Bill Gates recalls that, when he and Paul Allen read about the Altair in the January 1975 issue of Popular Electronics, they understood that the price of computers would soon drop to the point that selling software for them would be a profitable business.

Allen adapted this emulator based on the Altair programmer guide, and they developed and tested the interpreter on Harvard's PDP-10.

[8] Gates and Allen bought computer time from a timesharing service in Boston to complete their BASIC program debugging.

In preparation for the demo, they stored the finished interpreter on a punched tape that the Altair could read, and Paul Allen flew to Albuquerque.

While on final approach into the Albuquerque airport, Allen realized that they had forgotten to write a bootloader to read the tape into memory.

The smallest version, 4K BASIC, could run within a 4K RAM machine, leaving only about 790 bytes free for program code.

In order to fit the language into such a small space, the 4K version lacked string manipulation and a number of common mathematical functions.

Gates responded in 1976 with a strongly worded Open Letter to Hobbyists that accused the copiers of theft and declared that he could not continue developing computer software that people did not pay for.