At 19, Bill Gates sold code for a computer he'd never seen.
In December 1974, Gates, a Harvard student, and his friend Paul Allen, 22, saw the Altair 8800 on a magazine cover. It was a computer sold as a kit. They wrote to MITS, the Albuquerque company that made it, and offered it BASIC, a programming language. They had no code and no Altair.
Allen made Harvard's PDP-10 computer imitate the Intel chip inside the Altair. Gates led the coding. In a 1993 Smithsonian interview, Gates said that "if we had made any mistake in terms of how the instructions worked, the thing never would have run."
Allen flew to Albuquerque. On the plane he wrote the missing loader, a small program that reads the software into memory. At MITS the first load failed. The second worked: PRINT 2+2 printed 4. MITS became Microsoft's first customer, and sales that year were $16,005. In fiscal 2026, Microsoft reported revenue of $331.8 billion.
Anyone who has led an ERP go-live, the day new software goes into real use, will know this. Their testing depended on how faithfully they had imitated a machine they did not have. The real test came on the customer's machine, with the customer watching.
AI-generated illustration with narration by an AI voice.
Microsoft
What the captions say
- This is the story of how Bill Gates, at 19, sold software for a computer he had never seen, and started Microsoft.
- In December 1974, Gates, a Harvard student, and his friend Paul Allen, 22, saw a magazine cover. It showed the Altair, a kit computer built on one chip.
- In Gates' words, it was happening without us. So they wrote to its maker, MITS, offering BASIC, a programming language.
- They had no code and no Altair. MITS called back: were they serious?
- For 8 weeks they worked day and night. Allen made Harvard's big computer act like the Altair's chip.
- Gates led the coding. Their BASIC, plus the user's programs and data, had to fit in 4 kilobytes.
- It had never run on one. If they had misunderstood the chip, Gates said, it never would have run.
- Gates stayed up the night before the flight, checking the manual for mistakes. He punched the program onto paper tape.
- Before landing in Albuquerque, Allen realised he had forgotten the loader, a tiny program that reads the tape. He scribbled it on a steno pad.
- At MITS, founder Ed Roberts watched Allen feed in the tape. The first time, it did not work.
- They loaded it again. It said READY. Allen typed PRINT 2+2. It printed 4.
- MITS became their first customer. Their sales that year totalled $16,005.
- In fiscal 2026, Microsoft's revenue was $331.8 billion.
- In 1993, Gates called their BASIC the first real piece of software ever written for a PC.
