Operating System 24643

7/31/2019 Operating System 24643

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Chapter 2: Operating-System Structures


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2.2 Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 14, 2005

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

Operating System Generation


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OS Views

Interface Components

services

OS


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Operating System Services

One set of operating-system services provides functions that arehelpful to the user:

User interface - Almost all operating systems have a user interface (UI)

Varies between Command-Line (CLI), Graphics User Interface(GUI), Batch

Program execution - The system must be able to load a program intomemory and to run that program, end execution, either normally orabnormally (indicating error)

I/O operations - A running program may require I/O, which mayinvolve a file or an I/O device.

File-system manipulation - The file system is of particular interest.Obviously, programs need to read and write files and directories, createand delete them, search them, list file Information, permissionmanagement.


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Operating System Services (Cont.)

One set of operating-system services provides functions that arehelpful to the user (Cont):

Communications Processes may exchange information, on the samecomputer or between computers over a network

Communications may be via shared memory or through messagepassing (packets moved by the OS)

Error detection OS needs to be constantly aware of possible errors

May occur in the CPU and memory hardware, in I/O devices, in userprogram

For each type of error, OS should take the appropriate action to

ensure correct and consistent computing Debugging facilities can greatly enhance the users and

programmers abilities to efficiently use the system


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Operating System Services (Cont.)

Another set of OS functions exists for ensuring the efficient operation of thesystem itself via resource sharing

Resource allocation - When multiple users or multiple jobs runningconcurrently, resources must be allocated to each of them

Many types of resources - Some (such as CPU cycles,main memory,and file storage) may have special allocation code, others (such as I/Odevices) may have general request and release code.

Accounting - To keep track of which users use how much and what kindsof computer resources

Protection and security - The owners of information stored in a multiuseror networked computer system may want to control use of that information,concurrent processes should not interfere with each other

Protection involves ensuring that all access to system resources is

controlled Security of the system from outsiders requires user authentication,

extends to defending external I/O devices from invalid access attempts

If a system is to be protected and secure, precautions must beinstituted throughout it.


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User Operating System Interface

CLI, GUI, and Batch


8/512.8 Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 14, 2005

User Operating System Interface - CLI

CLI allows direct command entry Sometimes implemented in kernel, sometimes by systems

program

Sometimes multiple flavors implementedshells

Primarily fetches a command from user and executes it

Sometimes commands built-in, sometimes just names ofprograms

If the latter, adding new features doesnt require shell

modification



User Operating System Interface - GUI

User-friendly desktop metaphor interface

Usually mouse, keyboard, and monitor

Icons represent files, programs, actions, etc

Various mouse buttons over objects in the interface causevarious actions (provide information, options, execute function,

open directory (known as a folder) Invented at Xerox PARC

Many systems now include both CLI and GUI interfaces

Microsoft Windows is GUI with CLI command shell

Apple Mac OS X as Aqua GUI interface with UNIX kernel

underneath and shells available

Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)


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System Calls



System Calls

Programming interface to the services provided by the OS

Typically written in a high-level language (C or C++)

Mostly accessed by programs via a high-level Application ProgramInterface (API) rather than direct system call use

Three most common APIs are Win32 API for Windows, POSIX API forPOSIX-based systems (including virtually all versions of UNIX, Linux, and

Mac OS X), and Java API for the Java virtual machine (JVM) Why use APIs rather than system calls?

Portability

System calls are more detailed and difficult to work with.

(Note that the system-call names used throughout this text are generic)



Example of System Calls

System call sequence to copy the contents of one file to anotherfile



Example of Standard API

Consider the ReadFile() function in the

Win32 APIa function for reading from a file

A description of the parameters passed to ReadFile()

HANDLE filethe file to be read

LPVOID buffera buffer where the data will be read into and writtenfrom

DWORD bytesToReadthe number of bytes to be read into the buffer

LPDWORD bytesReadthe number of bytes read during the last read

LPOVERLAPPED ovlindicates if overlapped I/O is being used


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System Calls Implementation



System Call Implementation

Typically, a number associated with each system call

System-call interface maintains a table indexed according tothese numbers

The system call interface invokes intended system call in OS kerneland returns status of the system call and any return values

The caller need know nothing about how the system call isimplemented

Just needs to obey API and understand what OS will do as aresult call

Most details of OS interface hidden from programmer by API

Managed by run-time support library (set of functions builtinto libraries included with compiler)



API System Call OS Relationship



Standard C Library Example

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



System Call Parameter Passing

Often, more information is required than simply identity of desired

system call

Exact type and amount of information vary according to OSand call

Three general methods used to pass parameters to the OS

Simplest: pass the parameters in registers

In some cases, may be more parameters than registers

Parameters stored in a block, or table, in memory, and addressof block passed as a parameter in a register

This approach taken by Linux and Solaris

Parameters placed, or pushed, onto the stackby the program

and poppedoff the stack by the operating system Block and stack methods do not limit the number or length of

parameters being passed



Parameter Passing via Table


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Process control

Load, execute, create process, wait, etc.

Differs between single-tasking and multi-tasking.

File management

Create/delete file, open/close, read/write, etc.

Device management

Read, write, reposition, attach/detach device, etc.

Information maintenance.

Get time/date/process/file, set time/date/process/file, etc.

Communications

Send/receive messages , create/delete communication,etc.

Two models for IPC (interprocess communication):messages-passing and shared-memory.


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System programs


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System Programs

System programs provide a convenient environment forprogram development and execution. They can be dividedinto:

File manipulation

Status information

File modification

Programming language support

Program loading and execution

Communications

Application programs

Most users view of the operation system is defined by

system programs, not the actual system calls


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OS Design and Implementation


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OS structure

Its not always clear how to stitch OS modules together:

MemoryManagement

I/O System

Secondary StorageManagement

File System

Protection System

Accounting System

Process Management

Command Interpreter

Information Services

Error Handling


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Operating System Design and Implementation

Design and Implementation of OS not solvable, but some

approaches have proven successful

Internal structure of different Operating Systems can varywidely

Start by defining goals and specifications

Affected by choice of hardware, type of system

Usergoals and Systemgoals

User goals operating system should be convenient touse, easy to learn, reliable, safe, and fast

System goals operating system should be easy todesign, implement, and maintain, as well as flexible,reliable, error-free, and efficient


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Operating System Design and Implementation (Cont.)

Important principle to separate

Policy: What will be done? E.g timer construct. how longdoes a timer need to be set

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 policydecisions are to be changed later

Implementation: usually using a combination of high-level

and low level programming languages (i.e. Assembly,C/C++).


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OS Structure


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Operating Systems Structure(What is the organizational Principle?)

Simple (i.e. monolithic) Only one or two levels of code

Layered

Lower levels independent of upper levels

Microkernel

OS built from many user-level processes

Modular

Core kernel with Dynamically loadable modules


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Early structure: Monolithic

Traditionally, OSs (like UNIX, DOS) were built as a monolithicentity:

everything

user programs

hardware

OS


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MS-DOS Layer Structure

MS-DOS written to provide the

most functionality in the leastspace

Not divided into modules

Interfaces and levels offunctionality not wellseparated

S S


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UNIX: Also Simple Structure

UNIX limited by hardware functionality

Original UNIX operating system consists of two separable parts:

Systems programs

The kernel

Consists of everything below the system-call interface andabove the physical hardware

Provides the file system, CPU scheduling, memorymanagement, and other operating-system functions;

Many interacting functions for one level


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UNIX System Structure

User Mode

Kernel Mode

Hardware

Applications

Standard Libs


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Monolithic design

Major advantage: cost of module interactions is low (procedure call)

Disadvantages:

hard to understand

hard to modify

unreliable (no isolation between system modules)

hard to maintain

What is the alternative?

find a way to organize the OS in order to simplify its designand implementation


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Layered Approach

The operating system is divided into a number oflayers (levels), each built on top of lower layers.The bottom layer (layer 0), is the hardware; thehighest (layer N) is the user interface.

With modularity, layers are selected such thateach uses functions (operations) and services ofonly lower-level layers

Advantage: modularity Easier debugging/Maintenance

Not always possible: Does process scheduler lie above or

below virtual memory layer?


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Layered Operating System


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Microkernel System Structure

Moves as much from the kernel into user space

Communication takes place between user modules using messagepassing

Benefits:

Easier to extend a microkernel

Easier to port the operating system to new architectures More reliable (less code is running in kernel mode)

More secure

Detriments:

Performance overhead of user space to kernel spacecommunication


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Microkernel vs. Monolithic


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Modules-based Structure

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

Overall, similar to layers but with more flexibility and more efficientthan microkernel


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Virtual Machines


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Virtual Machines

A virtual machinetakes the layered approachto its logical conclusion. It treats hardware andthe operating system kernel as though theywere all hardware

A virtual machine provides an interfaceidenticalto the underlying bare hardware

The operating system creates the illusion ofmultiple processes, each executing on its own

processor with its own (virtual) memory


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Virtual Machines (Cont.)

The resources of the physical computer areshared to create the virtual machines

CPU scheduling can create the appearancethat users have their own processor

Spooling and a file system can provide virtualcard readers and virtual line printers

A normal user time-sharing terminal serves asthe virtual machine operators console


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Virtual Machines Implementation

Virtual-machine software -> kernel mode(VMWare)

Virtual-machine itself -> user mode (Ubuntu)

Inside Virtual-machine itself we have virtual kernel

mode and virtual user mode.


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Virtual Machines (Cont.)

(a) Non-virtual machine (b) virtual machine

Non-virtual MachineVirtual Machine

Virt al Machines (b fit )


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Virtual Machines (benefits)

The virtual-machine concept provides completeprotection of system resources since each virtualmachine is isolated from all other virtual machines.This isolation, however, permits no direct sharing ofresources.

A virtual-machine system is a perfect vehicle foroperating-systems research and development.System development is done on the virtual machine,instead of on a physical machine and so does not

disrupt normal system operation.

The virtual machine concept is difficult to implementdue to the effort required to provide an exactduplicate to the underlying machine


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VMware Architecture


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The Java Virtual Machine


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Operating System Generation

Operating systems are designed to run on any ofa class of machines; the system must beconfigured for each specific computer site

SYSGEN program obtains information concerningthe specific configuration of the hardware system

C


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Conclusion

Rapid Change in Hardware Leads to changing OS

Standard Components and Services

Process Control

Main Memory

I/O

File System

UI

Policy vs Mechanism

Crucial division: not always properly separated!

Complexity is always out of control

R f


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References

Pictures & some slidesProf. Kubiatowicz, Berkeley university,

Prof. Kimura & Zbikowski, WashingtonUniversity

Wikipedia

Content

Text book


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End of Chapter 2

Operating System 24643

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