To build the Calculator app, you’ll need a Windows 10 computer and Visual Studio. You might think that this is the same code that’s been shipping for 30 years — it’s a simple calculator ...

Microsoft is making available the Windows Calculator source code, build system, tests and roadmap as open source under an MIT license.

To mark the occasion, Gates has released the source code he and Allen wrote for the Altair 8800 – dubbed Altair BASIC – which became the company's first product.