Exokernel

An exokernel is a minimalist operating system kernel architecture that aims to maximize performance and flexibility by minimizing the amount of service logic implemented in the kernel. The kernel's primary task is to securely multiplex hardware resources—memory, disks, networks, and I/O devices—and to enforce protection, while avoiding the traditional high-level abstractions found in conventional kernels.

On top of an exokernel, applications or specialized library operating systems, known as libOS components, implement OS services according to the needs of the application. Rather than providing a fixed policy, the exokernel export resources as fine-grained primitives (such as memory frames, disk blocks, and network channels) along with safe protection mechanisms. LibOS layers reassemble these primitives into filesystems, IPC, networking, and other services, enabling specialization or customization for different workloads. The security model typically relies on capabilities or similar tokens to authorize use of resources, preventing untrusted code from misusing hardware.

Origin and influence: The exokernel concept was developed in the 1990s in university research, with ExOS as a prominent demonstration of how an extremely small kernel could safely multiplex resources while letting libraries implement higher-level policies. Although exokernel systems have not become mainstream in commercial operating systems, the approach has influenced subsequent research in microkernel design, library OSes, and performance-oriented OS architectures. Critics point to the complexity and maintenance burden of libOS stacks and to challenges achieving broad hardware support and portability.