Source Code

ARPANET (Advanced Research Projects Agency Network) was an early computer network developed by DARPA in the late 1960s. It introduced packet switching, a method that allowed computers to send small packets of data across networks efficiently, rather than requiring dedicated connections. ARPANET is widely considered the precursor to the modern internet, as it laid the groundwork for TCP/IP protocols, which still govern how data move across global networks today.

Gates encountered ARPANET through Harvard’s Aiken Computation Laboratory, which had a direct connection to the network. This gave him access to high-powered computing resources that were originally designed for government and defense purposes, underscoring the transition of computing from military to commercial and personal use.

Since its founding, the Defense Advanced Research Projects Agency (DARPA) has played a pivotal role in the evolution of computing and modern technology. Established in 1958 in response to the Soviet launch of Sputnik, DARPA was created to ensure that the United States maintained technological superiority in defense and scientific research. Over the decades, its funding and initiatives have led to groundbreaking advancements in computing, networking, and artificial intelligence. One of DARPA’s most influential contributions was the development of ARPANET in the late 1960s, a pioneering computer network that laid the foundation for the modern internet. ARPANET introduced packet-switching technology, enabling computers to communicate over long distances efficiently. This concept became the basis for the TCP/IP protocols that still govern internet communication today.

DARPA also played a crucial role in expanding access to advanced computing resources by loaning computers to universities such as Harvard, MIT, and Stanford for research purposes. These loans allowed researchers to develop early computing theories, programming languages, and artificial-intelligence models, accelerating progress in the field. The agency’s support of university research helped establish key innovations in timesharing systems, which enabled multiple users to access a single computer simultaneously, a fundamental concept in modern computing. In Source Code, Gates describes his early experience using a DARPA-funded computer at Harvard’s Aiken Computation Laboratory. Gates faced an administrative hearing after the Aiken Lab’s associate director learned that he was developing commercial software on DARPA computers intended for research purposes. This tension reflects the shift in using computers for defense and government purposes to commercial and personal use.

Before personal computers revolutionized technology, mainframe computers dominated computing from the 1950s through the 1970s. These large, powerful machines were used by governments, universities, and corporations for complex calculations, data storage, and business applications. Because mainframes were expensive and required specialized training, they were only accessible to select groups of users.

Gates’s first experience with computers came from mainframe access at Lakeside School, where he used a teletype machine to remotely connect to a mainframe in California. His exposure to these systems at an early age helped him see the limitations of centralized computing and motivated his later work in bringing software to personal computers.

Microprocessors are small, integrated circuits that serve as the brains of computers, handling calculations and executing instructions. The development of Intel’s 4004 microprocessor in 1971 paved the way for smaller, more affordable computers by replacing large, room-sized processors with a single chip.

Gates and Allen recognized that microprocessors would drive the future of computing. Their decision to buy an Intel 8080 microprocessor for personal experimentation foreshadowed the personal computing boom that Microsoft would later capitalize on. Their work on software for the Altair 8800—a computer powered by an Intel 8080 chip—was a direct result of this foresight.

The history of computer programming languages began in the 1940s and early 1950s when programmers wrote instructions in machine language, which consisted of binary code specific to a computer’s hardware. This process was tedious and error-prone, leading to the development of assembly language, which used symbolic instructions that were easier to read and write. However, assembly language was still tied to specific machines, so researchers sought to develop more abstract, high-level programming languages that could be used across different systems. One of the first major high-level languages was FORTRAN (Formula Translation), developed by IBM in 1957 for scientific and engineering applications. Created by John Backus and his team, FORTRAN introduced concepts such as loops, conditionals, and subroutines, making it significantly easier to write complex mathematical programs. FORTRAN became the dominant language in scientific computing and is still used today in specialized fields.

In 1964, John Kemeny and Thomas Kurtz at Dartmouth College created BASIC (Beginner’s All-purpose Symbolic Instruction Code) to make programming more accessible to students and non-experts. BASIC was simple, with easy-to-learn commands, making it widely adopted in educational institutions and, later, personal computing. In Source Code, Gates describes his early education in FORTRAN programming while using teletype machines as a student at Lakeside School and Harvard. After leaving Harvard, Gates and his future co-founder, Allen, developed a BASIC language specifically for the Altair 8800, an early personal computer.