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.

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.