Chapter 2: Operating-System Structures

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Chapter 2 Operating-System Structures
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Chapter 2 Operating-System Structures

• Operating System Services
• User Operating System Interface
• System Calls
• Types of System Calls
• System Programs
• Operating System Design and Implementation
• Operating System Structure
• Virtual Machines

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Objectives

• Describe the services an operating system
  provides to users, processes, and other systems
• Discuss various ways of structuring an operating
  system
• Explain how operating systems are installed and
  customized and how they boot

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Operating System Services

• Provide functions that are helpful to users
• User interface
• Batch, Command-Line (CLI), Graphics User
  Interface (GUI)
• Program execution
• Load a program into memory, Run it, End
  execution normally or abnormally
• I/O operations
• File-system manipulation
• Create, delete, read, write, and list files and
  directories
• Change permissions

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Operating System Services (Cont.)

• Provide functions that are helpful to users
• Communications
• Exchange information on the same computer or
  between computers over a network
• Shared memory or message passing
• Error detection
• Hardware CPU, memory, I/O devices, etc.
• User program
• For each type of error, take an appropriate
  action to ensure correct and consistent computing
• Debugging

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Operating System Services (Cont.)

• Ensure efficient system operation via resource
  sharing
• Resource allocation
• CPU cycles, main memory, file storage, I/O
  devices
• Accounting To keep track of which users use how
  much and what kinds of computer resources
• Protection and security
• Essential in multi-user/networked environment
• Protection Ensure that all access to system
  resources is controlled, e.g., mode bit, memory
  management
• Security User authentication, access control, ...

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User Operating System Interface CLI (Command
Line Interface)

• Direct command entry
• Implemented in kernel or by systems program
• Multiple flavors can be implemented, e.g.,
  several command shells for UNIX
• Fetch a command from user and execute it
• Built-in (i.e., implemented in kernel)
• Invoke user-level programs
• Adding new features requires no shell modification

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User Operating System Interface GUI

• User-friendly desktop metaphor interface
• Icons represent files, programs, actions, etc
• Mouse buttons over objects cause various actions
• Invented at Xerox PARC
• Many systems have both CLI and GUI
• Microsoft Windows is GUI with CLI
• UNIX/LINUX is CLI with optional GUI interfaces
• Apple Mac OS X has Aqua GUI interface with UNIX
  kernel underneath

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System Calls

• Programming interface to OS services
• Typically written in C or C
• Usually use high-level Application Program
  Interface (API) rather than direct system calls
• Win32 API for Windows
• POSIX API for POSIX-based systems (including
  virtually all versions of UNIX, Linux, and Mac OS
  X)
• Java API for the Java virtual machine (JVM)

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Example System Calls

• System call sequence to copy the contents of one
  file to another file

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Example of Standard API

• Consider the ReadFile() function in Win32 API

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System Call Implementation

• System call interface invokes intended system
  call in OS and returns system status and any
  return values
• Caller needs to know nothing about how the system
  call is implemented
• Use API
• Hide implementation details

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API System Call OS Relationship
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Standard C Library Example

• C program invoking printf() library call, which
  calls write() system call

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System Call Parameter Passing

• Often, more information is required than simple
  identity of desired system call
• How to pass parameters to OS
• Pass the parameters in registers
• Number length of parameters can be limited
• Parameters stored in a block or table in memory,
  and pass the blocks address as a parameter in a
  register
• Push parameters onto stack

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Parameter Passing via Table
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Types of System Calls

• Process control
• File management
• Device management
• Information maintenance
• Communications

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MS-DOS execution
(a) At system startup (b) running a program
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FreeBSD Running Multiple Programs
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System Programs

• Convenient environment for program development
  and execution
• Program loading and execution
• File manipulation
• Programming language support
• Communications
• Application programs
• Most users view of the OS is defined by system
  programs, not the actual system calls

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System Programs

• Provide a convenient environment for program
  development and execution
• Some of them are simply user interfaces to system
  calls others are considerably more complex
• File management
• Create, delete, copy, rename, print, dump, list,
• Status information
• File modification
• Text editors to create and modify files
• Commands to search file contents, e.g., grep in
  UNIX/Linux

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System Programs (contd)

• Programming language support
• Compilers/interpreters, assemblers, debuggers
• Program loading and execution
• Absolute loaders, relocatable loaders, linkage
  editors, and overlay loaders
• Communications
• Creating virtual connections among processes,
  users, and computer systems
• Send messages to somebodys screen, send email,
  browse web pages, remote log in, file transfer

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Operating System Design and Implementation

• No silver bullet but some approaches have been
  successful
• Internal structure of different OSs can vary
  widely
• Affected by choice of hardware OS type
• First define goals and specifications
• User goals and System goals
• User goals OS should be convenient, easy to
  learn, reliable, safe, and fast
• System goals OS should be easy to design,
  implement, and maintain, as well as flexible,
  reliable, error-free, and efficient

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Operating System Design and Implementation (Cont.)

• Important to separate
• Policy What will be done? Mechanism How to
  do it?
• Mechanisms determine how to do something,
  policies decide what will be done
• The separation of policy from mechanism is a very
  important principle, it allows maximum
  flexibility if policy decisions are to be changed
  later

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Simple Structure

• MS-DOS
• Written to provide the most functionality in the
  least space
• Not divided into modules
• Although MS-DOS has some structure, its
  interfaces and levels of functionality are not
  well separated

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Layered Approach

• The operating system is divided into a number of
  layers (levels), each built on top of lower
  layers. The bottom layer (layer 0), is the
  hardware the highest (layer N) is the user
  interface.
• With modularity, layers are selected such that
  each uses functions (operations) and services of
  only lower-level layers

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Layered Operating System
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UNIX

• UNIX
• Limited by hardware functionality, the original
  UNIX had limited structuring consisted of two
  separable parts
• Systems programs
• Kernel
• Consists of everything below the system-call
  interface and above the physical hardware
• Provides a large number of functions for one
  level CPU scheduling, memory management, file
  system and other OS functions

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UNIX System Structure
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Microkernel System Structure

• Moves as much from the kernel into user space
  as possible
• Message passing between user modules
• Benefits
• Easier to extend a microkernel
• Easier to port the OS to new architectures
• More reliable (less code is running in kernel
  mode)
• More secure
• Problem
• Performance overhead due to communications
  between user space to kernel space

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Mac OS X Structure

• Hybrid approach (microkernel layered approach)
• Mach microkernel Memory management, RPC IPC
• BSD kernel BSD command line interface,
  networking file system, POSIX APIs such as
  Pthreads
• In addition, kernel environment provides an I/O
  kit dynamically loadable modues

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Modules

• Most modern operating systems implement kernel
  modules
• Uses object oriented approach
• Each core component is separate
• Each talks to the others over known interfaces
• Each is loadable as needed within the kernel,
  e.g., device drivers system calls
• Similar to layers but more flexible

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Solaris Modular Approach
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Virtual Machines

• Take the layered approach
• Treat hardware and OS kernel as though they were
  all hardware
• Provide an interface identical to the underlying
  hardware
• An OS creates the illusion of multiple processes
  executing on its own processor with its own
  (virtual) memory

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Virtual Machines (Cont.)

• (a) Nonvirtual machine (b) virtual machine

Non-virtual Machine
Virtual Machine
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Virtual Machines (Cont.)

• Provide complete protection of system resources
• Each virtual machine is isolated from all other
  virtual machines
• No direct sharing of resources
• A perfect vehicle for operating systems research
  and development
• Develop an OS on a virtual machine
• Do not disrupt normal system operation.
• Difficult to implement due to the effort required
  to provide an exact duplicate to the underlying
  machine

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VMware Architecture
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The Java Virtual Machine
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End of Chapter 2 Questions?