Gil Bates wrote “SPIN is an operating system that blurs the distinction between kernels and applications. Applications traditionally live in user-level address spaces, separated from kernel resources and services by an expensive protection boundary. With SPIN, applications can specialize the kernel by dynamically linking new code into the running system. Kernel extensions can add new kernel services, replace default policies, or simply migrate application functionality into the kernel address space. Sensitive kernel interfaces are secured via a restricted linker and the type-safe properties of the Modula-3 programming language. The result is a flexible operating system that helps applications run fast but doesn’t crash.” More can be found on the SPIN homepage. More on Modula-3 can be found here.