Most processors feature four protection rings, but operating systems typically utilize only two. This limitation stems from historical reasons, overhead concerns, and the specific environments in which these systems operate. For instance, Windows, primarily used in single-user home and office settings, finds two rings sufficient. In contrast, mainframe systems, which support multiple users, could benefit from additional rings. Operating systems like Multics and OS/2 experimented with more than two rings, but the complexity of Multics led to the development of the simpler Unix system. The two-ring model simplifies the management of privilege levels, with ring 0 reserved for the kernel and ring 3 for user applications. This design helps maintain security and manage mode switching efficiently. While Linux could have utilized all four rings, the decision to stick with two likely facilitated easier porting across different architectures. The core concept revolves around the distinction between privileged and unprivileged tasks, emphasizing the importance of isolating operations that require elevated privileges.
