1' Introduction

1
1. Introduction

• 1.1 The Role of Operating Systems
• - Bridge the Semantic Gap between
  Hardware and Application
• - Three Views of Operating Systems
• 1.2 Organization of Operating Systems
• - Structural Organization
• - The Hardware Interface
• - The Programming Interface
• - The User Interface
• - Runtime Organization
• 1.3 Operating System Evolution Concepts

2
Single CPU System
Figure 1-1
3
Bridging the Semantic Gap

• Hardware capabilities are very low level
• Arithmetic and logical operators
• Comparison of two bit-strings
• Branching, reading, and writing bytes
• User needs to think in terms of problem to be
  solved
• High-level data structures and corresponding
  operations
• Simple, uniform interfaces to subsystems,
• Treat programs and data files as single entities
• Use software to bridge this gap
• Language processors (e.g., assemblers, compilers,
  interpreters).
• Editors and text processors, linkers and loaders.
• Application programs, utility and service
  programs.
• Operating Systems

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The role of OSs

• Bridge Hardware/Application Gap
• Machine instruction vs high level operation
• compiler bridges gap
• Linear memory vs data structures
• compiler bridges gap
• Limited CPU memory vs more needed
• OS bridges gap
• Secondary memory devices vs files
• OS bridges gap
• I/O devices vs high level I/O commands
• OS bridges gap

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Multiprocessor Systems
Figure 1-2a
Figure 1-2b
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Multicomputer System
Figure 1-2c
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PC Hardware Organization
Figure 1-7
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Three views of OSs

• OS is an extended machine
• Principle of abstraction hides complexity
• OS provides high level operations using lower
  level operations
• OS is a virtual machine
• Principle of virtualization supports sharing
• OS provides virtual CPU, memory, devices
• OS is a resource manager
• Balance overall performance with individual needs
  (response time, deadlines)

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Structural Organization of OSs

• Monolithic vs Layered

Figure 1-8
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Organization of OSs

• Hardware Interface
• Applications and OS compiled into machine
  instructions
• Interrupts and Traps allow OS to seize control
• process management (time-sharing)
• device management (I/O completion)

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Principles of Interupts and Traps
Figure 1-9
12
Organization of OSs

• Hardware interface (continued)
• Modes of CPU execution
• Privileged/Nonprivileged
• SVC (supervisor call) causes trap
• Control transferred to OS in privileged mode
• OS exits privileged mode when returning to user

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Organization of OSs

• Programming Interface
• Invoking system services
• Library call (nonprivileged)
• Kernel call (privileged)

Figure 1-8
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Invoking System Services
Figure 1-10
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Organization of OSs

• User interface (cf. Fig. 1-8)
• Text-based shell (e.g. Unix)
• command interpreter
• shell scripts
• Graphics-based GUI (e.g. Mac, MS Windows)
• Windows
• Icons
• Menus
• Pointer

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Organization of OSs

• Runtime organization
• Service is a Subroutine
• Service is an Autonomous Process
  (client-server)

Figure 1-12
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OS Evolution and Concepts

• Early systems
• Bootstrapping
• Batch OSs
• I/O processors
• Interrupts
• Relocatable code
• Multiprogramming

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Multiprogramming

• Basic problem
• Some programs are compute-bound, some I/O-bound
• Even balanced programs are balance only over
  time
• No one program can make full use of the system
• Solution Multiprogramming
• Have more than one active (running) program in
  memory at any one time
• Multiprogramming requires
• Bridging the semantic gap
• Sharing resources among different programs
• Hiding from each program the fact of this sharing

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OS Evolution and Concepts

• Multiprogramming Systems
• Overlap CPU and I/O
• Protection
• Synchronization and Communication
• Dynamic Memory Management (swapping and
  paging)
• Interactive OSs
• Guaranteed response time
• Time-sharing (quantum)

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Batch Processing

• Uses multiprogramming
• Job (file of OS commands) prepared offline
• Batch of jobs given to OS at one time
• OS processes jobs one-after-the-other
• No human-computer interaction
• OS optimizes resource utilization
• Batch processing (as an option) still used
• today

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Shell Command Line Interpreter
Interactive User
Application System Software
Shell Program
OS System Call Interface
OS
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The Shell Strategy
grep first f3
fork a process
read keyboard
Shell Process
Process to execute command
f3
read file
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Initializing a UNIX Machine
Serial Port A
login
Serial Port B
login
Serial Port C
login
Serial Port Z
login
getty
/etc/passwd
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A Shell Script Batch File

• gcc -g -c menu.c
• gcc -g -o driver driver.c menu.o
• driver lt test_data gt test_out
• lpr Ppr0 test_out
• tar cvf driver_test.tar menu.c driver.c
  test_data test_out
• uuencode driver_test.tar driver_test.tar
  gtdriver_test.encode

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UNIX Files

• UNIX and NT try to make every resource (except
  CPU and RAM) look like a file
• Then can use a common interface

open Specifies file name to be used close
Release file descriptor read Input a block of
information write Output a block of
information lseek Position file for
read/write ioctl Device-specific operations
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UNIX File Example
include ltstdio.hgt include ltfcntl.hgt int
main() int inFile, outFile char
inFileName in_test char outFileName
out_test int len char c inFile
open(inFileName, O_RDONLY) outFile
open(outFileName, O_WRONLY) / Loop through the
input file / while ((len read(inFile, c,
1)) gt 0) write(outFile, c, 1) / Close
files and quite / close(inFile)
close(outFile)
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OS Evolution and Concepts

• PC and workstation OSs
• GUI
• Real-time OSs
• Deadlines (scheduling)
• Distributed OSs
• Loosely coupled/tightly coupled
• Consistent timeline (logical clocks, time stamps)

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Examples of Modern OS

• UNIX variants (e.g. Linux) -- have evolved since
  1970
• Windows NT/2K -- has evolved since 1989
• VxWorks for real-time
• Research OSes still evolving
• Small computer OSes still evolving

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Requirements from a Modern OS

• 1. Micro-kernel structure
• Scheduling
• Networking
• Device Drivers
• Memory Management
• Scheduling
• IP Communication
• Everything else is built as utilities

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Requirements (cont.)

• 2. Multi-threading
• - A process consists of threads
• - Threads run concurrently
• - A thread is schedulable and interruptable
• - User and kernel threads
• 3. Symmetric Multiprocessing
• - More than one processors -gt speed and fault
  tol.
• - Incremental growth is possible

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Requirements (cont.)

• 4. Distributed Operating System
• - Running on many processors (clusters)
• - Vision of one OS
• - Fault Tolerance increased
• - Performance, resource utilization
• - Still a research issue not many around
• - Distributed and Mobile and Real-time ???
• that seems to be the future of OS !

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Processor Modes

• Mode bit Supervisor or User mode
• 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

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Kernels

• The part of the OS critical to correct operation
  (trusted software)
• Executes in supervisor mode
• The trap instruction is used to switch from user
  to supervisor mode, entering the OS

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Supervisor and User Memory
User Space
User Process
Supervisor Process
Supervisor Space
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Procedure Call and Message Passing Operating
Systems
call()
trap
return
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System Call Using the trap Instruction
fork()
Kernel
Trap Table
fork() trap N_SYS_FORK()
sys_fork()
sys_fork() / system function / return
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A Thread Performing a System Call
User Space
Kernel Space
Thread
fork()
sys_fork()
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Basic Operating System Organization
Process, Thread Resource Manager
Processor(s)
Main Memory
Devices
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The UNIX Architecture
Interactive User
Libraries
Commands
Application Programs

OS System Call Interface
Trap Table
Device Driver

• Monolithic Kernel Module
• Process Management
• Memory Management
• File Management
• Device Mgmt Infrastructure

Device Driver

Driver Interface
Device Driver
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Microkernel Organization
Process
Process
Libraries
Process
User
Supervisor
Server
Server
Server
Device Drivers
Microkernel
Processor(s)
Main Memory
Devices
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Windows NT Organization
Process
Process
T
T
Process
T
T
T
T
Libraries
T
T
T
Process Management Memory Management File
Management Device Mgmt Infrastructure
Subsystem
Subsystem
Subsystem
User
Supervisor
I/O Subsystem
NT Executive
NT Kernel
Hardware Abstraction Layer
Processor(s)
Main Memory
Devices
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Abstraction

• E.W.Dijkstra, The Humble Programmer (1972)
  The purpose of abstraction is not to be vague,
  but to create a new semantic levelin which one
  can be absolutely precise.
• abstraction is created by distinguishing
• Essential characteristics fromUnimportant
  details
• Build levels (layers) of abstractions
• What is unimportant detail at one levelis an
  essential characteristic at a lower one.