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Choices
“Our object-oriented system architecture
embodies the notion of customizing operating
systems to tailor them to support particular
hardware configuration and particular
applications.”
--from Choices homepage
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What is Choices?
• Choices is an object-oriented operating system
• Written in C++
• Machine and processor dependent portions are in
assembler
• It has an object-oriented interface
• Operating system entities are objects
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Where could Choices run?
• Bare networked SPARC stations
• Shared memory Encore Multimaxes
• Personal computers using the Intel 386 and
MC68030 processors
• Tapestry hypercubes
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Frameworks
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An architectural design of a system
Components and interactions
From general to specific
Advantages
– Code reuse, Portability, Rapid prototyping
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Building Frameworks
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Abstract classes for generalized interfaces
Inclusion polymorphism
Constraints between classes
Dynamic code loading
Delayed binding and virtual functions
Conversions
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Choices Framework
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A hierarchy of framework representing the OS
Abstract classes as configurable components
Single inheritance
Subtyping to express inclusion polymorphism
A particular OS is a possible instantiation of the
framework
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Choices Framework
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MemoryObject
Process
Domain (Kernel domain, User domain)
Relationship between these three
NameServer
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Choices Subsystems
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Virtual memory
Persistent storage
Process management
Message passing
Device management
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Virtual Memory
• Features
– Multiple 32 bit virtual memory address spaces
– One and two level paging
– Shared memory
• Components
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MemoryObject
Domain
MemoryObjectCache
Address Translation
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Process Management
• Features
– Multi-threading support
– Grouping of processes (Gang)
• Components
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Process (System, Application, Interrupt)
ProcessContext (machine dependent state)
Processsor
Gang
ProcessContainer (implements scheduling)
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Persistent Storage
• Features
– Support both standard and customized storage systems
• Components
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PersistentStore (random access)
PersistentObject
PersistentStoreContainer
PersistentStoreDictionary
PersistentArray, RecordFile, AutoloadPersistentObject
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Message Passing System
• Features
– Different reliability models
– Support multiprocessor and distributed system
• Components
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MessageContainer (Mach ports, communication entity)
ContainerRepresentative
MessageSystemInterface
Transport
DataTransfer (buffering strategy)
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Device Management
• Features
– Processes communicate with devices
– Similarities and differences with UNIX
• Components
– Device (server)
– DevicesControllers
– DevicesManager
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Further Work
• uChoices
– Micro kernel OS
• Adaptive file systems that provide different
caching and network protocol support
• A fast process migration facility
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uChoices (1)
• “To support modern OS services such as user level
and gang scheduling, distributed customizable
virtual memory, and multimedia in a completely
modular architecture, while maintaining high
performance.”
• Redesign of Choices as a micro-kernel
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uChoices (2)
• Nano-kernel encapsulates hardware dependencies
• Micro-kernel provides the interface to the
remainder of the OS
– Encapsulates the micro-kernel data structures
– Single standard interface to nano-kernel
• Intermediate levels of OS
– Client, server, file system components, etc.
• Application interfaces
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Comparisons (1)
• Choices vs. OS-Kit
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general purposes
OS-Kit also support finer granularity
Different attitudes toward legacy code
OS-Kit supports COM interface
Choices supports ORB
OS-Kit tries to minimize the interaction and
dependencies between components
– OS-Kit has a minimal API
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Comparisons (2)
• Choices vs. MMLite
– General purpose (MMLite is also used in embedded
systems)
– The distributed file system extension of Choices is
close to the mechanism used in MMLite (proxy)
– MMLite focuses on support for transparently replacing
components in use
– MMLite supports COM
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Comparisons (3)
• Choices vs. PURE
– PURE is for deeply embedded systems
– PURE extension has an OSEK API
– PURE uses the concept of components at the design
level, and avoids artificical boundaries in
implementation concerning run time and code size
– PURE is fine-grained
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Comparisons (4)
• Choices vs. Pebble
– Pebble is for complex embedded system
– Concerns about safe extensibility and low interrupt
latency
– Pebbles uses a micro-kernel stucture
– Pebble uses fine-grained components
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Comparisons (5)
• Choices vs. eCos
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eCos is designed for embedded system
eCos has its own published API
eCos is fine-grained
Statically reconfigurable
eCos has very good tool support (analysis and
configuration tool)
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Conclusion
• Relatively old research work
• Limited work on constraints
• No clear description of how to build a system
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Choices
• Reference:
– Choices, Frameworks and Refinement
– Getting Started with Choices
– An Object-Oriented Nano-Kernel for Operating System
Hardware Support
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