Chapter 2: System Structures
Operating System Concepts – 8th Edition,
Silberschatz, Galvin and Gagne ©2009
Chapter 2: 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 Debugging
 Operating System Generation
 System Boot
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Objectives
 To describe the services an operating system provides to users, processes,
and other systems
 To discuss the various ways of structuring an operating system
 To explain how operating systems are installed and customized and how
they boot
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Operating System Services
 One set of operating-system services provides functions that are
helpful 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 into
memory and to run that program, end execution, either normally or
abnormally (indicating error)

I/O operations - A running program may require I/O, which may involve
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, create
and delete them, search them, list file Information, permission
management.
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A View of Operating System Services
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Operating System Services (Cont)
 One set of operating-system services provides functions that are
helpful to the user (Cont):

Communications – Processes may exchange information, on the same
computer or between computers over a network


Communications may be via shared memory or through message
passing (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 user
program

For each type of error, OS should take the appropriate action to
ensure correct and consistent computing

Debugging facilities can greatly enhance the user’s and
programmer’s 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 the
system itself via resource sharing

Resource allocation - When multiple users or multiple jobs running
concurrently, 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/O
devices) may have general request and release code

Accounting - To keep track of which users use how much and what kinds
of computer resources

Protection and security - The owners of information stored in a multiuser
or 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 be
instituted throughout it.
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User Operating System Interface - CLI
Command Line Interface (CLI) or command interpreter allows direct command
entry

Sometimes implemented in kernel, sometimes by systems program

Sometimes multiple flavors implemented – shells

Primarily fetches a command from user and executes it, commands
can be implemented in two ways:
–
Commands built-in, number of commands determines the size
of command interpreter since each command has its own code.
–
Command interpreter does not understand command, it uses
only the command to identify a file to be loaded into memory
and executed.
–
Example rm file.txt  search for file rm, load it into memory and
execute it with the parameter file.txt
»
If the latter, adding new features doesn’t require shell
modification. Command interpreter can be small.
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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 cause various
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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Bourne Shell Command Interpreter
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The Mac OS X GUI
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System Calls
 Programming interface to the services provided by the OS
 Typically written in a high-level language (C or C++) “routines” some low
level tasks may need to be written in assembly language.
(Note that the system-call names used throughout this text are generic)
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System Calls

Example of a system call: writing a program to read data from a file and copy
them to another file:

Need to know two files names, requires a sequence of system call

A prompt message, read characters from keyboard.

Define two files

other option can have a mouse which requires another system calls

Open the input file and create the output file (system calls)

Possible errors:

input file does not exist or has protection: Print a message on the
console to user (system calls) then terminate abnormally (system call)

Output file: exist  may cause abort (a system call) and create new one
(a system call) in interactive system: may ask to replace or abort the
program (system calls)
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System Calls

Both files are ready:

We enter a loop read from input file (system call) and write to output
file (system call)

After entire file is copied:
–

program may close both files (system call)
–
Write a message to the console or window (more system calls)
–
Terminate normally (final system calls)
Usually system executes thousands of system calls per second see
Figure (2.4)
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System Calls
 Mostly accessed by programs via a high-level Application Program Interface
(API) rather than direct system call use
 Three most common APIs are Win32 API for Windows, POSIX API for
POSIX-based systems (including virtually all versions of UNIX, Linux, and
Mac OS X), and Java API for the Java virtual machine (JVM)
 So functions (made from API) invoke the actual system calls on behalf of
the application programmers
 Why use APIs rather than system calls?

An application programmer designing a program using an API can
expect her program to compile and run on any system that supports the
same API. ( works on different systems)

Actual system calls can often be more detailed and difficult to work with
than the API available to an application programmer.
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Example of 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 the

Win32 API—a function for reading from a file

A description of the parameters passed to ReadFile()

HANDLE file—the file to be read

LPVOID buffer—a buffer where the data will be read into and written from

DWORD bytesToRead—the number of bytes to be read into the buffer

LPDWORD bytesRead—the number of bytes read during the last read

LPOVERLAPPED ovl—indicates if overlapped I/O is being used
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System Call Implementation
 Typically, a number associated with each system call

System-call interface maintains a table indexed according to these
numbers
 The system call interface invokes intended system call in OS kernel and
returns status of the system call and any return values
 The caller need know nothing about how the system call is implemented

Just needs to obey API and understand what OS will do as a result call

Most details of OS interface hidden from programmer by API

Managed by run-time support library (set of functions built into
libraries included with compiler)
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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, with system call, more information is required than simply
identity of desired system call

Exact type and amount of information vary according to OS and call
 For example to get input we may need:
Specify the fie or the device as the source
 Address and length of memory buffer into in which the input should
be read.

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System Call Parameter Passing
 Three general methods used to pass parameters to the OS
1.
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 address of block
passed as a parameter in a register
 This approach taken by Linux and Solaris
3. Parameters placed, or pushed, onto the stack by the program and
popped off the stack by the operating system
 Block and stack methods do not limit the number or length of
parameters being passed
2.
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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
 Protection
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Examples of Windows and Unix System Calls
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Process control

end / abort

A running program needs to be able to halt its execution either normally
(end) or abnormally (abort)
 Load / execute

A process or job executing one program may want to load and execute
another program
 Create process / terminate a process

Process may need to create a new process and terminate it.
 Get process attributes/ set process attributes

If we create new job or process, we need to control the execution.

This control requires the ability to determine and reset the attributes of a
job or process, including the process’s priority, time execution and so
on.
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Process control

wait for time/ wait event/ signal event

Having created new processes, we may need to wait for them to finish
their execution, we may want to wait for a certain amount of time to
pass (wait time)

We will want to wait for a specific event to occur (wait event)

The process should then signal when the event has occurred (signal
event)
 Lock / release lock

When two or more process share data, OS provides system calls allow
a process to lock shared data to prevent another process from
accessing the data while its locked. Release lock, remove prevention
from locked data.

Such system calls lock/ release lock used in concurrent process
execution
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Process control
 Variations of process control:


Single-tasking system

MS-Dos

Execute only one process

Process cannot create another process
Multi-tasking system

FreeBSD (derived from Berkeley UNIX)

It accept commands from user and return back to command line

To run another process, shell execute fork() system call, then the
selected program loaded into memory via exec() system call

When process is done, it executes an exit() system call to terminate
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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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File Management
 Create file/ delete file

Need the files name and perhaps their attributes
 Open/ close/ read/ write/ reposition

Once created, we need to open it and use it

We may also need to read, write, reposition (rewinding or skipping to
the end of the file)

We need to close the file

Same as the directories
 Get file attributes/ set file attributes

Attributes such as file name, file type, protection code, accounting
information and so on.
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Device Management
 A process may need several resources to execute.
 Resources  devices  physical (disk drives) virtual devices (files)
 Request device/ release device

When program needs to use some resources.

Once it done, it release it

Functions are similar to open and close system calls for files
 Read/ write/ reposition device

Same as files
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Information Maintenace
 Many system calls exist to transfer information between user program and
OS
 Get time/ get date
 Other system may return other information such as number of users, OS
version, free memory and so on
 OS keep information about all processes and system calls are used to
access this information.

Get (process/ file / device) attributes

Set (process/ file / device) attributes
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Communication
 Two common models of interprocesses communication

Message-passing model

Shared-memory model
 Message-passing model

Use messages between processes to transfer information

Messages exchanged through a common mail-box

First: a connection must be opened, the name of other communicator
muse be known

Can be a process in the same system or other computer in the
network

Each computer has a host name, and a network identifier (IP
address)

Each process has a process name.
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Communication
Message-passing model –continued
 The get hostid and get processid system calls (from process name to
process identifier) do this translation
 The identifiers then passed to:

General-purpose open and close calls provided by file system or

Specific open connection and close connection system calls
depending on the system model of communication.
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Communication
Message-passing model –continued
 The recipient process must give its permission for communication to take
place with an accept connection call.

Most processes receiving connection are special-purpose daemons.

They execute a wait for connection call and are awakened when the
connection is made.

Source process known as client and receiving daemon known as
server.

Then exchange message by using read message and write message
system calls

The close connection call terminates the communication
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Communication
Shared memory model

Processes use shared memory create and shared memory attach system calls to
create and gain access to regions of memory owned by other process.

Once permission granted, processes can exchange information by reading and
writing data in the shared memory.

This form of data is controlled by the process not OS.

Processes ensure that they are not using the same location of memory
simultaneously

Comparisons:

Both models are used, the first one is useful for smaller amount of data, and
easier to implement than shared memory model for inter-computer
communication.

Shared-memory model allow maximum speed since it is done at the memory
transfer speed. Problems (protection –synchronization between processes
sharing the memory)
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Protection
 Protection provide a mechanism for controlling access to the resources of
the system
 With the advent of networking and Internet, protection becomes more
important.
 Typical system calls for protection include

Set permission

Get permission
 Which manipulate the permission settings of resources such as files and
disks.

Allow user and Deny user system calls decide what user has the right
to access certain resources.
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End of Chapter 2
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