Unit 1: OS overview and structure
Operating Systems notes · PTU syllabus (UGCC2508)
On this page
Unit summary
An operating system (OS) is the software that manages a computer's hardware and provides services to programs and users. Without it, every program would have to control the hardware directly. This unit covers what an OS is, how it evolved, its components, services, structure, types and system calls.
After this unit you can
- Define an operating system and describe its functions and services
- Trace the evolution and generations of operating systems
- Compare batch, multiprogramming, time-sharing, parallel, distributed and real-time systems
- Explain system calls and system programs
PTU syllabus topics
- Definition
- evolution
- components and services of an OS
- structure and architecture
- batch systems
- multiprogramming
- time-sharing
- parallel
- distributed and real-time systems
- OS services and system calls
- system programs
- OS generations
Batch
Jobs grouped and run without interaction
Multiprogramming
Several programs in memory share the CPU
Time-sharing
Fast switching gives each user a share
Real-time
Guaranteed response within a deadline
Distributed
Many machines work as one system
Topic 1
What is an operating system?
An operating system is system software that acts as an interface between the user and the hardware, and as a resource manager that allocates the CPU, memory, files and devices among programs. Examples: Windows, Linux, macOS, Android.
- Users
People and other computers
- Application programs
Browsers, editors, games
- Operating system
Controls and coordinates hardware use
- Hardware
CPU, memory, I/O devices
Goals: convenience (easy to use) and efficiency (good use of resources).
Topic 2
Components and services of an OS
Process management
Create, schedule, terminate processes
Memory management
Allocate and free memory
File management
Create, delete, organise files
I/O and device management
Drivers, buffering
Secondary storage management
Disk space and scheduling
Protection and security
Control access
Services an OS gives programs and users: user interface (CLI or GUI), program execution, I/O operations, file-system manipulation, communication between processes, error detection, resource allocation, accounting, and protection.
Topic 3
Evolution and types of operating systems
| Type | Key idea | Drawback or feature |
|---|---|---|
| Batch | Similar jobs grouped and run one after another without user interaction | CPU idle during I/O; no interaction |
| Multiprogramming | Several programs in memory; CPU switches when one waits for I/O | Better CPU use |
| Time-sharing (multitasking) | CPU time shared in small slices among many users | Quick response; interactive |
| Parallel (multiprocessor) | Several CPUs share memory and work together | Higher throughput, reliability |
| Distributed | Many independent computers connected by a network act as one system | Resource sharing, fault tolerance |
| Real-time | Results must arrive within strict time limits | Hard (missile control) vs soft (video streaming) |
Generations: 1st (1945–55) vacuum tubes, no OS; 2nd (1955–65) transistors and batch systems; 3rd (1965–80) ICs, multiprogramming and time-sharing; 4th (1980–present) personal computers, GUIs, networked and mobile OS.
Topic 4
OS structure and architecture
- Simple/monolithic structure: the whole OS runs as one large program in kernel mode (early UNIX, MS-DOS). Fast but hard to maintain.
- Layered structure: OS divided into layers, each using only the layer below. Easy to debug.
- Microkernel: only essential services (IPC, scheduling) in the kernel; others run as user processes. Reliable and extensible.
- Modular: a core kernel with loadable modules (modern Linux).
Topic 5
System calls and system programs
A system call is the way a program requests a service from the OS kernel. It switches the CPU from user mode to kernel mode.
Process control
Create, end, wait for processes
fork(), exit(), wait()
File management
Create, open, read, write files
open(), read(), write(), close()
Device management
Request and release devices
ioctl(), read(), write()
Information maintenance
Get or set time and data
getpid(), time()
Communication
Send and receive messages
pipe(), send(), recv()
System programs (utilities) provide a convenient environment: file managers, compilers, editors, loaders and shells.
Key terms
- Operating system
- Software that manages hardware and serves users and programs
- Kernel
- The core part of the OS that runs in privileged mode
- System call
- A request from a program to the OS kernel
- Multiprogramming
- Keeping several programs in memory to keep the CPU busy
- Real-time system
- A system that must respond within fixed time limits
Quick revision
- OS = interface + resource manager.
- Batch → multiprogramming → time-sharing → distributed/real-time.
- Hard real-time: deadlines must be met; soft: occasional misses allowed.
- System calls move from user mode to kernel mode.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Define an operating system and give two examples.
- Q2.List four services of an OS.
- Q3.Differentiate between multiprogramming and time-sharing.
- Q4.What is a real-time OS? Distinguish hard and soft real-time.
- Q5.What is a system call?
- Q6.What is a microkernel?
Long-answer questions
- Q1.Explain the functions and services of an operating system.
- Q2.Explain the different types of operating systems with their advantages and disadvantages.
- Q3.Describe the evolution and generations of operating systems.
- Q4.What are system calls? Explain their types with examples.
Stuck on this unit?
Message SBS on WhatsApp for help with Operating Systems, or to ask about studying BCA at Synetic.