Chapter 3: OS Organization

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Chapter 3 OS Organization
Prof. Steven A. Demurjian, Sr. Computer Science
Engineering Department The University of
Connecticut 191 Auditorium Road, Box
U-155 Storrs, CT 06269-3155
steve_at_engr.uconn.edu http//www.engr.uconn.edu/st
eve (860) 486 - 4818
These slides have been modified from a set
of originals by Dr. Gary Nutt.
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Purpose of this Chapter

• Concentrate on Designers Perspective of an OS
• Examine OS Design by Focusing on
• What Factors are Critical to Design an OS?
• Recall SW Qualities and Principles
• How do they relate to Designing an OS?
• What are Major OS Functions?
• Nuts and Bolts - What is under the hood?
• How are OSs Implemented?
• Organizational Strategies for Resources and their
  Interactions
• Strong Parallels with Software Design
  Engineering Practices

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Recall OS Requirements

• Provide Resource Abstractions
• Concrete Resource Definitions
• Design and Runtime Support/Management
• Provide Process Abstraction
• Conceptualization of Unit of Computation
• Model for Definition and Interaction
• Manage Sharing
• Permit/Prohibit when Dictated by Design
• Authorization and Security Play Significant Role
• Ensure Isolation Between Processes
• Insure Independent Executions
• Allow Controlled Interaction

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What Impacts the Design and Subsequent Evolution
of an OS?

• Performance Fast, Response Time
• Protection/Security
• Autonomous and Non-Interfering
• Preventing Malicious Access
• Correctness Work as Expected at All Times?
• Maintainability Easy to Correct, Change, and
  Evolve?
• Commercial Factors
• HW Platforms, Market Trends, Free?
• Lessons of Unix and C
• Standards/Open Systems Compatibility, Easy to
  Use and Extend, Conformity, etc.

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Performance

• What is Cost of Abstractions Provided by OS?
• Is it Easier to Design and Write Code?
• Is Ease of Use at Expense of Speed?
• Tradeoff Between Utility of Abstraction vs.
  Impact on Computers Performance
• What is the Role of Continued Advances in
• Hardware Speedup (400-500MhZ PCs)?
• Main Memory Capacity/Price/Speed?
• Disk and Other I/O Devices?
• Moving Graphics Capabilities to Separate
  Processing Computer?
• OS Must Not Overwhelm Machine with Overhead!

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Performance

• In 1997, Early Releases of Java/JDK Suffered
  Greatly w.r.t. Performance
• JIT Compilers Faster PCs Lessen Impact of Issue
• What about JavaOS and JINI?
• What is Performance Issue in Both Situations?

MULTIPLE LAYERS CAN DEGRADE PERFORMANCE!!
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Protection and Security

• What are Relevant Issues in an OS?
• Processes Share Resources (Files, Data, etc.)
• Processes Cannot Interfere with One Another
• Processes Know How to Obtain Exclusive Access to
  Resources
• Processes Prohibited from Malicious Access
• Security Policy Protection Mechanisms
• Policy Sharing Strategy of ComputerUser
  Passwords/Privileges/File Sharing
• Protection Implement Security PolicyDefined
  Policy for Resource Access
• Emergence of Security at Programming Level with
  Java Security within JDK/JRE

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Correctness

• Recall Functional Correctness - Software Behaves
  According to Requirements Specification
• When is Correctness Important in an OS?
• For Select Trusted Resources
• For Resources or Functions Defined with Precise
  Requirements Specifications
• What are Some Examples of Trusted Resources?
• Process Scheduler that Dictates Time-Sharing of
  CPU Based on Process Priority/Resources
• Concurrency Control/Deadlock Prevention Strategy
  for File Access
• OSs Must Embrace Formal Correctness over Ad-hoc
  Correctness

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Maintainability

• Recall Maintenance - Corrective, Perfective,
  and Adaptive, Modifications After Product Release
• Key Question from Textbook
• How could one change the OS software and have
  any assurance the result could be trusted, was
  correct, and did not introduce new bugs?
• Has this Been True in Commercial OSs?
• Must OS Designers Choose Between
• Design for Maintainability at Expense of
• Design for Performance?
• Correctness/Maintenance Key for Life Critical
  Applications
• Guiding Spaceship/Managing Nuclear Reactor

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Commercial Factors

• Mid 1970s into 1980s, Multi-programmed
  Time-Shared OSs Dominant (Unix, VMS, etc.)
• Advent of Networking has Evolved These OSs to
  their Network-Based Successors
• 1970s/1980s OS Wars Fought on Free vs. Stable
• BSD vs. ATT Unix
• BSD Unix vs. Digital VMS
• From Late 1980s on, Digitals Unix Support
  Personnel Outnumbered VMS Personnel
• 1990s/2000s New OS Wars
• Win95, 98, NT, 2000 (MS at War with Itself?)
• WinXX vs. Linux/BSD Unix vs. JavaOS?
• Who will Win (pun intended) the OS Wars?

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Standards and Open Systems

• 1990s Seen a Resurgence of Free Software and Open
  Systems
• Unix Community
• CORBA Community
• UML Community for OO Design
• Computing Model in Most Companies Embraces
  Heterogeneous Network of Interacting Nodes
• Legacy, COTS, Databases, etc.
• New Clients, Java Servers, Web Servers, etc.
• Key Issues Application Integration, Portability,
  and Interoperability
• Role of OS? Distributed OS? Java? JINI? CORBA?

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Single Process OS Organization
Program
Program
Libraries
Program
OS Services
ROM Routines
Processor(s)
Main Memory
Devices
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Basic OS FunctionsFour Broad Categories

• Device Management
• Disks, Terminals, Printers, etc.
• Emergence of New Media
• Process and Resource Management
• Execution Environment of Programs
• Sharing, Interaction, etc.
• Memory Management
• Local Memory for Program Execution
• Virtual Memory Management
• File Management
• Perspectives of End-User vs. Software Engineer
• Executing User/System Process vs. OS Routine

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Basic OS Functional Decomposition
File Manager
Process Resource Manager
Device Manager
Memory Manager
Processor(s)
Main Memory
Devices
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Device Management

• Disks (Hard, 3.5in, CD, DVD, etc.), Tapes,
  Terminals, etc. All Managed in Similar Fashion
• Management of Allocation, Isolation, Sharing
• Management Based on Pre-Defined Policies
• OS Must React to Change
• Ability to Include New Devices
• Load and Dynamically/Install Drives
• No Re-Compilation of OS
• Re-Configurable Device Drivers
• In Modern PC-Based OS, Wide Variety of Devices
  Supported is Staggering!
• WinNT has 60 Printer Manufactures
• If Each Average 20 Printer Types - 1200 Total!

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Process and Resource Management

• Process Management Track All Existing Processes
  and Their States
• Resource Management
• Track Resources Being Used by Processes
• Determine Conditions Under Which Process can
  Receive Resource
• In Practice, Combined into P R Management
• Major Responsibilities Include
• Enforce Isolation of Resource Among Multiple
  Processes According to Policy
• Determine Conditions When Circumvention by
  Process can Occur to Support Sharing

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Memory Management

• Responsible for
• Allocation/Use of Primary Memory by Process
• Allow Sharing of Memory by Processes
• Promote Isolation of Memory by Processes
• Modern OS Includes Virtual Memory
• Combination of Primary Memory and Secondary
  Storage (Disk, Tape, etc.)
• Staging of File, Program, etc. Off Secondary
  Storage When Memory Exceeded
• Requires Management of Virtual Memory Space in
  Conjunction with Physical Memory Space
• VM Needs Policies and Mechanisms Integrated with
  File Management

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Memory Management

• Virtual Memory in PDP-11/44 circa 1983
• 64K Virtual Memory/128K Primary Memory
• Utilization of Memory Overlays Tells OS
  Where/When Different Modules Execute
• Virtual Memory Management Brings in a Program,
  File, etc., Block-by-Block on Demand
• VMM Integrated with Caching/LRU Stategies
• In Todays Networked File Systems
• Process Can Utilize Non-Local Resources
• Yields Distributed Shared Memory Abstraction
• What Technology Allows Interaction Over Network
  Between Processes and/or Data Sources?

JAVA RMI!!!
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File Management

• Managing Abstract Resources for Storage Devices
• Files Remain in Primary Memory Until Released
  by Process
• Files Copied to Magnetic Disk, Tape, Writable CD
• Abstractions Range from ASCII Byte Steam to
  Indexed Records to Relational Databases
• In Todays OS, Commonly Distributed
• Distribution Transparent to User/Process
• Machine/User Utilizes Local and Remote Files
• E.g., Logical Mounting of Unix File System on NTs
  in Learning Center

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OS Organization RevisitedFunctional Interactions
File Manager
Process Resource Manager
Device Manager
Memory Manager
Processor(s)
Main Memory
Devices
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Implementation Considerations

• What are Core Tools Utilized to Design and
  Construct a Modern OS?
• Processor Modes
• Mode Bit of Processor
• Distinguish Between User/Supervisor Execution
• Kernels
• Critical, Core Portion of OS
• Represents Trusted Software
• Minimize to Alleviate Potential Performance
  Impact
• Method of Requesting System Service
• Procedure Call vs. Message Passing
• What is Role/Impact of Each in Implementing OS?

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Processor Modes

• Mode Bit
• Define Execution Capabilities of Program on a
  Processor
• Included in Modern Micro-Processors
• Supervisor Mode
• Can Execute All Machine Instructions
• Can Reference All Memory Locations
• User Mode
• Can Only Execute a Subset of Instructions
• Can Only Reference a Subset of Memory Locations
• Both Tied to Security Instructions that are
  Supervisor, Privileged, or Protected

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Kernels

• The Part of the OS Critical to Correct Operation
  (Trusted Software)
• Recall Trusted Software Bound to Verifiable
  Requirements!
• Kernel Guaranteed to Protect Covert or Malicious
  Changes by Untrusted Software
• Kernel Executes in Supervisor Mode
• Placing Function in Kernel
• May Impact Performance
• Allows Function to Interact with Other Kernel
  Capabilities
• The Trap Instruction is Used to Switch From User
  to Supervisor Mode, Entering the OS

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Demonstrating Modes and KernelsThe trap
Instruction

• Traps Allow User Process to Interact with Kernel
  and Executed Trusted Code
• Instruction Sets Mode Bit Branches to Code
• Similar in Concept to Hardware Interrupt

Mode
U
Trusted Code
User
Supervisor
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System Call Approach

• User Process Traps to OS Routine for Function to
  be Invoked
• Trap Does Context Switch to Supervisor Mode
• Function Called as Procedure
• Upon Completion, Switch Back to User Mode and
  Return Control to User Process

call()
trap
return
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Message Passing Approach

• User Process Constructs a Message and Sends
• A send Function Interacts with Trusted OS Process
  
• If OK, Switch to Supervisor Mode Deliver to
  Process that Implements Functionality
• User Process Waits for receive Message

send(, A, ) receive(, B, )
send/receive
receive(A, ) send(, B, )
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The UNIX Organization
Process
Process
Libraries
Process
User
Super- visor
Device Drivers
Kernel
Main Memory
Processor(s)
Devices
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Microkernel Organization

• Microkernel Contains Mechanism Dependent,
  Hardware Dependent Portion of OS - Non-Portable

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NT Organization
Process
Process
T
T
Process
T
T
T
T
Libraries
T
T
T
Subsystem
Subsystem
Subsystem
User
Super- visor
I/O Subsystem
NT Executive
NT Kernel
Hardware Abstraction Layer
Main Memory
Processor(s)
Devices
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Concluding Remarks/Looking Ahead

• Review of Designer Perspective of OS
• Critical Issues and Concepts that Impact Design
• Tradeoffs of Performance, Maintainability, etc.
• Interesting Exercise 1 in Section 3.5
• Design/Develop PC OS with 5 year Lifetime
• Relevance of Requirements (slide 4)
• What about Mainframe OS with 20 year LT?
• Looking Ahead to
• Review of Computer Organization
• Relevance of CO to OS Design/Usage
• Device Management
• Process Management
• Project Status