Chapter 2: Operating System Structures

CS330 - Operating Systems

Understanding OS Services, Interfaces, and Architecture

2.1 Operating System Services

Operating systems provide an environment for execution of programs and services to programs and users. Services are divided into those that are helpful to the user and those that ensure efficient operation of the system.

Services for Users:

🖥️ User Interface (UI)

Almost all operating systems have a user interface. Varies between Command-Line Interface (CLI), Graphical User Interface (GUI), or both.

📂 Program Execution

System capability to load a program into memory and run it. OS able to end execution either normally or abnormally.

💾 I/O Operations

Since user programs cannot execute I/O operations directly, the OS must provide means to perform I/O.

📁 File System Manipulation

Ability to read, write, create, open, close and delete files/directories.

🔗 Communications

Exchange of information between processes on the same computer or different systems via shared memory or message passing.

⚠️ Error Detection

Ensure correct computing by detecting errors in CPU, memory hardware, I/O devices, or user programs.

Services for System Efficiency:

Resource Allocation: When multiple users or jobs run concurrently, resources must be allocated to each
Accounting: Keep track of which users use how much and what kinds of resources
Protection and Security:
- Protection - ensures all access to system resources is controlled
- Security - defends system from external and internal attacks

📊 A View of Operating System Services

Shows the relationship between User, OS Services, and System

2.2 User Operating System Interface

Command Line Interface (CLI)

Direct command entry
Implemented in kernel or system program
Multiple shells (bash, zsh, cmd)
Commands can be built-in or program names
Examples: Linux terminal, Windows cmd

Graphical User Interface (GUI)

User-friendly desktop metaphor
Mouse, keyboard, monitor interaction
Icons, windows, menus
Invented at Xerox PARC (1970s)
Examples: Windows, macOS Aqua, GNOME

Aspect	CLI	GUI
Speed	Faster for experienced users	Slower but more intuitive
Resource Usage	Low memory/CPU usage	Higher resource requirements
Learning Curve	Steep - must memorize commands	Gentle - visual and intuitive
Automation	Easy to script and automate	Harder to automate tasks
Precision	Very precise control	Less precise, more general

Modern OS Examples:

Windows: GUI with CLI "command" shell
macOS: Aqua GUI with Unix kernel and shells available
Linux: CLI with optional GUI (KDE, GNOME, XFCE)

2.3 System Calls

System calls provide an interface between a running program and the operating system. They are generally available as assembly language instructions and accessed through high-level language API.

System Call Mechanism:

User Program

→

API Call

→

System Call Interface

→

Kernel Mode

→

OS Service

📊 API - System Call - OS Relationship

Shows transition from user mode to kernel mode

Types of System Calls:

Category	Functions	Examples
Process Control	end, abort, load, execute, create/terminate process, get/set attributes, wait	fork(), exit(), wait(), exec()
File Management	create, delete, open, close, read, write, reposition, get/set attributes	open(), close(), read(), write(), lseek()
Device Management	request/release device, read, write, reposition, get/set attributes	ioctl(), read(), write()
Information Maintenance	get/set time or date, get/set system data, get/set process attributes	getpid(), alarm(), sleep()
Communications	create/delete connection, send/receive messages, transfer status	pipe(), shmget(), socket()
Protection	get/set permissions, allow/deny access	chmod(), umask(), chown()

                // Example: System calls in C (Linux)
                #include <stdio.h>
                #include <unistd.h>
                #include <sys/types.h>
                #include <sys/wait.h>

                int main() {
                pid_t pid;

                // fork() system call - Process Control
                pid = fork();

                if (pid == 0) {
                // Child process
                printf("Child Process: PID = %d\n", getpid());

                // exec() system call - replaces process image
                execlp("/bin/ls", "ls", "-l", NULL);

                } else if (pid > 0) {
                // Parent process
                printf("Parent Process: PID = %d\n", getpid());

                // wait() system call - wait for child
                wait(NULL);
                printf("Child completed\n");
                }

                return 0;
                }
            

2.4 Dual-Mode Operation

Dual-mode operation allows OS to protect itself and other system components. The hardware provides a mode bit to distinguish between user code and kernel code execution.

User Mode (1)

✓ Executing user applications
✓ Limited CPU instructions
✗ No direct hardware access
✗ No privileged instructions

⇄

Kernel Mode (0)

✓ Executing OS code
✓ All CPU instructions
✓ Direct hardware access
✓ Privileged instructions

⚠️ Important for Exams:

Mode bit: 0 = Kernel Mode, 1 = User Mode
System call changes mode from user to kernel
Return from system call resets to user mode
Privileged instructions only executable in kernel mode

📝 Practice Question:

Q: What happens when a user program tries to execute a privileged instruction in user mode?

a) The instruction executes normally
b) The system switches to kernel mode automatically
c) A trap is generated and the OS handles the exception
d) The instruction is ignored

2.5 Operating System Structure

General-purpose OS is a very large program. Various ways exist to structure operating systems, each with advantages and disadvantages.

1. Simple Structure (Monolithic)

Example: MS-DOS, Original UNIX

No well-defined structure
Minimal layers
Direct hardware access from applications
Fast but not secure or reliable

2. Layered Approach

Layer N: User Interface

Layer N-1: User Programs

Layer 3: I/O Management

Layer 2: Memory Management

Layer 1: Process Management

Layer 0: Hardware

Characteristics:

OS divided into layers (levels)
Each layer built on top of lower layers
Layer 0 is hardware, Layer N is user interface
Each layer uses only lower-level layer services
Modularity with specific and limited functionality

3. Microkernel System Structure

Application Program

File System

Device Driver

Microkernel
(IPC, Memory Mgmt, CPU Scheduling)

Microkernel Characteristics:

Removes all nonessential components from kernel
Implements removed components as user-level programs
Communication via message passing
Benefits:
- Easier to extend OS
- Easier to port to different hardware
- More reliable (less code in kernel mode)
- More secure (services run as user processes)
Example: Mach, QNX

4. Modular Structure (Loadable Kernel Modules)

Modular OS structure with loadable kernel modules

Module System Features:

Kernel has core components - Only essential services in base kernel
Additional services via modules - Loaded at boot time or dynamically at runtime
Object-oriented approach - Each component is separate and encapsulated
Components communicate over known interfaces - Well-defined APIs between modules
Loadable as needed within kernel - Can be added or removed without rebooting
More flexible than layered structure - Any module can call any other module
Examples: Linux, macOS, Solaris, Windows

5. Hybrid Systems

Most modern operating systems are not one pure model but combine multiple approaches:

Linux: Monolithic plus modular design
Windows: Mostly monolithic, plus microkernel for subsystems
macOS: Hybrid with Mach microkernel + BSD subsystems

Structure	Advantages	Disadvantages	Examples
Simple/Monolithic	Fast, efficient	Hard to maintain, not secure	MS-DOS, early UNIX
Layered	Modular, easy to debug	Performance overhead, hard to define layers	THE, OS/2
Microkernel	Reliable, secure, portable	Performance overhead from messaging	Mach, QNX, Minix
Modular	Flexible, efficient, maintainable	Complexity in module interfaces	Linux, Solaris, Windows

2.6 System Programs

System programs provide a convenient environment for program development and execution. They can be divided into several categories.

📁 File Management

Create, delete, copy, rename, print, dump, list files and directories

ℹ️ Status Information

Date, time, memory usage, disk space, number of users, performance

✏️ File Modification

Text editors to create and modify files, search contents

🔧 Programming Support

Compilers, assemblers, debuggers, interpreters

📚 Program Loading

Loaders, linkers, and execution utilities

💬 Communications

Email, web browsers, remote login, file transfer

2.7 System Boot

The process of starting a computer by loading the kernel is known as booting the system.

Boot Process Steps:

1. Power On

→

2. BIOS/UEFI

→

3. Bootstrap Loader

→

4. Kernel Load

→

5. Init Process

Power-on: Execution starts at predefined memory location (ROM)
Firmware (BIOS/UEFI):
- Initializes hardware
- Runs diagnostics (POST)
- Loads bootstrap loader
Bootstrap Loader:
- Located in boot block/partition
- Loads kernel into memory
- May load secondary bootstrap (GRUB)
Kernel Initialization:
- Initialize data structures
- Start system daemons
- Mount root file system
System Ready: Login prompt or GUI appears

2.8 Exam-Style Practice Questions

Multiple Choice Questions

Q1. Which of the following is NOT a benefit of a microkernel architecture?

a) Easier to extend the OS
b) More reliable and secure
c) Better performance than monolithic kernels
d) Easier to port to different hardware

Q2. In a layered operating system structure, each layer can only use services of:

a) Higher-level layers
b) Lower-level layers
c) Any other layer
d) Adjacent layers only

Q3. Which system call category would chmod() belong to?

a) Process Control
b) File Management
c) Device Management
d) Protection

Q4. When a user program makes a system call, the mode bit changes from:

a) 0 to 1
b) 1 to 0
c) Remains at 0
d) Remains at 1

Q5. Which of the following was invented at Xerox PARC?

a) Command Line Interface
b) Graphical User Interface
c) System Calls
d) Microkernel Architecture

Short Answer Questions

Q6. Explain the difference between user mode and kernel mode. Why is this distinction important? (5 marks)

Answer Key:

User Mode: Restricted mode where user applications run. Limited access to system resources and cannot execute privileged instructions.
Kernel Mode: Unrestricted mode where OS kernel runs. Full access to hardware and can execute all CPU instructions.
Importance:
- Protection: Prevents user programs from damaging the system
- Security: Restricts access to critical resources
- Stability: Isolates user program crashes from system

Q7. Compare and contrast monolithic kernel and microkernel architectures. Give one advantage and disadvantage of each. (8 marks)

Answer Key:

Aspect	Monolithic Kernel	Microkernel
Structure	All OS services in kernel space	Minimal kernel, services in user space
Advantage	Fast performance (no message passing)	More reliable and secure
Disadvantage	Hard to maintain, less secure	Performance overhead from IPC
Example	Linux, Unix	Mach, QNX

Q8. List and briefly describe five types of system calls with one example each. (10 marks)

Answer Key:

Process Control: Manage processes - Example: fork()
File Management: Handle files - Example: open()
Device Management: Control devices - Example: ioctl()
Information Maintenance: Get/set system info - Example: getpid()
Communications: IPC and networking - Example: socket()

2.9 Key Terms and Definitions

Term	Definition
System Call	Interface between running program and OS kernel
API	Application Programming Interface - set of functions available to programmers
Shell	Command interpreter that provides user interface to OS
Kernel Module	Loadable kernel component that extends OS functionality
Mode Bit	Hardware bit distinguishing user mode (1) from kernel mode (0)
Privileged Instruction	CPU instruction only executable in kernel mode
Bootstrap Loader	Program that loads OS kernel during system boot
IPC	Inter-Process Communication - message passing between processes
System Program	Program that provides environment for program development/execution

2.1 Operating System Services

Services for Users:

🖥️ User Interface (UI)

📂 Program Execution

💾 I/O Operations

📁 File System Manipulation

🔗 Communications

⚠️ Error Detection

Services for System Efficiency:

📊 A View of Operating System Services

2.2 User Operating System Interface

Command Line Interface (CLI)

Graphical User Interface (GUI)

Modern OS Examples:

2.3 System Calls

System Call Mechanism:

📊 API - System Call - OS Relationship

Types of System Calls:

2.4 Dual-Mode Operation

User Mode (1)

Kernel Mode (0)

⚠️ Important for Exams:

📝 Practice Question:

2.5 Operating System Structure

1. Simple Structure (Monolithic)

2. Layered Approach

Characteristics:

3. Microkernel System Structure

Microkernel Characteristics:

4. Modular Structure (Loadable Kernel Modules)

Module System Features:

5. Hybrid Systems

2.6 System Programs

📁 File Management

ℹ️ Status Information

✏️ File Modification

🔧 Programming Support

📚 Program Loading

💬 Communications

2.7 System Boot

Boot Process Steps:

2.8 Exam-Style Practice Questions

Multiple Choice Questions

Short Answer Questions

Answer Key:

Answer Key:

Answer Key:

2.9 Key Terms and Definitions

2.10 Diagrams from Slides

📊 Layered OS Structure Diagram

📊 Microkernel Architecture