Despite what you (or I, for that matter) might think about Microsoft, this is pretty amazing. Edited for clarity and emphasis mine:
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In preparation for the 1978 demo with computer manufacturer Altair, Bill Gates and Paul Allen stored the finished BASIC interpreter on a punched tape that the Altair computer could read and flew to Albuquerque.
On final approach, Allen realized that they had forgotten to write a bootstrap program to read the tape into memory. Writing in 8080 machine language, Allen finished the program before the plane landed.
Because Gates and Allen had no access to an Altair system on which to develop, only when they loaded the program onto a computer at Altair’s office and saw a prompt asking for the system’s memory size did they know that their interpreter worked.