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 ...

In the 1970s, Gates and his Microsoft co-founder, Paul Allen, used a computer in Harvard's lab to compose what he calls the 'coolest code I've written.' It's now public for the first time.

Now you can get the once-coveted Microsoft Basic source code for the 8086/8088 directly from Microsoft in the state you would have found it in 1983.

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.

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