Operating Systems (AD-405) Unit 1 Notes - RGPV

How unit 1 is examined

This unit covers what an OS is and does, how it evolved, its services and system calls; Evolution (with time-sharing, batch, demand paging) and Function carry the marks.

Function

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Definition. <mark>An operating system is system software that acts as an interface between the user and the hardware, manages all hardware and software resources, and provides a convenient and efficient environment for running programs.</mark>

Key points.

The OS is a resource manager: it allocates CPU, memory, files and I/O devices among competing programs fairly and efficiently.
The OS is an extended machine: it hides the ugly hardware details behind simple abstractions, for example a disk appears as files and folders.
The OS is a control program: it controls execution of user programs to prevent errors and improper use of the computer.
Goals are convenience (easy to use), efficiency (best use of hardware) and ability to evolve (new features and hardware without disturbing services).
User goals are that the system is easy to learn, reliable, safe and fast; system goals are that it is easy to design, implement and maintain, flexible and error-free.
Functions are process management, memory management, file management, device (I/O) management, and security and protection.
Management tasks are done as resource allocator: for example the scheduler picks which process gets the CPU and coordinates processes that share a resource.

Answer frame. Open with the definition; list goals, then user versus system goals; list the five functions with one line each; close with "the OS is the resource manager that makes the computer convenient and efficient".

Asked: [7 marks] (Jun 2023, Jun 2025) Define an operating system. What are the goals of an operating system? Explain. List the functions of OS. How operating system perform management task in a system?

Evolution

Definition. <mark>Evolution of operating systems is the progression from no OS to serial, batch, multiprogrammed, time-sharing, personal, parallel, distributed, real-time and mobile systems, driven by the need for better CPU use, response and convenience.</mark>

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Key points.

Serial processing (1940s) had no OS: the programmer loaded programs by hand, so setup time was large and the CPU sat idle.
Batch processing groups similar jobs into a batch that a resident monitor runs one after another with no user interaction, which cuts setup time but leaves the CPU idle during I/O. Examples are payroll and bank statements.
Multiprogramming keeps several jobs in memory, and when one waits for I/O the CPU switches to another, which raises CPU utilisation.
Time-sharing is multiprogramming with a small time slice (quantum) given to each user in turn, so each user gets quick response and the system feels interactive. Examples are UNIX and Multics.
Personal computer OS serves a single user with a friendly interface (MS-DOS, Windows); parallel OS uses many CPUs sharing memory to compute faster.
Distributed OS joins independent computers by a network so they behave as one system; real-time OS guarantees response within a deadline (missile control); mobile OS runs on phones (Android, iOS).
The goals throughout are efficiency, convenience and better resource management.

Short-note extras (Nov 2023).

Item	Point
Virus	A program that attaches itself to a host file and spreads only when that file is run by the user. It corrupts or deletes data.
Worm	A standalone program that copies itself over a network without a host file or user action, consuming bandwidth and memory. Example: Morris worm.
Demand paging	A page is brought into memory only when it is referenced. A page fault traps to the OS, which loads the page from disk and restarts the instruction. Advantages: less memory and I/O, more processes.

Answer frame. For 7 marks open with the OS definition and why it evolved; draw the stage chain; develop points 1-6 in order; close with point 7. For the 3-mark time-sharing note give the definition, the quantum, the advantage of quick interactive response, and UNIX as example. For the 14-mark note give each of the four parts about 3 lines plus one example.

Pitfall: Confusing batch (no interaction, jobs one after another) with multiprogramming (jobs overlap in memory).

Asked: [7 marks] (Nov 2023, Jun 2026) Explain the evolution of operating system in detail. Asked: [14 marks] (Nov 2023) Write short notes: a) Worms and Virus b) Demand paging c) Batch processing d) Time-sharing Asked: [3 marks] (Jun 2023) Write short notes: Time Sharing

Desirable Characteristics and features of an O/S

Definition. Desirable characteristics are the qualities a good OS must have to serve users and hardware well.

Key points.

It should be efficient, using CPU, memory and devices with minimum waste and quick response.
It should be reliable and robust, continuing to work despite errors and protecting itself and users' data.
It should be convenient, secure, portable and easy to maintain, with a friendly interface.
It should be extensible so new hardware and features can be added.

Operating Systems Services: Types of Services

Definition. <mark>OS services are the functions the OS provides to users and programs: program execution, I/O operations, file-system manipulation, communication, error detection, resource allocation, protection and accounting.</mark>

Process management (five activities).

Creation and deletion of user and system processes, tracked using the Process Control Block (PCB).
Scheduling: the scheduler decides which process gets the CPU and when.
Synchronization of processes so shared data stays consistent.
Communication between processes using shared memory or message passing.
Deadlock handling: prevention, avoidance, detection and recovery.

Memory management (five activities).

Allocation and deallocation of memory to processes as they start and end.
Tracking which parts of memory are used and by whom.
Protection so one process cannot access another's memory.
Swapping processes between memory and disk to make room.
Managing paging and segmentation and deciding which processes are loaded.

Answer frame. Open with the definition of process (or memory) management; list the five activities in order with one explaining sentence each; close with the role of the PCB and scheduler (or a memory manager sitting between processes and RAM).

Asked: [7 marks] (Jun 2024) What are the five major activities of an operating system in regards to process management? Asked: [7 marks] (Jun 2025) What are the five major activities of an operating system in regards to memory management?

Different ways of providing these Services – Utility Programs

Definition. Utility programs are system programs that perform common housekeeping tasks and give users a convenient way to use OS services.

Key points.

Services are given in two ways: through system calls made by programs, and through system programs (utilities) used by people.
Utilities include file managers, text editors, compilers, debuggers, loaders, disk formatters, backup tools and antivirus.
A command interpreter (shell) or GUI lets the user reach these services, and to the user the utilities look like the OS.

System Calls

Definition. <mark>A system call is the programmatic interface through which a user program requests a service from the OS kernel.</mark>

Key points.

System calls are used for process control (fork, exit), file management (open, read, write, close), device management, information maintenance and communication.
Programs use them through library functions or APIs such as the C library, which hide the details.
Execution steps: the program puts the parameters in registers or on the stack, executes a trap (software interrupt), the CPU switches to kernel mode, the kernel runs the service, and control returns to the program with the result.
Example: printf calls write, which traps into the kernel to output data.

Asked: [7 marks] (Jun 2023) What is meant by system call? How it can be used. How does an application program use these call during execution?

Last-minute revision

OS = resource manager + extended machine + control program.
Goals: convenience, efficiency, ability to evolve.
Functions: process, memory, file, device, security management.
Evolution: serial, batch, multiprogramming, time-sharing, personal, parallel, distributed, real-time, mobile.
Time-sharing = multiprogramming plus a small quantum per user.
Virus needs a host file; worm spreads by itself.
Demand paging loads a page only on a page fault.
Process management: create/delete, schedule, synchronize, communicate, deadlock.
Memory management: allocate, track, protect, swap, paging/segmentation.
System call: trap to kernel mode, run service, return.

Memory hooks

Evolution: "Some Big Machines Take Power, Providing Dependable Results, Mobile" for Serial, Batch, Multiprogramming, Time-sharing, Personal, Parallel, Distributed, Real-time, Mobile.
Process five: "C S S C D" = Create, Schedule, Synchronize, Communicate, Deadlock.
Memory five: "A T P S P" = Allocate, Track, Protect, Swap, Page.
Virus needs a host; worm walks alone.

Coverage checklist

Function: Q3 (Jun 2023, Jun 2025).
Evolution: Q1 (Nov 2023), Q2 (Nov 2023, Jun 2026), Q7 (Jun 2023).
Desirable Characteristics and features of an O/S: no past questions.
Operating Systems Services: Types of Services: Q4 (Jun 2024), Q5 (Jun 2025).
Different ways of providing these Services – Utility Programs: no past questions.
System Calls: Q6 (Jun 2023).

Account

Operating Systems (AD-405) - Unit 1 Short Notes

How unit 1 is examined

Function

Evolution

Desirable Characteristics and features of an O/S

Operating Systems Services: Types of Services

Different ways of providing these Services – Utility Programs

System Calls

Last-minute revision

Memory hooks

Coverage checklist