Understanding Exokernel

1. Open Course by Ferdian Sonatha , Padepokan Kanuragan Blankon Malang
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Exokernel
Exokernel is an operating system kernel developed by the MIT Parallel and Distributed
Operating Systems group, and also a class of similar operating systems.
Operating System Kernels generally present the hardware-resources to applications
through high level abstractions such as the (virtual) file-system. The idea behind
exokernels is to force as few abstractions as possible on developers, enabling them to
make as many decisions as possible about hardware abstractions. Exokernels are tiny,
since functionality is limited to ensuring protection and multiplexing of resources, which
are vastly simpler than conventional microkernels' implementation of message passing
and monolithic kernels' implementation of abstractions.
Implemented applications are called library operating systems; they may request specific
memory addresses, disk blocks, etc. The kernel only ensures that the requested
resource is free, and the application is allowed to access it. This low-level hardware
access allows the programmer to implement custom abstractions, and omit unnecessary
ones, most commonly to improve a program's performance. It also allows programmers
to choose what level of abstraction they want, high, or low.

2. Open Course by Ferdian Sonatha , Padepokan Kanuragan Blankon Malang
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Exokernels can be seen as an application of the end-to-end principle to operating
systems, in that they do not force an application program to layer its abstractions on top
of other abstractions that were designed with different requirements in mind. For
example, in the MIT Exokernel project, the Cheetah web server stores
preformatted Internet Protocol packets on the disk, the kernel provides safe access to
the disk by preventing unauthorized reading and writing, but how the disk is abstracted is
up to the application or the libraries the application uses.
Motivation
Traditionally kernel designers have sought to make individual hardware resources
invisible to application programs by requiring the programs to interact with the hardware
via some abstraction model. These models include file systems for disk storage, virtual
address spaces for memory, schedulers for task management, and sockets for network
communication. These abstractions of the hardware make it easier to write programs in
general, but limit performance and stifle experimentation in new abstractions. A security-
oriented application might need a file system that does not leave old data on the disk,
while a reliability-oriented application might need a file system that keeps such data for
failure recovery.
One option is to remove the kernel completely and program directly to the hardware, but
then the entire machine would be dedicated to the application being written (and,
conversely, the entire application codebase would be dedicated to that machine). The
exokernel concept is a compromise: let the kernel allocate the basic physical resources
of the machine (e.g. disk blocks, memory pages, and processor time) to multiple
application programs, and let each program decide what to do with these resources. The
program can then link to a support library that implements the abstractions it needs (or it
can implement its own).
MIT exokernel
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:

3. Open Course by Ferdian Sonatha , Padepokan Kanuragan Blankon Malang
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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 interrupts, 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
acapability 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 byte
code language designed for easy security-checking by the kernel.
Applications
The available library operating systems for Exokernel include the custom ExOS system
and an emulator for BSD. In addition to these, the exokernel team created the
Cheetahweb server, which uses the kernel directly.
History
The exokernel concept has been around since at least 1994,[1]
but as of 2010 exokernels
are still a research effort and have not been used in any major commercial operating
systems. A concept operating exokernel system is Nemesis, written by University of
Cambridge, University of Glasgow, Citrix Systems, and the Swedish Institute of
Computer Science. MIT has also built several exokernel based systems, including
ExOS.

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Bibliography
 Engler, Dawson R (1998). The Exokernel Operating System Architecture (PostScript).
MIT. Retrieved 2006-09-22.
 ———; Kaashoek, M. Frans (1996). Exterminate All Operating System
Abstractions (PostScript). MIT. Retrieved 2006-09-22.
 ———; Kaashoek, M. Frans; O’Toole, James Jr (1995). "Exokernel: An Operating
System Architecture for Application-Level Resource Management" (PDF). Proceedings of the
fifteenth ACM symposium on Operating systems principles: 251–
66. doi:10.1145/230000/224076/p251-engler.pdf. ISBN 0-89791-715-4. Retrieved 2006-09-
22.
External links
 Erlingsson, Úlfar; Kyparlis, Athanasios, Microkernels, Cornell, "The extent to
which simple, efficient operations are a good choice in a kernel interface design".
 Exokernel Operating System, MIT. Putting the Application in Control.
 Nemesis, UK: Cambridge. An operating system with principles.
 BareMetal OS, Return Infinity.
 XOmB.
 ExAmour, FR. IA-32 exokernel.