The microkernel pattern
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The Microkernel Pattern. What is the Microkernel Pattern?. Similar to Layers pattern Separate minimal functionality from extended functionality Able to adapt to customer-specific parts Originates from OS theory and research. Why use the Microkernel Pattern?.

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The Microkernel Pattern

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The microkernel pattern

The Microkernel Pattern


What is the microkernel pattern

What is the Microkernel Pattern?

  • Similar to Layers pattern

  • Separate minimal functionality from extended functionality

  • Able to adapt to customer-specific parts

  • Originates from OS theory and research


Why use the microkernel pattern

Why use the Microkernel Pattern?

  • When there is a need to adapt to changing system requirements

  • Portability, extensibility and adaptability

  • Easy integration of emerging technologies (e.g. emulation)


Roles to be played

Roles to be played

  • Internal server

  • External server

  • Adapter

  • Client

  • Microkernel


The microkernel in more detail

The Microkernel in more detail

  • Implements central services

  • Other components build on these basic services

  • Encapsulation of system-specific dependencies

  • These basic services are called atomic services (or mechanisms)


Internal server subsystem in more detail

Internal Server (Subsystem) in more detail

  • Extends functionality offered by the microkernel

  • Separate component

  • Encapsulates some dependencies on underlying hardware/software system

  • Example: device drivers


External server personality in more detail

External Server (Personality) in more detail

  • Uses microkernel to implement its own view of the underlying application domain

  • Layer on top of atomic services provided by microkernel

  • Different external servers implement different policies from different application domains

  • Example: OS/2 Warp


Client in more detail

Client in more detail

  • Application that is associated with exactly one external server

  • It accesses the APIs provided by the external server


Adapter emulator in more detail

Adapter (Emulator) in more detail

  • Protect clients and external server from direct dependencies

  • Use adapter interface

  • Example: no distinction between a native OS/2 Warp and microkernel support

  • Implement as a Proxy (since Adapter refers to one external server)


Example the chorus system

Example: The CHORUS System

  • Originated in INRIA (France) in 1990

  • High-performance UNIX emulation

  • Distributed environments

  • RT applications

  • Integration with OOT


Chorus architecture

Chorus Architecture


Example of a microkernel system chorus

Example of aMicrokernel System (Chorus)


Test case windows nt architecture

Test CaseWindows NT Architecture

  • OS designed to run programs from different environments

  • DOS, 16-bit Windows, OS/2

  • Client-server model (programs run in User mode)

  • Calls packaged and sent to Kernel via LPC


System services

System Services


General features

General Features

  • NT Executive is layered

  • Some objects in charge of policy (e.g. Security Monitor)

  • The Kernel is responsible for implementation


General features continued

General Features (continued)

  • Relaxed layered system (Kernel and I/O manager may access hardware directly)

  • NT has a subsystem for each operating system


The layers

The Layers

  • System services: interface layer between subsystems and NT Executive

  • Resource management: Object Manager, Virtual Memory Manager etc.

  • Kernel: basic functions (interrupt, thread dispatching...)


The layers continued

The Layers (continued)

  • HAL: hides hardware differences between different processor families

  • Hardware layer


The kernel

The Kernel

  • Bottom of the food chain

  • Fundamental functions and objects

  • Cannot be pre-empted


Responsibilities of the kernel

Responsibilities of the Kernel

  • Scheduling threads

  • Transferring control on interrupts and exceptions

  • Synchronising multiprocessor operations

  • Initiating system recovery after a power failure


Objects in kernel

Objects in Kernel

  • Dispatcher objects (e.g. Mutex, Semaphore, Thread)

  • Control objects (Process, I/O, Asynchronous Procedure Call)

  • Represent more primitive attribute subset provided by corresponding User objects


Final remarks

Final Remarks

  • Microkernel is a variant of Layers pattern

  • Use Microkernel when clients have different views of underlying business logic (Buschmann)


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