The Computer History Museum has released something rather special for you to download: the original DOS source code for the 1978 Apple II.
The hardware was incredible for the time: the version with 4K of memory cost $US1300 and had colour, graphics, sound, expansion slots, game paddles, and a built-in BASIC programming language.
It needed correspondingly great software but, with a team of just 15, Apple lacked the expertise to produce the goods. Instead, it hired contractor Paul Laughton in return for a one-off payment of $US13,000, who produced the OS in just seven weeks. The result was a kind of bridge between old-school file management systems and modern-day operating systems, and it gave applications a way to access disk files through BASIC commands. Needless to say, it was game-changing.