Chapter 2: Operating System Structures¶

System Components of an Operating System¶

When discussing the system components of an operating system (OS), we are referring to the major parts that manage computer hardware and software.

Kernel¶

The core of the operating system.
Manages communication between hardware and software.
Responsibilities:
- Process management (creating, scheduling, terminating processes)
- Memory management (allocating and freeing RAM)
- Device management (drivers for I/O devices)
- System calls (interface between programs and OS functions)

Process Management¶

Handles the creation, scheduling, and termination of processes.
Ensures CPU time is distributed fairly and efficiently.
Supports multitasking, synchronization, and deadlock handling.

Memory Management¶

Controls and coordinates computer memory.
Allocates space to programs and data.
Handles virtual memory, paging, and segmentation.

File System Management¶

Organizes and manages data on storage devices.
Provides ways to create, read, write, and delete files.
Handles file permissions and directories.

Device Management¶

Controls input/output devices (keyboard, mouse, disk drives, printers).
Uses device drivers to translate OS commands into hardware actions.
Manages device communication and buffering.

I/O System¶

Provides a uniform interface for input/output operations.
Shields programs from low-level hardware complexities.

Security and Protection¶

Ensures only authorized users and programs can access system resources.
Handles authentication, access control, and encryption.

User Interface (UI)¶

Provides interaction between the user and the OS.
Types:
- Command-Line Interface (CLI) – e.g., Linux terminal, Windows PowerShell
- Graphical User Interface (GUI) – e.g., Windows, macOS, GNOME

Summary¶

An operating system’s system components work together to manage hardware (CPU, memory, devices), provide essential services (processes, file system, I/O), and allow users and programs to interact safely and efficiently.

Operating System Services¶

An operating system provides a set of essential services that make using the computer convenient, efficient, and secure. These services act as an interface between the user, application programs, and hardware.

Program Execution¶

Loads programs into memory and runs them.
Handles program termination (normal or abnormal).

I/O Operations¶

Manages input/output with devices such as keyboard, mouse, printer, and storage.
Provides uniform interfaces for device communication.

File System Manipulation¶

Allows programs and users to create, read, write, and delete files.
Provides access control and directory management.

Communication¶

Enables processes to exchange data and information.
Supports:
- Shared memory
- Message passing (between processes, or over a network)

Error Detection¶

Monitors system activity for errors (hardware or software).
Ensures proper action is taken (log, recovery, or graceful shutdown).

Resource Allocation¶

Distributes system resources (CPU, memory, I/O devices) among multiple users and processes.
Uses scheduling and priority policies to optimize performance.

Protection and Security¶

Protects data and resources from unauthorized access.
Provides authentication, permissions, and encryption.

Summary¶

Operating system services ensure that applications can execute smoothly, resources are used efficiently, and the system remains stable and secure.

System Calls in Operating Systems¶

A system call is the mechanism that allows a program (running in user mode) to request a service from the operating system’s kernel (privileged mode). They act as the interface between user programs and the OS, enabling controlled access to hardware and low-level resources.

Main Categories of System Calls¶

Process Control¶

Create, execute, and terminate processes.
Examples: fork(), exec(), exit(), wait()
Functions: load programs into memory, manage execution, and handle errors.

File Management¶

Operations on files and directories.
Examples: open(), read(), write(), close(), unlink()
Functions: create, delete, read/write, reposition file pointer, set/get file attributes.

Device Management¶

Request and release devices.
Examples: ioctl(), read(), write()
Functions: get device attributes, perform I/O operations, buffer management.

Information Maintenance¶

Retrieve or set system data.
Examples: getpid(), alarm(), time()
Functions: system time/date, process information, system data settings.

Communication¶

Exchange information between processes.
Examples: pipe(), shmget(), mmap(), send(), recv()
Functions:
- Shared memory
- Message passing
- Socket communication

Protection¶

Control access to resources.
Examples: chmod(), umask(), setuid()
Functions: provide authentication, permissions, and ownership management.

How System Calls Work¶

A user program makes a request (e.g., to read a file).
The system call instruction switches the CPU from user mode to kernel mode.
The OS kernel executes the requested operation.
Control is returned to the user program.

Summary¶

System calls are the bridge between user programs and the operating system kernel, allowing programs to safely and efficiently access system resources.

Layered and Microkernel Architectures¶

This section explains two important operating system architectures: Layered Architecture and Microkernel Architecture.

Layered Architecture¶

The operating system is divided into layers (levels), each built on top of the lower one.
Each layer only interacts with the one directly beneath it.
Higher layers use services provided by lower layers.

Characteristics¶

Easy to design and implement.
Provides modularity, making it easier to test and debug.
Each layer hides the details of lower layers.

Disadvantages¶

Rigid structure; modification requires changes across multiple layers.
Performance overhead due to multiple layers of communication.

Examples¶

THE Operating System (early layered OS).
Modern OSes often have layered influences.

Microkernel Architecture¶

Instead of a large monolithic kernel, the OS keeps only minimal essential functions in the kernel.
Other services (file system, device drivers, networking, etc.) run in user space as separate processes.

Kernel Responsibilities (Minimal)¶

Inter-process communication (IPC)
Basic process management
Basic memory management

Advantages¶

High reliability (a failure in one service does not crash the whole system).
Easier to extend and maintain.
Improved security (less code runs in privileged mode).

Disadvantages¶

Performance overhead due to frequent IPC (context switching between kernel and user processes).

Examples¶

Mach (basis of macOS and iOS kernel, XNU hybrid)
Minix
QNX

Comparison¶

Aspect	Layered Architecture	Microkernel Architecture
Structure	OS divided into layers	Minimal kernel, services in user space
Modularity	High (layers hide details)	Very high (services isolated)
Reliability	Moderate	High (failures isolated)
Performance	Can suffer due to extra layers	Can suffer due to IPC overhead
Examples	THE OS, layered designs	Mach, Minix, QNX

Summary¶

Layered architecture organizes the OS into levels of abstraction.
Microkernel architecture minimizes kernel size, moving most services to user space.
The trade-off lies between modularity and reliability versus performance efficiency.