An operating system is a program that manages computer hardware and works as an intermediary between the user and the hardware. It gives the base on which application programs run.
The OS has two views: user view and system view. In user view, it makes interaction easy; in system view, it manages resources like CPU, memory, and I/O devices. Example: a PC focuses on easy use, while a mainframe focuses on resource sharing.
The OS controls and coordinates hardware resources among users and programs. It allocates CPU time, memory space, file-storage space, and I/O devices when many jobs run at the same time.
The OS aims to make the system easy to use and to run the computer efficiently by managing resources, protecting them, and supporting execution of programs. This is reflected in the listed OS services such as resource allocation, protection, and program execution.
The OS functions include program execution, I/O operations, file-system manipulation, communication, error detection, resource allocation, accounting, and protection. Example: it loads a program into memory and also handles file read/write operations.
OS evolved from serial processing systems to batch systems, then to multiprogramming systems, time-sharing systems, distributed systems, and real-time systems. The file also shows the growth from first generation to fourth generation operating systems.
Batch OS executes jobs in batches. Multiprogramming keeps several jobs in memory. Time-sharing/multitasking switches CPU quickly among users. Multiprocessing uses two or more processors. Distributed and network OS share resources across connected computers. Real-time OS works with strict timing, and it is of two types: hard real-time and soft real-time.
Batch OS: jobs are grouped and executed together.
Multiprogramming OS: keeps a job ready so CPU is not idle.
Time-sharing OS: many users share the CPU with quick switching.
Multiprocessing OS: has two or more processors working together.
Distributed OS: multiple computers cooperate through a network.
Network OS: provides file sharing and message exchange across computers.
Real-Time OS: works under strict deadlines; hard real-time is for critical tasks, soft real-time is less strict.
The file mentions layered approach and microkernel as OS structures. In layered structure, each layer uses services of the lower layer. In microkernel structure, only the minimal functions stay in the kernel and other services run in user space.
The kernel is the core part of the operating system. In monolithic architecture, file management, memory management, device management, and process management are directly controlled inside the kernel.
The file explains this mainly as user space and kernel space. User programs and services run in user space, while the kernel runs in kernel space and handles core OS work. They communicate through IPC/message passing. Example: a file request goes through the file server and microkernel, not directly.
OS services make programming easier and provide an execution environment. They include program execution, I/O operations, file-system manipulation, communication, error detection, resource allocation, accounting, and protection.
A system call is a request made by a program to the OS kernel for a service. It is the main interface through which programs interact with the operating system. Example: a program uses system calls to create a process or read a file.
The common categories are process control, file management, device management, information maintenance, and communication. Example: create, terminate, open, close, read, write, send, and receive are system-call examples.
Monolithic architecture: all major services run inside kernel space; it is fast, but if one service fails, the whole system may fail.
Layered architecture: OS is divided into layers, and each layer uses only lower layers.
Microkernel architecture: only minimum services remain in the kernel; other services run in user space, giving better security and reliability.
Hybrid architecture: the file does not give a direct definition, but it shows Windows NT moving from a layered microkernel style toward a more monolithic design, which supports the idea of a mixed or hybrid structure.
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