In short, operating systems are complex, fragile, inflexible, and slow, because they have dabbled in the practice of providing a general purpose virtual machine. The operating system is basically hardware masquerading as software: it cannot be changed, all applications must use it, and the information it hides cannot be recovered. Operating system designers should learn what hardware designers learned a decade ago during the transition from CISC to RISC: hardware should provide primitives, not high-level abstractions...

In this section we give a general sketch of an OS structure that embodies a "abstraction-free", low-level interface. We call such a structure an exokernel. The sole function of an exokernel is to allocate, deallocate, and multiplex physical resources in a secure way. The resources exported by this kernel are those provided by the underlying hardware: physical memory (divided into pages), the CPU (divided into time-slices), disk memory (divided into blocks), DMA channels, I/O devices, translation look-aside buffer, addressing context identifiers, and interrupt/trap events.

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