MS-DOS is now Open Source

I feel I should comment and say that MSR-LA is 'technically open source'. It really limits what you can do with changes to the software, and any modifications can only be re-licensed and sold (or used commercially) by Microsoft. Other wise they must carry the same license.

"You may not use or distribute this Software or any derivative works in any form for commercial purposes. Examples of commercial purposes would be running business operations, licensing, leasing, or selling the Software, distributing the Software for use with commercial products, using the Software in the creation or use of commercial products or any other activity which purpose is to procure a commercial gain to you or other" [1]

Technically Open Source, the source is open for studying and learning. And it can be distributed, and redistributed.

[1] https://research.microsoft.com/en-us/projects/pex/msr-la.txt

No, it's not open source.

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