Unit 4: Memory and I/O management
Operating Systems notes · PTU syllabus (UGCC2508)
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Unit summary
Memory management decides how programs are placed in main memory, and I/O management controls devices and disks. This unit covers address binding, swapping, contiguous allocation (MFT and MVT), fragmentation, paging, segmentation, virtual memory with demand paging and page replacement, thrashing, and disk scheduling.
After this unit you can
- Distinguish logical and physical addresses and explain swapping
- Compare MFT and MVT and internal and external fragmentation
- Explain paging, segmentation and demand paging
- Solve page replacement and disk scheduling problems
PTU syllabus topics
- Logical and physical address mapping
- swapping
- memory allocation
- MFT/MVT
- internal/external fragmentation and compaction
- paging
- segmentation
- demand paging
- page replacement algorithms
- frame allocation
- thrashing
- I/O hardware principles
- disk structure and scheduling algorithms
Division
Fixed-size pages and frames
Variable-size logical segments
Fragmentation
Internal
External
Programmer view
Invisible to the programmer
Matches program structure (code, data, stack)
Address
Page number + offset
Segment number + offset
Topic 1
Logical and physical addresses, swapping
A logical address is generated by the CPU; a physical address is the actual location in memory. The Memory Management Unit (MMU) maps logical to physical addresses at run time, for example by adding a relocation (base) register value. Swapping temporarily moves a process from memory to disk (backing store) and back, so that more processes can run than fit in memory at once.
Topic 2
Contiguous allocation: MFT and MVT
Partitions
Memory divided into fixed-size parts in advance
Created to fit each process as it arrives
Fragmentation
Internal
External
Degree of multiprogramming
Limited by the number of partitions
Flexible
Placement strategies for variable partitions: first fit (first hole big enough — fast), best fit (smallest hole big enough — least leftover), worst fit (largest hole).
- Internal fragmentation: wasted space inside an allocated partition.
- External fragmentation: enough total free memory exists, but not in one contiguous block. Compaction shuffles processes together to create one large free block.
Topic 3
Paging and segmentation
Paging divides logical memory into fixed-size pages and physical memory into frames of the same size. A page table maps each page to a frame, so a process need not be contiguous. Paging removes external fragmentation.
- Logical address = (page number p, offset d); physical address = frame number × page size + d.
Segmentation divides a program into variable-size logical segments (code, data, stack). A segment table stores each segment's base and limit.
Unit
Fixed-size pages
Variable-size segments
View
Physical, invisible to the programmer
Logical, matches program structure
Fragmentation
Internal (last page)
External
Table
Page table
Segment table with base and limit
Topic 4
Virtual memory, page replacement and thrashing
Virtual memory lets a process run even if only part of it is in memory. Demand paging loads a page only when it is needed; accessing a page not in memory causes a page fault.
FIFO
The oldest page in memory
Simple; Belady's anomaly possible
Optimal
The page not needed for the longest time
Lowest faults; needs future knowledge
LRU
The least recently used page
Good approximation of optimal
Example
Reference string 7, 0, 1, 2, 0, 3, 0, 4 with 3 frames under FIFO gives 7 page faults: 7, 0, 1, 2, 3, 0, 4 (only the 5th reference, 0, is a hit).
Frame allocation can be equal or proportional to process size. Thrashing happens when processes have too few frames and spend more time paging than executing; it is controlled with the working-set model or by reducing multiprogramming.
Topic 5
I/O hardware and disk scheduling
I/O devices connect through controllers; data moves by programmed I/O, interrupts or DMA. A disk has platters, tracks, sectors and cylinders; access time = seek time + rotational latency + transfer time. Disk scheduling reduces seek time.
| Algorithm | How it moves the head |
|---|---|
| FCFS | Serves requests in arrival order |
| SSTF | Serves the nearest request next (may starve far ones) |
| SCAN (elevator) | Moves end to end, serving requests on the way |
| C-SCAN | Serves in one direction only, then jumps back |
| LOOK / C-LOOK | Like SCAN / C-SCAN but turns at the last request |
Exam tip
For disk problems, draw the head movement on a number line and add up every cylinder moved.
Key terms
- Logical address
- An address generated by the CPU
- Page fault
- An access to a page not currently in memory
- Fragmentation
- Wasted memory, internal or external
- Thrashing
- Excessive paging that stops useful work
- Seek time
- Time to move the disk head to the required track
Quick revision
- MMU maps logical to physical addresses.
- MFT → internal fragmentation; MVT → external; compaction fixes external.
- Paging: fixed pages, no external fragmentation; segmentation: logical, variable segments.
- FIFO, Optimal, LRU; Belady's anomaly in FIFO.
- Disk scheduling: FCFS, SSTF, SCAN, C-SCAN, LOOK.
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.Differentiate between logical and physical addresses.
- Q2.What is swapping?
- Q3.Differentiate between internal and external fragmentation.
- Q4.What is a page fault?
- Q5.What is thrashing?
- Q6.What is Belady's anomaly?
Long-answer questions
- Q1.Explain paging with a diagram and compare it with segmentation.
- Q2.Explain contiguous memory allocation with first fit, best fit and worst fit.
- Q3.Solve a page replacement problem using FIFO, Optimal and LRU and compare page faults.
- Q4.Explain disk scheduling algorithms and calculate total head movement for a given request queue.
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