In January 1975 Popular Electronics put the Altair 8800 computer on its cover. Bill Gates and Paul Allen saw it. Gates was a 19-year-old Harvard student. Allen worked at Honeywell.
They called the maker, MITS, and said they had a BASIC for the Altair. They had no Altair and no code. They wrote the program in eight weeks on a simulator. Allen finished the loader on the plane to Albuquerque.
The first time the code ran on a real Altair was the demonstration itself. The machine answered two plus two with four. Microsoft was founded that April.