection mechanism that utilized reference counting and a concurrent mark-and-sweep algorithm. The purpose of the hybrid approach was to mitigate cases where reference counting fails (e.g. cyclic references). The algorithm in this project fully implements reference counting, but at present has a naïve synchronous mark-and-sweep implementation.

It has been observed during usage of the Limbo compiler running in DisVM, that the mark-and-sweep implementation has a 20 - 25 % performance impact. This is clearly unacceptable for a modern virtual machine implementation, but was written to satisfy the current needs of the virtual machine as a learning tool. Even though the current mark-and-sweep implementation has poor performance characteristics, the original reason for it was cyclical references, so if data structures are written so these types of references don't or can't exist, reference counting will suffice. The option to disable the mark-and-sweep garbage collector is exposed as a flag on the disvm-exec program or if consumed directly - during instantiation of the VM.

This component can also be replaced with a custom implementation if the DisVM is consumed as a library.

Scheduler - `src/vm/scheduler.cpp`

The DisVM default scheduler supports utilization of 1 to 4 system threads, which is useful if parallelism is desired at runtime. The current default is for the scheduler to use 1 system thread, but this can be altered from the disvm-exec command line or programmatically.

Like the garbage collector, this component can also be replaced with a custom implementation.

Just-In-Time compilation

There is currently no support for the JIT compilation described in the Dis virtual machine specification.

Build instructions

Requirements

gyp - http://gyp.gsrc.io
Python 2.7 - http://www.python.org/download/releases/2.7/

DisVM build scripts can be generated using gyp (Generate-Your-Project)

Install Python 2.7
Get current source for gyp
Generate build file for your platform
- Windows -- gyp.bat src\exec\exec.gyp
- Other -- gyp_main.py src/exec/exec.gyp

Python and gyp are only needed for build file generation.

Source

The source tree is as follows:

limbo/ - Source code written in Limbo for testing DisVM
- compiler/ - Copied and slightly modified source code for the official Limbo compiler
src/
- asm/ - Library for manipulating byte code
- include/ - Global include files
- exec/ - Hosting binary for DisVM (includes debugger)
- vm/ - DisVM as a static library

Comment prefix	Description
`[TODO]`	Work to be done in the future.
`[SPEC]`	A departure or undocumented assumption in the Dis VM specification.
`[PERF]`	Potential for performance improvement.
`[PAL]`	Work that should be done in a platform abstraction layer (PAL).

See readme.md or license.txt in source directories for further details or specific licensing.

References

Dis virtual machine specification: http://www.vitanuova.com/inferno/papers/dis.html

Limbo language addendum: http://www.vitanuova.com/inferno/papers/addendum.pdf

Inferno OS - Original Dis virtual machine implementation: https://bitbucket.org/inferno-os/inferno-os/

Licenses

See source directory for a license.txt that defines the license, otherwise licenses for the DisVM project are defined below.

RFC1321

License to copy and use this software is granted provided that it is identified as the "RSA Data Security, Inc. MD5 Message-Digest Algorithm" in all material mentioning or referencing this software or this function.

License is also granted to make and use derivative works provided that such works are identified as "derived from the RSA Data Security, Inc. MD5 Message-Digest Algorithm" in all material mentioning or referencing the derived work.

RSA Data Security, Inc. makes no representations concerning either the merchantability of this software or the suitability of this software for any particular purpose. It is provided "as is" without express or implied warranty of any kind.

These notices must be retained in any copies of any part of this documentation and/or software.

UTF8 Parsing

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

LAPACK

Copyright (c) 1992-2013 The University of Tennessee and The University of Tennessee Research Foundation. All rights reserved. Copyright (c) 2000-2013 The University of California Berkeley. All rights reserved. Copyright (c) 2006-2013 The University of Colorado Denver. All rights reserved.

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Additional copyrights may follow

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Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer listed in this license in the documentation and/or other materials provided with the distribution.
Neither the name of the copyright holders nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

The copyright holders provide no reassurances that the source code provided does not infringe any patent, copyright, or any other intellectual property rights of third parties. The copyright holders disclaim any liability to any recipient for claims brought against recipient by any third party for infringement of that parties intellectual property rights.

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Unless otherwise stated in a `license.txt` or above - MIT License