Key takeaways
- CPU scheduling, page replacement, Banker's algorithm and semaphores carry most marks.
- Learn TAT = CT - AT and WT = TAT - BT; every scheduling numerical uses them.
- Each section has exam tips and formula boxes, and the last page lists the most asked practice questions.
- These are original Uncodemy revision notes, not official university notes; check your own syllabus.
About these OS handwritten notes
This page offers Uncodemy's Operating Systems notes as a free handwritten-style PDF. Operating Systems explains how the OS manages processes, CPU time, memory, files and devices so many programs can run safely together. The notes are written in short points with neat tables, labelled diagrams, boxed formulas and exam tips, so you can revise a full unit in one sitting instead of reading a long textbook chapter.
Please note: these are original Uncodemy revision notes. They follow the common syllabus that most Indian universities share for this subject, but they are not official university notes. Check your own university syllabus and add anything it lists that is missing here.
What's inside the OS notes
The PDF is arranged in 8 sections plus a final page of most asked practice questions. The sections follow the usual unit order of a Operating Systems syllabus, so you can map them to your own units quickly. Here is what each section covers.
Introduction
The main jobs of an OS, batch, multiprogramming, time-sharing, real-time and distributed OS, kernel and system calls, dual mode, and monolithic vs microkernel.
Process
Program vs process, process states and the PCB, context switch, long, short and medium-term schedulers, threads and the fork() formula.
CPU scheduling
Arrival, burst, completion, turnaround and waiting time, FCFS, SJF, SRTF, priority and Round Robin, with a solved FCFS example.
Process synchronisation
The critical section problem and its three requirements, Peterson's solution, semaphores and mutex, and classic problems such as producer-consumer.
Deadlock
The four necessary conditions, prevention, avoidance with Banker's algorithm and its formulas, detection and recovery.
Memory management
Contiguous allocation, fragmentation, paging with address translation and segmentation.
Virtual memory
Demand paging, page faults, FIFO, LRU and Optimal replacement with a worked reference string, Belady's anomaly and thrashing.
File system and disk scheduling
File allocation methods, directory structures, and FCFS, SSTF, SCAN, C-SCAN and LOOK disk scheduling.
Most asked questions (practice)
Ten exam-style questions, from process vs thread to monolithic vs microkernel.
How to use handwritten notes to revise OS
Operating Systems is half theory and half numericals, so make sure you redo every solved example in the notes, not just read it.
- Solve one scheduling problem with FCFS, SJF, SRTF and Round Robin and draw the Gantt chart for each.
- Find page faults for one reference string with FIFO, LRU and Optimal in the same table.
- Run Banker's algorithm on a five-process example and write the safe sequence.
- Write the producer-consumer solution with semaphores and explain each wait and signal.
A simple three-pass method works well with handwritten notes. In the first pass, read one section and underline only what you do not already know. In the second pass, close the notes and write the definitions, diagrams and steps for that section on a blank sheet, then compare and correct in a different colour. In the third pass, a day or two later, answer two or three exam-style OS questions in timed conditions using only memory. Add your own examples and doubts in the margins so the PDF becomes your personal revision copy.
Formulas, diagrams and definitions to memorise
- TAT = CT - AT, WT = TAT - BT, and response time = first run - AT.
- n fork() calls create 2^n processes in total, of which 2^n - 1 are children.
- Banker's algorithm: Need = Max - Allocation, and the safety check using Work and Finish.
- Effective memory access time with a TLB, and logical address split into page number and offset.
- The four deadlock conditions: mutual exclusion, hold and wait, no preemption and circular wait.
Common mistakes that cost marks
- Forgetting idle CPU time in the Gantt chart when no process has arrived.
- Using burst time instead of remaining time in SRTF after a preemption.
- Counting the first loads into empty frames incorrectly in page replacement questions.
- Saying a deadlock happens when only one of the four conditions holds; all four must hold together.
Exam focus: OS topics that carry the most marks
These Operating Systems topics carry the most marks in university papers and in GATE.
- CPU scheduling numericals with Gantt charts.
- Process synchronisation and semaphores.
- Deadlock conditions and Banker's algorithm.
- Paging, page replacement, thrashing and disk scheduling.
Typical long questions
- Differentiate a process and a thread.
- Explain process states with a diagram and describe the PCB.
- Solve FCFS, SJF, SRTF and Round Robin for given processes and find the average WT and TAT.
- Explain the four necessary conditions for deadlock and Banker's algorithm with an example.
- Solve the producer-consumer problem using semaphores.
- Find page faults using FIFO, LRU and Optimal, and explain Belady's anomaly.
- Explain SCAN and C-SCAN disk scheduling with an example.
Marks per question change between universities and years, so confirm the pattern on a recent official paper. In long answers, start with a definition, add a labelled diagram or table, explain the steps and finish with an example or comparison. Underline the key term in each answer and leave a line between steps, because examiners mark faster when the structure is easy to see.
A 7-day revision plan for OS
- Day 1: Introduction, Process.
- Day 2: CPU scheduling, Process synchronisation.
- Day 3: Deadlock, Memory management.
- Day 4: Virtual memory.
- Day 5: File system and disk scheduling.
- Day 6: redo the numericals: four scheduling algorithms, three page replacement methods, one Banker's problem and one disk scheduling problem.
- Day 7: answer the most asked practice questions at the end of the notes from memory, then solve one official previous year paper in three hours and mark the units that cost you time.
Keep a single sheet of formulas, definitions and diagrams from the notes for the last hour before the exam, and do not start new topics on the final day.
Who these notes are for
- B.Tech CSE and IT students preparing for the semester Operating Systems exam.
- BCA and MCA students with an OS paper.
- GATE CS aspirants revising scheduling, memory and deadlock numericals.
- Freshers revising OS basics before developer, DevOps or system interviews.
The notes are a revision aid, not a first textbook. If a OS topic is new to you, learn it from your prescribed book or class lectures first, then come back to the notes to compress and revise it. For practice, pair them with the Operating System Important Questions PDF page and your university's official previous year papers. If your syllabus has a topic that is not covered here, you can ask for it through the handwritten notes request page linked below.
Download Operating Systems Handwritten Notes PDF
Operating Systems Handwritten Notes PDF (Free)
The PDF carries the same content as this page in a clean, print-ready layout, so you can read it offline and keep it for revision.
8 pages · 291 KB · PDF · Updated 5 Nov 2026