LICENSE
About this file
xv6 is released under the MIT license, one of the shortest and most permissive open-source licenses. In plain words: you may do almost anything with xv6 (use it, study it, change it, sell it, put it inside your own project, even a closed-source one) as long as you keep this copyright and permission notice with any copy of it you pass on. In return, the authors promise nothing: if it does not work or causes harm, the authors disclaim responsibility.
This page explains the terms in everyday language. It is not legal advice; the text of the license itself is what counts.
Who holds the copyright
The copyright belongs to the three main authors, Frans Kaashoek, Robert Morris and Russ Cox, and to MIT, for work done from 2006 (when xv6 was first written, for x86) to 2024. Copyright exists automatically when someone writes code; a license is how the owners give other people permission to use it. Without this file, you could read xv6 but would have no clear right to copy or modify it.
What you are allowed to do
Anyone who obtains a copy gets permission, free of charge, to “deal in the Software without restriction”. The list spells out what that includes: use it, copy it, modify it, merge it into other works, publish it, distribute it, sublicense it (grant others rights to it under terms of your own, while still including this notice), and sell copies. The same rights extend to anyone you give it to. Students’ modified versions of xv6 for course labs fall well within this.
The one condition
Wherever you copy xv6, or a substantial part of it, the copyright notice and this permission notice must come along. That is the only obligation. Unlike “copyleft” licenses such as the GNU GPL, the MIT license does not require you to publish the source code of your own changes or to release your project under the same license.
No warranty, no liability
The capital letters are a legal convention for making important clauses conspicuous. The first sentence says the software comes “as is”: the authors do not promise it works, is good for any purpose, or does not infringe anyone else’s rights. The second sentence states that the authors are not liable for damage that results from using it, whatever the legal theory (contract, “tort” meaning a civil wrong such as negligence, or anything else). For a teaching kernel this is the natural deal: you get it free and complete, and you take it as it is.