Operating System Services

Operating System Services

• Operating systems create an environment for programs to execute and provide services to programs and users.
• OS services provide user functions such as user interface (UI), varying between Command-Line (CLI), Graphics User Interface (GUI), and Batch.
• Program execution: systems must load and run programs and end execution normally or with errors.
• I/O operations involve file or I/O device interaction.
• File-system manipulation: programs should be able to read, write, create, delete, search, list file information and manage permissions for files and directories.
• Processes can exchange information either on the same computer or across a network.
• Communications can occur via shared memory or through message passing, where packets are moved by the OS.
• Error detection is a constant OS task for CPU, memory hardware, I/O devices, and user program errors.
• The OS should act appropriately to ensure correct computing for each error.
• Debugging facilities can greatly improve user and programmer system efficiency.
• Another set of OS functions ensure that resource sharing makes the system operate efficiently.
• Resource allocation is required when multiple users or jobs run concurrently for resources like CPU cycles, main memory, and file storage, which may need special allocation code.
• Other resources like I/O devices require general request and release code.
• Accounting tracks how much and what kinds of computer resources are used.
• Protection and security measures are needed to control information use in multi-user or networked systems, preventing interference between concurrent processes.
• Protection ensures that all access to system resources is controlled.
• System security from outsiders involves user authentication and defending against invalid access attempts on external I/O devices.

User Operating System Interface

• Command Line Interface (CLI), or a command interpreter, allows direct command entry.
• The CLI is sometimes implemented in the kernel or as a systems program, and multiple flavors (shells) are available.
• CLI primarily fetches a command from the user and executes it, and commands can be built-in or programs.
• Adding new features does not require shell modification if the commands are programs.
• User-friendly desktop metaphor interfaces use a mouse, keyboard, and monitor
• Icons represent files, programs, actions, etc.
• The interface has objects that when used with the mouse buttons cause actions like providing information, options, executing a function, or opening a directory or folder.
• Xerox PARC invented the GUI, now many systems include both CLI and GUI interfaces.
• Microsoft Windows uses GUI with CLI "command" shell.
• Apple Mac OS X uses an "Aqua" GUI interface with a UNIX kernel underneath with shells.
• Solaris uses a CLI with optional GUI interfaces like Java Desktop (KDE).

System Calls

• System calls are the programming interface for OS-provided services.
• System calls are typically written in a high-level language like C or C++.
• System calls are mostly accessed by programs through a high-level Application Program Interface (API) instead of direct system calls.
• Three common APIs are Win32 API for Windows, POSIX API for Linux, UNIX, and Mac OS X, and Java API for the Java virtual machine (JVM).
• A system call sequence copies the contents of one file to another.
• Acquiring input file name, writing prompts to the screen and accepting input are all actions taken within this system.
• The system opens the input file, aborting if the file does not exist, and creates an output file, aborting if the file already exists.
• In a loop, the system reads from the input file, writes to the output file until reading fails, and then closes the output file.
• Finally, the system call sequence writes a completion message to the screen and terminates normally.

Operating systems offer an environment for program execution and provide services to programs and users. Key services include user interface management (CLI, GUI, Batch), program execution, I/O operations, and file system manipulation. Additionally, processes facilitate inter-process communication across networks, error detection, and debugging facilities.