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The Microkernel Pattern

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s 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

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s 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)

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Roles to be played

Internal server External server Adapter Client Microkernel

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s 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)

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s 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

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s 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

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Client in more detail

Application that is associated with exactly one external server

It accesses the APIs provided by the external server

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s 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)

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s Example: The CHORUS System

Originated in INRIA (France) in 1990 High-performance UNIX emulation Distributed environments RT applications Integration with OOT

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Chorus Architecture

U1 U2 U3 User process

Microkernel

UserAddressSpace

KernelAddressSpace

S1 S2

K1

Unix Subsystem

S3

K2 Kernel process

System process

Object-Oriented Subsystem{{

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s Example of aMicrokernel System (Chorus)

Unix Process

PM

Microkernel

Microkernel Microkernel

Microkernel

PM

SM

IPCM

PMPM SM

SMOM

IPCM

Embedded System

Xterminal Diskless Workstation

Complete System

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s 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

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System Services

Kernel Mode}User Mode

System Services

LocalProcedureCall Facility

(LPC)

VirtualMemoryManager

I/OManager

FileSystemCache

SystemsDrivers

Kernel

Hardware Abstraction Layer(HAL)

Hardware

ObjectManager

. . .

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General Features

NT Executive is layered Some objects in charge of policy

(e.g. Security Monitor) The Kernel is responsible for

implementation

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General Features (continued)

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

NT has a subsystem for each operating system

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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...)

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The Layers (continued)

HAL: hides hardware differences between different processor families

Hardware layer

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The Kernel

Bottom of the food chain Fundamental functions and objects Cannot be pre-empted

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s Responsibilities of the Kernel

Scheduling threads Transferring control on interrupts

and exceptions Synchronising multiprocessor

operations Initiating system recovery after a

power failure

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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

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Final Remarks

Microkernel is a variant of Layers pattern

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