AMD uses the Intel instruction set architecture (ISA), primarily because it ensures compatibility with the vast majority of software written for personal computers, which has historically been designed for the x86 architecture created by Intel. The x86 ISA became the dominant standard for desktop and laptop computers since the 1980s due to the widespread adoption of IBM's personal computers, which were powered by Intel's x86 processors. This created a positive feedback loop, with software developers focusing on x86 to ensure their programs could reach the largest possible market, and users purchasing x86-based systems to run the widest range of software. In 1982, AMD signed a contract with Intel to become a second-source manufacturer of x86 processors, which legally allowed AMD to produce processors that were compatible with Intel's technology. This relationship was integral in AMD's ability to build CPUs that could compete in the market with compatibility for the popular operating systems and applications that were designed to run on Intel's x86 processors. As technology evolved, both companies engaged in cross-licensing agreements, and AMD introduced the x86-64 extension (also known as AMD64), which extended the x86 architecture to 64-bit. Intel eventually adopted a compatible version of this extension under the name Intel 64 (formerly EM64T), further entangling their ecosystems. Ensuring compatibility with the x86 (and consequently x86-64) ISA allows AMD to participate in the broader PC market, leveraging existing software and ecosystems without forcing users or software developers to deal with compatibility issues that would arise from a different, incompatible ISA. This compatibility has been a major factor in AMD's success in the consumer, server, and enterprise markets, where software compatibility is a fundamental requirement. In essence, the use of the Intel instruction set by AMD enables it to create CPUs that are directly competitive with Intel's offerings, often differentiating on performance, price, or features, while maintaining the essential compatibility that the market demands.