Editing Exokernel (section)

==MIT exokernels==
MIT developed two exokernel-based operating systems, using two kernels: Aegis, a proof of concept with limited support for storage, and XOK, which applied the exokernel concept more thoroughly.

An essential idea of the MIT exokernel system is that the operating system should act as an executive for small programs provided by the application software, which are constrained only by the requirement that the exokernel must be able to guarantee that they use the hardware safely.

===Design===
The MIT exokernel manages hardware resources as follows:

; Processor
: The kernel represents the processor resources as a timeline from which programs can allocate intervals of time. A program can yield the rest of its time slice to another designated program. The kernel notifies programs of processor events, such as [[interrupt]]s, [[exception handling|hardware exceptions]], and the beginning or end of a time slice. If a program takes a long time to handle an event, the kernel will penalize it on subsequent time slice allocations; in extreme cases the kernel can abort the program.

; Memory
: The kernel allocates physical memory pages to programs and controls the [[translation lookaside buffer]]. A program can share a page with another program by sending it a ''[[capability-based security|capability]]'' to access that page. The kernel ensures that programs access only pages for which they have a capability.

; Disk storage
: The kernel identifies disk blocks to the application program by their physical block address, allowing the application to optimize data placement. When the program initializes its use of the disk, it provides the kernel with a function that the kernel can use to determine which blocks the program controls. The kernel uses this callback to verify that when it allocates a new block, the program claims only the block that was allocated in addition to those it already controlled.

; Networking
: The kernel implements a programmable [[packet filter]], which executes programs in a [[bytecode]] language designed for easy security-checking by the kernel.

===Applications===
The available library operating systems for the exokernel include the custom ExOS system and an emulator for [[Berkeley Software Distribution|BSD]]. In addition to these, the exokernel team created the Cheetah [[web server]], which uses the kernel directly.