Operating Systems Overview

Study Notes

The operating system provides services to control and manage computer resources for efficient use, protection, and security.
Protection involves ensuring controlled access to system resources.
Security safeguards the system from unauthorized external access through user authentication and protection of external input/output devices.

User-friendly desktop metaphor, often involving the mouse, keyboard, and monitor.
Icons represent files, programs, actions, and other entities.
The mouse interacts with objects in GUI for various actions like providing information, options, executing functions, and navigating directories.
Invented at Xerox PARC.
Many systems include both CLI and GUI interfaces.

Provide programming interface to OS services.
System calls are usually implemented as functions, typically written in a high-level language like C or C++.
For lower-level access, they are implemented in Assembly Language.

Used by application developers for seamless interactions with the operating system.
Provides a set of functions with specific parameters and return values.
Examples of common APIs include Win32 API for Windows, POSIX API for POSIX-based systems, and Java API for the Java Virtual Machine.
Often involves behind-the-scenes calls to system calls.

Program portability: Applications can be compiled and run on any system supporting the same API.
Ease of programming: Actual system calls offer greater detail than the API, making programming straightforward.

System programs provide a convenient environment for program development and execution.
File manipulation: Creating, deleting, copying, renaming, printing, dumping, listing, and manipulating files and directories.
Status information: Obtaining system information like date, time, available memory, disk space, and number of users.
Programming language support: Offering compilers, assemblers, debuggers, and interpreters.
Program loading and execution: Providing absolute loaders, relocatable loaders, linkage editors, and overlay-loaders.
Communications: Establishing virtual connections and facilitating messaging, browsing, emailing, remote login, and file transferring.
Background Services: Launching at boot time and providing facilities like disk checking, process scheduling, error logging, and printing. These services operate in user context. - Also known as services, subsystems, or daemons.
Application programs: User-run programs that are not typically considered part of the OS.

Source code is compiled into relocatable object files, which can be loaded into any physical memory location.
The linker combines these object files into a single executable file, including necessary libraries.
The loader brings the executable file from secondary storage into memory for execution.
Relocation assigns final addresses to program parts and adjusts code and data accordingly.
Dynamically linked libraries (DLLs in Windows) are loaded only when needed and shared by all users utilizing the same library version.

Applications compiled on one operating system are often not executable on another due to differences in system calls, file formats, and other OS-specific features.