MODULE-1_Operating System Services - ppt

MODULE-1
Operating System Services
Department of CSE- Data Science

Contents
 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.

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

Operating System Services
 Operating systems provide an environment for execution of programs.
 It provides certain services to programs and to the users of those programs.
 These operating-system services are provided for the convenience of the
programmer, to make the programming task easier.

Fig. A view of 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 interface
(BI).
• 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. Programs need
to read and write files and directories, create and delete them, search them, list file
Information, permission management.

• 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

 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. A chain is only as strong as its weakest link

User Operating-System Interface
 There are several ways for users to interface with the operating system.
1. Command-line interface (CLI), or command interpreter, allows users to
directly enter commands to be performed by the operating system.
2. Graphical user interface (GUI), allows users to interface with the
operating system using pointer device and menu system.

Command-line interface (CLI)
 Command Interpreters are used to give commands to the OS. The main function of the command
interpreter is to get and execute the user-specified command. Many of the commands manipulate
files: create, delete, list, print, copy, execute, and so on.
 The commands can be implemented in two general ways-
1. The command interpreter itself contains the code to execute the command. For example, a
command to delete a file may cause the command interpreter to jump to a particular section
of its code that sets up the parameters and makes the appropriate system call.
2. The code to implement the command is in a function in a separate file. The interpreter
searches for the file and loads it into the memory and executes it by passing the parameter.
Thus by adding new functions new commands can be added easily to the interpreter without
disturbing it.

Graphical User Interfaces (GUI)
 GUI provides a mouse-based window-and-menu system as an interface.
 A GUI provides a desktop metaphor where the mouse is moved to position its
pointer on images, or icons, on the screen (the desktop) that represent programs,
files, directories, and system functions.
 Depending on the mouse pointer's location, clicking a button on the mouse can
invoke a program, select a file or directory—known as a folder— or pull down a
menu that contains commands.

 Many systems now include both CLI and GUI interfaces
‣ Microsoft Windows is GUI with CLI “command” shell
‣ Apple Mac OS X is “Aqua” GUI interface with UNIX kernel underneath
and shells available
‣ Unix and Linux have CLI with optional GUI interfaces (CDE, KDE,
GNOME)

System Calls
 System calls provides an interface to the services of the operating system.
 These are generally written in C or C++
 The figure illustrates the sequence of system calls required to copy a file
content from one file (input file) to another file (output file).

 An example to illustrate how system calls are used: writing a
simple program to read data from one file and copy them to another
file
‣ There are number of system calls used to finish this task. The
first system call is to write a message on the screen (monitor).
Then to accept the input filename. Then another system call to
write message on the screen, then to accept the output
filename.
‣ When the program tries to open the input file, it may find that
there is no file of that name or that the file is protected against
access. In these cases, the program should print a message on
the console (another system call) and then terminate
abnormally (another system call) and create a new one
(another system call).
‣ Now that both the files are opened, we enter a loop that reads
from the input file (another system call) and writes to output
file (another system call).
‣ Finally, after the entire file is copied, the program may close
both files (another system call), write a message to the
console or window (system call), and finally terminate
normally (final system call).

 Most programmers do not use the low-level system calls directly, but instead use an
"Application Programming Interface", API.
 Instead of direct system calls, APIs provides for greater program portability between
different systems. The API then makes the appropriate system calls through the system
call interface, using a system call table to access specific numbered system calls.
 Each system call has a specific numbered system call. The system call table (consisting
of system call number and address of the particular service) invokes a particular service
routine for a specific system call.
 The caller need know nothing about how the system call is implemented or what it
does during execution.

Fig. The handling of a user application invoking the open() system call
 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

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

System Calls - Parameter Passing
 Three general methods used to pass
parameters to OS are –
1. To pass parameters in registers.
2. If parameters are large blocks,
address of block (where parameters
are stored in memory) is sent to OS
in the register. (Linux & Solaris).
3. Parameters can be pushed onto the
stack by program and popped off the
stack by OS.

Types of System calls
 The system calls can be categorized into six major categories:
1. Process Control
2. File management
3. Device management
4. Information management
5. Communications
6. Protection

 Process control
‣ end, abort
‣ load, execute
‣ create process, terminate process
‣ get process attributes, set process attributes
‣ wait for time
‣ wait event, signal event
‣ allocate and free memory
 File management
‣ create file, delete file
‣ open, close file
‣ read, write, reposition
‣ get and set file attributes

 Device management
‣ request device, release device
‣ read, write, reposition
‣ get device attributes, set device attributes
‣ logically attach or detach devices
 Information maintenance
‣ get time or date, set time or date
‣ get system data, set system data
‣ get and set process, file, or device attributes

 Communications
‣ create, delete communication connection
‣ send, receive messages
‣ transfer status information
‣ attach and detach remote devices

MODULE-1_Operating System Services - ppt

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MODULE-1_Operating System Services - ppt