Advanced Design and System Patterns
The Microkernel Pattern
Advanced Design and System Patterns
What is the Microkernel Pattern?
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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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Advanced Design and System Patterns
Why use the Microkernel
Pattern?
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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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Advanced Design and System Patterns
Roles to be played
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Internal server
External server
Adapter
Client
Microkernel
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Advanced Design and System Patterns
The Microkernel in more detail
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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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Advanced Design and System Patterns
Internal Server (Subsystem) in
more detail
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Extends functionality offered by the
microkernel
Separate component
Encapsulates some dependencies on
underlying hardware/software system
Example: device drivers
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Advanced Design and System Patterns
External Server (Personality) in
more detail
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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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Advanced Design and System Patterns
Client in more detail
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Application that is associated with exactly
one external server
It accesses the APIs provided by the
external server
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Advanced Design and System Patterns
Adapter (Emulator) in more
detail
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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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Advanced Design and System Patterns
Example: The CHORUS System
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Originated in INRIA (France) in 1990
High-performance UNIX emulation
Distributed environments
RT applications
Integration with OOT
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Advanced Design and System Patterns
Chorus Architecture
{
{
U1
U2
U3
User process
User
Address
Space
Kernel
Address
Space
Unix Subsystem
S1
Object-Oriented Subsystem
S2
S3
System process
K2
Kernel process
K1
Microkernel
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Advanced Design and System Patterns
Example of a
Microkernel System (Chorus)
Unix Process
PM
PM
Microkernel
Xterminal
IPCM
Complete System
SM
Microkernel
SM
Microkernel
Embedded System
PM
OM
PM
SM
IPCM
Microkernel
Diskless Workstation
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Advanced Design and System Patterns
Test Case
Windows NT Architecture
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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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Advanced Design and System Patterns
System Services
User Mode
System Services
Object
Manager
...
Virtual
Local
Memory
Procedure
Call Facility Manager
(LPC)
I/O
Manager
File
System
Cache
Systems
Drivers
}
Kernel Mode
Kernel
Hardware Abstraction Layer
(HAL)
Hardware
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Advanced Design and System Patterns
General Features
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NT Executive is layered
Some objects in charge of policy (e.g.
Security Monitor)
The Kernel is responsible for
implementation
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Advanced Design and System Patterns
General Features (continued)
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Relaxed layered system (Kernel and I/O
manager may access hardware directly)
NT has a subsystem for each operating
system
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Advanced Design and System Patterns
The Layers
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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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Advanced Design and System Patterns
The Layers (continued)
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HAL: hides hardware differences between
different processor families
Hardware layer
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Advanced Design and System Patterns
The Kernel
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Bottom of the food chain
Fundamental functions and objects
Cannot be pre-empted
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Advanced Design and System Patterns
Responsibilities of the Kernel
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Scheduling threads
Transferring control on interrupts and
exceptions
Synchronising multiprocessor operations
Initiating system recovery after a power
failure
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Advanced Design and System Patterns
Objects in Kernel
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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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Advanced Design and System Patterns
Final Remarks
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Microkernel is a variant of Layers pattern
Use Microkernel when clients have
different views of underlying business
logic (Buschmann)
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