Operating systems: Single & Multi User, Features of MS-DOS, MS Windows, Linux, UNIX, Windows NT.

1. Introduction to Operating Systems (OS)

An Operating System (OS) is system software that acts as an intermediary between users/applications and computer hardware. It manages resources (CPU, memory, I/O devices) and provides a user-friendly interface.

Core Functions of an OS:

• Process Management
• Memory Management
• File System Management
• Device Management
• Security & Access Control
• User Interface (CLI/GUI)

2. Classification by User Support

🔹 A. Single-User Operating System

• Designed for one user at a time.
• May support single-tasking (one program at a time) or multi-tasking (multiple programs).
• Examples: MS-DOS (single-tasking), Windows 95/98 (multi-tasking but single-user).
• Use Case: Personal computers, early home systems.

✅ Note: Modern "single-user" systems (e.g., Windows 10 Home) often support multi-tasking but not concurrent multi-user logins.

🔹 B. Multi-User Operating System

• Allows multiple users to access the system simultaneously (via terminals or network).
• Requires robust security, resource allocation, and user isolation.
• Examples: UNIX, Linux, Windows Server, Windows NT (with limitations).
• Use Case: Servers, universities, corporate networks.

💡 Key Feature: Each user has a separate session, home directory, and permissions.

3. Overview of Key Operating Systems

4. Detailed Features of Each OS

🔹 A. MS-DOS (Microsoft Disk Operating System)

• Type: Single-user, single-tasking
• Interface: Command-Line Interface (CLI) only
• File System: FAT12/FAT16 (File Allocation Table)
• Key Commands:
  • DIR – list directory
  • COPY – copy files
  • FORMAT – format disk
  • CD – change directory
• Memory: Limited to 640 KB conventional memory (real-mode)
• Limitations:
  • No built-in security
  • No multi-tasking
  • No GUI
• Legacy: Foundation for early Windows (Windows 1.x–3.x ran on top of DOS)

🔹 B. MS Windows (Consumer Versions: 95, 98, XP, 10, 11)

• Type: Primarily single-user, but supports multi-tasking
• Interface: Graphical User Interface (GUI) – iconic Start Menu, Windows, Icons, Mouse
• Kernel: Hybrid (Windows 9x: DOS-based; Windows NT-based from Win 2000 onward)
• Key Features:
  • Plug and Play (PnP) hardware support
  • Wide software & driver compatibility
  • User-friendly for non-technical users
  • Integrated networking (from Windows 95)
• File Systems: FAT32, NTFS (New Technology File System – secure, journaling)
• Security: User accounts (limited in home editions), Windows Defender, BitLocker (Pro/Enterprise)
• Limitation: Historically more vulnerable to malware than UNIX/Linux

🔹 C. Linux

• Type: Multi-user, multi-tasking, open-source
• Interface: CLI (Bash, Zsh) + multiple GUIs (GNOME, KDE, XFCE)
• Kernel: Monolithic (developed by Linus Torvalds)
• Key Features:
  • Free and open-source (GNU/Linux)
  • High stability, security, and customizability
  • Strong command-line environment
  • Excellent networking and server capabilities
  • Package managers (APT, YUM, Pacman) for software installation
• File System: Ext4, XFS, Btrfs (journaling, permissions, ACLs)
• Security: Built-in user/group permissions, SELinux/AppArmor
• Distributions (Distros): Ubuntu, Fedora, Debian, CentOS, Arch

🔹 D. UNIX

• Type: Multi-user, multi-tasking, proprietary (original)
• Interface: CLI (Bourne Shell, C Shell) – GUIs like CDE available
• Developed: 1969 at AT&T Bell Labs (Ken Thompson, Dennis Ritchie)
• Key Features:
  • Portable (written in C)
  • Hierarchical file system
  • Everything is a file philosophy
  • Strong inter-process communication (IPC)
  • Built-in security model (users, groups, permissions)
• Variants (Unix-like): AIX (IBM), Solaris (Oracle), HP-UX (Hewlett-Packard)
• Legacy: Inspired Linux, macOS (Darwin is UNIX-certified)

🔹 E. Windows NT (New Technology)

• Type: Multi-user capable, multi-tasking, 32/64-bit
• Released: 1993 (Windows NT 3.1)
• Purpose: Enterprise/server OS (replaced DOS-based Windows for business)
• Key Innovations:
  • Preemptive multitasking & protected memory
  • NTFS file system (security, compression, encryption)
  • Hardware abstraction layer (HAL) – better portability
  • Integrated networking (TCP/IP, domain support)
  • Security: User accounts, ACLs, audit logs
• Architecture: Hybrid kernel (microkernel + monolithic elements)
• Legacy: Foundation for all modern Windows (2000, XP, 7, 10, 11, Server editions)

5. Comparative Analysis

6. Key Concepts & Terminology

• CLI (Command-Line Interface): Text-based user interface (e.g., DOS prompt, Linux terminal).
• GUI (Graphical User Interface): Visual interface with windows, icons, menus (e.g., Windows, macOS).
• Kernel: Core of the OS; manages hardware and system resources.
• Multi-tasking: Running multiple programs "simultaneously" (via time-sharing).
• Multi-user: Multiple users logged in and working at the same time.
• Open-source: Source code freely available for modification/redistribution (Linux).
• Proprietary: Source code owned and restricted by vendor (Windows, UNIX variants).
• NTFS: Advanced Windows file system with security, journaling, and large file support.
• Shell: Command interpreter in UNIX/Linux (e.g., Bash).

7. Glossary

• DOS: Disk Operating System – early CLI-based OS.
• NT: New Technology – Microsoft’s enterprise OS line.
• Kernel: Central OS component managing hardware.
• Shell: CLI interface in UNIX/Linux.
• FAT: File Allocation Table – simple DOS/Windows file system.
• NTFS: New Technology File System – secure, advanced Windows file system.
• Open-source: Software with publicly accessible source code.
• Multi-tasking: Concurrent execution of multiple processes.
• Multi-user: System supporting simultaneous user sessions.