Operating System Services Overview

### Operating System Services

• Operating systems provide environments for program execution, and services for users and programs.
• User Interface Different Operating Systems provide different user interfaces:
  • Command-Line Interface (CLI) : Keyboard input for commands
  • Graphics User Interface (GUI): Most commonly used interface
  • Touchscreen Interface: Used by mobile devices
  • Combination of all the above interfaces: Some OS provide multiple UI variations.
• Program execution:
  • Operating systems can load programs into memory and then execute them.
  • Processes can be ended by the Operating System normally as well as in the case of errors or abnormal conditions.
• I/O operations:
  • Programs can access I/O devices via Operating System services.
  • For efficiency and protection, users are prevented from using devices directly.
• File System Manipulation: The file system provides services to:
  • Read and write files and directories
  • Create and delete files and directories with the help of file names
  • Search files based on a given name
  • List file information
  • Manage file permissions based on file ownership
• Communications: Processes can exchange information with each other.
  • Processes on the same or different machines can communicate with each other via a network.
  • The Operating System facilitates communication through shared memory, where two or more processes can read and write to a shared section of memory.
  • Processes can also use message passing where information packets are exchanged.
• Error detection: The operating system constantly monitors for errors and attempts to correct them.
  • The operating system tracks memory errors, power failures, I/O device errors and user program errors.
  • The operating system responds to these errors by either attempting to correct them, terminating a process, or returning an error code to the process.
  • Debugging facilities assist users and programmers in using the system efficiently.
• The Operating System also provides functions for efficient system operation and resource sharing between processes:
  • Resource Allocation: When multiple processes are running concurrently, resources (e.g., CPU cycles, memory, file storage, I/O devices) need to be allocated to each process.
  • Logging: The operating system keeps track of user activity and resource usage for accounting and system configuration purposes.
  • Protection and Security: To protect information and the system itself, the operating system controls access to resources and security measures.
    • Protection ensures controlled access to system resources.
    • Security measures are taken to defend the system from external threats and unauthorized I/O device access.

User and Operating System Interface

• Command-Line Interface (CLI): The user interacts with the system by typing text commands in a specific format. The operating system responds to the commands.
• Graphics User Interface (GUI): Users interact with the system using graphical elements like windows, icons, and menus.
• Graphical User Interfaces (GUIs):
  • GUI use a mouse or a keyboard to interact with the system.
  • They are more user-friendly and intuitive for the average user.
  • They simplify system operations.
  • GUIs are easier to use than command-line interfaces for many users.
• Command-Line Interface (CLI):
  • CLI provide an efficient way to access system resources, and are commonly used by developers for their programmability.
  • They are more difficult to use for the average user.
  • CLIs make repetitive tasks easier because of their own programmability.
  • They can automate common tasks by storing command sequences in files.

System Calls

• System calls provide the programming interface to the OS services.
• They are generally written in high-level languages like C or C+.
• Low-level tasks that require hardware access may be written in assembly language.
• The OS uses system calls to manage its resources and operations.
• Programs frequently execute thousands of system calls every second.
• Instead of directly using system calls, programs usually make calls through a high-level Application Programming Interface (API).
• Each Operating System names its system calls differently.

Application Programming Interface (API)

• Applications are built according to an API, which specifies the functions and data structures that an application can use.
• The API provides the programmer with a set of functions, their parameters and expected return values.
  • Some popular APIs: Win32 API for Windows, POSIX API for POSIX-based systems (like UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM). The operating system provides a library of functions that implement the API.
• Applications access the API by calling functions in the library. Windows provides the API as a set of C functions, while UNIX and Linux use libc for C programs.
• While the API functions are invoked by applications, these functions usually call the system calls as well.