This paper introduces SPIN, an extensible operating system designed to meet the performance and functionality requirements of specific applications. SPIN provides a robust extension infrastructure and a core set of extensible services, allowing applications to safely modify the operating system's interface and implementation. The system uses language and link-time mechanisms to export fine-grained interfaces to operating system services, enabling extensions to access system resources with low overhead. Extensions are written in Modula-3, a type-safe language, and dynamically linked into the kernel, ensuring rapid access to system services while protecting the kernel code. The design of SPIN emphasizes extensibility, safety, and performance. Extensibility is achieved through fine-grained access to system services, enforced modularity, logical protection domains, and dynamic call binding. Safety is ensured by co-location, where extensions are dynamically linked into the kernel, and by logical protection domains, which isolate extensions from each other and from the kernel. Performance is optimized through low-overhead communication between extensions and the system. The paper also discusses the motivation for building an extensible operating system, related work, and the architecture of SPIN. It describes the core services provided by SPIN, including memory management and thread management, and evaluates the system's performance through microbenchmarks, networking protocols, and end-to-end applications. The results show that SPIN can efficiently support high-performance network protocols and applications, demonstrating its effectiveness in addressing the challenges of system specialization.This paper introduces SPIN, an extensible operating system designed to meet the performance and functionality requirements of specific applications. SPIN provides a robust extension infrastructure and a core set of extensible services, allowing applications to safely modify the operating system's interface and implementation. The system uses language and link-time mechanisms to export fine-grained interfaces to operating system services, enabling extensions to access system resources with low overhead. Extensions are written in Modula-3, a type-safe language, and dynamically linked into the kernel, ensuring rapid access to system services while protecting the kernel code. The design of SPIN emphasizes extensibility, safety, and performance. Extensibility is achieved through fine-grained access to system services, enforced modularity, logical protection domains, and dynamic call binding. Safety is ensured by co-location, where extensions are dynamically linked into the kernel, and by logical protection domains, which isolate extensions from each other and from the kernel. Performance is optimized through low-overhead communication between extensions and the system. The paper also discusses the motivation for building an extensible operating system, related work, and the architecture of SPIN. It describes the core services provided by SPIN, including memory management and thread management, and evaluates the system's performance through microbenchmarks, networking protocols, and end-to-end applications. The results show that SPIN can efficiently support high-performance network protocols and applications, demonstrating its effectiveness in addressing the challenges of system specialization.