Find solved example num here :
Module 4
57. What are the four conditions that create deadlock?
Deadlock can arise if four conditions hold simultaneously (Page 2 of attached document):
1.
Mutual Exclusion: This condition states that only one process can use a resource at a time. If a resource is already being used by a process, any other process requesting the resource must wait until it is released. This condition is inherent for non-sharable resources like printers or writeable files. Without mutual exclusion, processes could access and modify shared data concurrently, leading to inconsistent results.
2.
Hold and Wait: This occurs when a process is holding at least one resource and is waiting to acquire additional resources that are currently held by other processes. This creates a dependency where the process cannot proceed until it acquires the needed resources, and the resources it holds remain unavailable to others.
3.
No Preemption: This condition implies that a resource can only be released voluntarily by the process holding it, after the process has completed its task. The operating system cannot forcibly take a resource away from a process. If preemption were allowed, the OS could interrupt a process and reallocate resources, potentially breaking a deadlock cycle.
4.
Circular Wait: This is a situation where there exists a set of waiting processes {P0, P1, …, Pn} such that P0 is waiting for a resource held by P1, P1 is waiting for a resource held by P2, and so on, until Pn is waiting for a resource held by P0. This creates a circular dependency where no process can proceed because each is waiting for a resource held by another in the chain.
(Internet Source Addition) Deadlock is a critical issue in operating systems as it can bring the entire system to a standstill. These four conditions must all be present for a deadlock to occur. If even one of these conditions is not met, a deadlock cannot happen. Understanding these conditions is crucial for implementing deadlock prevention, avoidance, and detection techniques.
58. How can the hold-and-wait condition be prevented?
The hold-and-wait condition can be prevented by ensuring that whenever a process requests a resource, it does not hold any other resources (Page 14 of attached document). This can be achieved in two primary ways:
1.
Request all resources before execution: One way to prevent hold-and-wait is to require each process to request and be allocated all of its required resources before it begins execution. The process must make a single request for all the resources it will need. If all resources are available, they are granted to the process, and the process can run. If not, the process waits until all resources are available. This approach ensures that a process never holds some resources while waiting for others.
2.
Release resources before requesting: Another method is to allow a process to request resources only when the process has none allocated to it. Before requesting a new resource, the process must release all currently held resources. This ensures that a process is never in a state of holding some resources and waiting for additional ones.
(Internet Source Addition) These strategies, while effective at preventing hold-and-wait, have drawbacks. Requesting all resources upfront can lead to low resource utilization because resources are allocated to a process for its entire duration, even if they are not continuously used. This can also cause starvation, where a process is indefinitely postponed because the system can't allocate all its resources simultaneously. Frequent releasing and requesting of resources, on the other hand, introduces overhead and can reduce system efficiency.
59. List two ways in which the no preemption condition can be prevented.
The no-preemption condition can be prevented by allowing the operating system to preempt resources from a process (Page 15 of attached document). Here are two ways to achieve this:
1.
Resource Preemption: If a process is holding some resources and requests another resource that cannot be immediately allocated to it, then all resources currently held by that process are released. These preempted resources are added to the list of resources for which the process is waiting. The process is restarted only when it can regain its old resources, as well as the new ones it is requesting. This approach ensures that no process can hold onto resources indefinitely, preventing other processes from getting stuck.
2.
Process Suspension: If a resource is requested by a higher-priority process, the resource can be preempted from the current (lower-priority) process and allocated to the higher-priority process. This is commonly used in priority-based scheduling systems, where critical tasks need immediate access to resources.
(Internet Source Addition) Implementing preemption can be complex. When a resource is preempted, the state of the process using that resource must be saved, and the resource state might need to be rolled back to a safe point. Careful design is needed to minimize overhead and prevent data corruption or loss. Moreover, preemption is not always feasible, especially for resources like printers, where interrupting a task mid-way can lead to inconsistencies.
60. How can the circular wait condition be prevented?
The circular-wait condition can be prevented by imposing a total ordering of all resource types and requiring that each process requests resources in an increasing order of enumeration (Page 15 of attached document).
1.
Resource Ordering: Assign a unique number to each resource type. A process can request resources only in ascending order according to this numbering. If a process needs a resource with a lower number after acquiring a resource with a higher number, it must release the higher-numbered resource first. For example, if a process holds resource R5 and needs resource R3 (where R3 has a lower number than R5), it must release R5 before requesting R3.
(Internet Source Addition) This approach ensures that a circular dependency cannot occur because processes are forced to request resources in a predefined sequence, breaking any potential circular wait. One major challenge is determining an appropriate ordering of resources that satisfies the needs of all processes. This ordering can impact resource utilization because processes may need to release and re-request resources to comply with the ordering, leading to increased overhead. Also, it might not always be possible to find an ordering that works well for all possible resource request patterns.
61. What is the difference among deadlock avoidance, detection, and prevention?
Deadlock handling in operating systems can be approached in three main ways: prevention, avoidance, and detection, each with its own strategy and implications (Page 6 of attached document).
Deadlock Prevention: This approach aims to eliminate the possibility of deadlock by negating one or more of the four necessary conditions for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait (Page 14 of attached document). Prevention is a static approach, meaning it sets up conditions in advance to ensure deadlock cannot occur. For example, requiring a process to request all its resources at once prevents the "hold and wait" condition. However, prevention can lead to inefficient resource utilization.
Deadlock Avoidance: This method uses a more dynamic approach. The system requires information about the maximum resources a process may request during its execution (Page 7 of attached document). The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. The system makes decisions on whether to grant resource requests based on whether the resulting state would be "safe" (where all processes can complete) or "unsafe" (where deadlock is possible). The Banker's Algorithm is a well-known technique for deadlock avoidance. Avoidance allows more concurrency than prevention but requires the system to know future resource needs.
Deadlock Detection: This approach allows deadlocks to occur. The system does not attempt to prevent or avoid deadlocks but instead provides mechanisms to detect when a deadlock has occurred and recover from it (Page 12 of attached document). Detection algorithms periodically check the system for cycles in the resource allocation graph or use other methods to identify deadlocked processes. Once a deadlock is detected, the system employs recovery techniques such as process termination or resource preemption to break the deadlock. Detection is useful in systems where deadlocks are infrequent, as the overhead of prevention or avoidance may be higher than the cost of detecting and recovering from deadlocks.
(Internet Source Addition)
In summary, prevention acts proactively to constrain resource allocation and eliminate deadlock conditions. Avoidance dynamically assesses resource requests based on future needs to maintain a safe state. Detection identifies and resolves deadlocks after they occur, allowing for more flexible resource allocation at the cost of occasional deadlock resolution.
62. Numerical on Resource allocation graph
63. Numerical on banker’s algorithm?
64. What is thrashing? How is it handled?
Thrashing is a phenomenon in virtual memory operating systems where the system spends a disproportionate amount of time swapping pages in and out of memory, resulting in little productive work being done. (Not found in attached document)
Thrashing occurs when a process does not have "enough" pages in memory to support its execution. This leads to frequent page faults. When a page fault occurs, the operating system must retrieve the required page from disk, which is a slow operation. If the system is thrashing, processes are constantly faulting, leading to a high degree of disk activity and low CPU utilization. Processes spend more time waiting for pages to be swapped in than executing instructions.
Thrashing can be handled through several techniques:
Locality Model: By structuring code and data to improve locality of reference, the number of page faults can be reduced. Processes should access memory locations that are close to each other in time (temporal locality) or space (spatial locality).
Working-Set Model: This model is based on the assumption of locality. The working set is the set of pages a process is actively using. The OS tracks the working set of each process and allocates enough frames to hold that working set in memory. If a process's working set exceeds the available memory, the OS may suspend the process to reduce the overall memory demand.
Page-Fault Frequency (PFF) Control: This approach adjusts the number of frames allocated to a process based on its page-fault rate. If the page-fault rate is too high, the OS increases the number of frames allocated to the process. If the page-fault rate is too low, the OS can decrease the number of frames.
Load Control: Reducing the degree of multiprogramming can alleviate thrashing. By decreasing the number of processes competing for memory, each process receives a larger share of available frames, reducing the likelihood of page faults.
(Internet Source Addition)
Thrashing is a significant performance issue in virtual memory systems. Effective handling requires a combination of techniques to ensure that processes have adequate memory resources and that system resources are not wasted on excessive page swapping.
65. What is demand paging? What are the advantages?
Demand paging is a virtual memory technique where pages are loaded into physical memory only when they are needed (on demand), rather than loading all pages of a process at once (Not found in attached document).
In a demand-paging system, when a process is started, none of its pages are loaded into physical memory. Instead, the operating system sets up the page table so that any attempt to access a page that is not in memory will cause a page fault. When the process accesses a page that is not in memory (a page fault), the operating system retrieves the page from secondary storage (usually a disk) and loads it into a free frame in physical memory. The page table is updated to reflect the new location of the page, and the process can continue execution. If no free frames are available, a page replacement algorithm is used to select a page to be swapped out.
Advantages of demand paging:
Reduced Memory Usage: Only the pages that are actually needed by a process are loaded into memory. This results in lower memory usage compared to loading all pages upfront.
Faster Startup Times: Processes can start execution faster because the operating system does not need to load all pages into memory before starting the process.
Increased Multiprogramming: Demand paging allows more processes to run concurrently in the system because the memory usage per process is reduced.
Reduced I/O: Fewer pages are loaded into memory, which reduces the amount of I/O required to load processes.
Efficient use of Memory: The use of the memory is done so that only needed pages are fetched into the main memory.
(Internet Source Addition) Demand paging allows a program to run even if it is larger than the available physical memory, improving overall system efficiency. It is a fundamental technique in modern operating systems.
66. Explain the concept of virtual memory.
Virtual memory is a memory management technique that allows processes to execute even if they are not entirely loaded into physical memory (Not found in attached document). It creates an abstraction that provides each process with the illusion of having a large, contiguous address space, regardless of the amount of physical memory available.
Virtual memory is typically implemented using demand paging or demand segmentation. Each process has its own virtual address space, which is divided into pages (in demand paging) or segments (in demand segmentation). These pages or segments can be stored either in physical memory or on secondary storage (e.g., a hard disk).
The operating system maintains a mapping between the virtual addresses used by a process and the physical addresses in memory. This mapping is typically implemented using a page table or segment table. When a process accesses a virtual address, the operating system translates it into a physical address using the mapping table. If the corresponding page or segment is not in physical memory (a page fault or segment fault), the operating system retrieves it from secondary storage and loads it into memory.
Key aspects of virtual memory:
Address Translation: The operating system translates virtual addresses into physical addresses, allowing processes to access memory locations without needing to know their physical locations.
Paging or Segmentation: Virtual memory is typically implemented using paging or segmentation, which divide the virtual address space into fixed-size pages or variable-size segments.
Demand Paging/Segmentation: Pages or segments are loaded into physical memory only when they are needed, reducing memory usage and improving system efficiency.
Memory Protection: Virtual memory provides memory protection by ensuring that each process can only access its own virtual address space.
(Internet Source Addition) Virtual memory enables efficient memory utilization, supports larger-than-physical-memory processes, and provides memory protection, making it a cornerstone of modern operating systems.
Q67 Consider a system consisting of m resources of the same type ,
being shared by n processes. Resources can be requested and released by processes only
one at a time. Show that the system is deadlock free if the following two conditions are hold:
a. The maximum need of each process is between 1 and m resources.
b. The sum of all maximum needs is less than the m+n.
Notation
m = total identical resources
n = number of processes
for process Pᵢ:
Maxᵢ ∈ [1…m] its maximum demand
Aᵢ its current allocation
Needᵢ = Maxᵢ–Aᵢ its remaining need
Available = m – ∑₁ⁿ Aᵢ
Given
1.
1 ≤ Maxᵢ ≤ m for each i
2.
∑₁ⁿ Maxᵢ < m + n
We must show no deadlock can occur.
— Step 1: Show Available ≥ 1 in any reachable state
Assume, for contradiction, that Available = 0.
Then ∑₁ⁿ Aᵢ = m, so
∑₁ⁿ Needᵢ
= ∑₁ⁿ (Maxᵢ–Aᵢ)
= (∑Maxᵢ) – m
< (m + n) – m
= n.
But in a deadlock every blocked process must be waiting for at least one more unit, so Needᵢ ≥ 1 for each of the n processes, giving ∑Needᵢ ≥ n.
Hence ∑Needᵢ ≥ n and ∑Needᵢ < n—a contradiction.
Therefore Available ≥ 1 always.
— Step 2: Find a process whose Need ≤ Available
Suppose to the contrary that every process has Needᵢ > Available. Then
∑₁ⁿ Needᵢ
∑₁ⁿ (Available + 1)
On the other hand,
∑Needᵢ
= ∑Maxᵢ – ∑Aᵢ
= ∑Maxᵢ – (m – Available)
< (m + n) – (m – Available)
= n + Available.
So we get
n·Available + n < ∑Needᵢ < n + Available,
which is impossible for n ≥ 1.
Thus there must be at least one process Pₖ with
Needₖ ≤ Available.
— Step 3: Induction to finish everyone
1.
Available ≥ 1, so Step 2 gives us some Pₖ that can instantly acquire Needₖ units and run to completion.
2.
When Pₖ finishes, it releases all Aₖ + Needₖ = Maxₖ units, so Available increases.
3.
Remove Pₖ from consideration (now n–1 processes, total resources still m).
4.
The two conditions still hold for the remaining set (numeric inequalities are even looser), so we repeat.
By induction, every process eventually runs to completion—no deadlock.
Q68
Q69
FIFO :
Optimal replacement:
LRU same example :
70. What is paging? How is it different from segmentation? Explain hardware support for paging? (4 marks)
Paging is a memory management scheme that eliminates the problems of fitting varying sized memory chunks by dividing the physical memory into fixed-sized blocks called frames and logical memory into blocks of the same size called pages. When a process is to be executed, its pages are loaded into any available frames in physical memory, which need not be contiguous. This avoids external fragmentation and simplifies memory allocation. A page table is maintained to keep track of the mapping between logical pages and physical frames. However, paging can cause internal fragmentation because the last frame allocated to a process may not be fully used[1, pp. 20-24].
Difference between Paging and Segmentation:
Aspect
Paging
Segmentation
Memory Division
Divides memory into fixed-size pages/frames
Divides memory into variable-sized segments
Addressing
Logical address divided into page number and offset
Logical address divided into segment number and offset
Fragmentation
Avoids external fragmentation, causes internal fragmentation
Avoids internal fragmentation, may cause external fragmentation
View of Memory
Uniform view (all pages same size)
Logical view based on program structure (functions, data)
Hardware Support
Uses page tables and frame mapping
Uses segment tables with base and limit registers
Hardware support for paging:
1.
Page Table: Each process has a page table that maps page numbers to frame numbers in physical memory.
2.
Memory Management Unit (MMU): Translates logical addresses to physical addresses at runtime by using the page table.
3.
Page Table Base Register (PTBR): Holds the starting address of the page table in memory.
4.
Page Table Length Register (PTLR): Indicates the size of the page table.
5.
Address Translation: The logical address generated by the CPU is divided into a page number (p) and page offset (d). The MMU uses the page number to index into the page table to find the corresponding frame number, then combines the frame number with the offset to form the physical address.
6.
Translation Lookaside Buffer (TLB): A special fast cache that stores recent page table entries to reduce the two-memory-access problem (one for page table lookup and one for data access), thereby speeding up address translation[1, pp. 22-26].
71. Compare the following main memory organization schemes: contiguous memory allocation, pure segmentation, and pure paging with respect to external and internal fragmentation
Memory Scheme
External Fragmentation
Internal Fragmentation
Contiguous Memory Allocation
High external fragmentation due to variable-sized holes created when processes are loaded and removed
Low internal fragmentation as processes are allocated exactly the required size
Pure Segmentation
Possible external fragmentation because segments vary in size and may not fit into available holes
No internal fragmentation since segments are variable-sized and allocated exactly as needed
Pure Paging
No external fragmentation because pages are fixed size and can be placed anywhere in physical memory
Possible internal fragmentation due to last page/frame possibly not fully used
Explanation:
Contiguous allocation requires processes to be loaded into a single contiguous block of memory, which leads to external fragmentation as free memory gets split into small unusable holes.
Segmentation divides memory based on logical segments, avoiding internal fragmentation but still suffers from external fragmentation.
Paging divides memory into fixed-size blocks, eliminating external fragmentation but causing internal fragmentation when the last page is partially filled[1, pp. 15-25].
72. Explain hardware support with TLB paging with the help of a neat diagram
Hardware support for paging with TLB:
The Translation Lookaside Buffer (TLB) is a small, fast associative memory cache that stores recent translations of page numbers to frame numbers. When the CPU generates a logical address, the page number is first checked in the TLB:
If the page number is found in the TLB (TLB hit), the corresponding frame number is retrieved directly, and the physical address is formed by combining the frame number with the page offset. This avoids the need to access the page table in main memory, speeding up the address translation.
If the page number is not found in the TLB (TLB miss), the system accesses the page table in main memory to find the frame number, updates the TLB with this entry, and then forms the physical address.
Diagram:
This hardware support reduces the average memory access time by avoiding the two-memory-access problem inherent in paging without TLB[1, pp. 25-27].
73. Justify the statement: Demand paging can significantly affect performance of computer systems
Demand paging is a memory management technique where pages are loaded into memory only when they are needed (on demand), rather than loading the entire process at once. This can significantly affect system performance due to the following reasons:
Reduced initial loading time: Only necessary pages are loaded, reducing the time to start a process.
Efficient memory utilization: Memory is used only for pages currently needed, allowing more processes to reside in memory simultaneously.
Page faults: When a required page is not in memory, a page fault occurs, causing the system to fetch the page from secondary storage (disk), which is a slow operation and can stall the CPU.
Thrashing: Excessive page faults can lead to thrashing, where the system spends more time swapping pages in and out than executing processes, severely degrading performance.
Thus, while demand paging optimizes memory usage and process startup, it can degrade performance if page faults are frequent or if the system does not have enough physical memory to hold the working set of processes[1, pp. 28-30; additional explanation from internet sources].
Note: All page references are from the attached document "OS-M5-memory-management.pdf"1. Additional clarifications on demand paging are supplemented from standard operating system concepts available on internet sources.
75. Compare and contrast given allocation methods: contiguous allocation, linked allocation, indexed allocation. (4 marks)
Contiguous Allocation:
In contiguous allocation, each file occupies a set of contiguous blocks on the disk. This method is simple and supports fast sequential and direct access. However, it suffers from external fragmentation and may require knowing the file size in advance, making it difficult to grow files[1, p. 12, general OS knowledge].
Linked Allocation:
Linked allocation stores each file as a linked list of disk blocks, which may be scattered anywhere on the disk. Each block contains a pointer to the next block. This eliminates external fragmentation and allows files to grow easily. However, it is inefficient for direct access since all blocks must be traversed sequentially to reach a specific point in the file[1, p. 13, general OS knowledge].
Indexed Allocation:
In indexed allocation, each file has its own index block, which contains the addresses of all the file’s blocks. This allows direct access to file blocks and supports dynamic file growth. However, it requires extra space for the index block, and if the file is very large, multiple levels of indexing may be needed[1, p. 13, general OS knowledge].
Method
Advantages
Disadvantages
Contiguous Allocation
Fast access, simple implementation
External fragmentation, hard to grow files
Linked Allocation
No external fragmentation, easy to grow files
Slow direct access, pointer overhead
Indexed Allocation
Fast direct access, dynamic file growth
Index block overhead, complex for large files
76. Write short notes on: Memory segmentation (4 marks)
Memory segmentation is a memory management technique in which the memory is divided into variable-sized segments, each representing a logical unit such as a function, object, or data structure. Each segment has a segment number and an offset, and the logical address is specified as a pair (segment number, offset)[1, p. 19].
Segmentation provides a user’s view of memory, making it easier to organize and protect code and data. It supports sharing and protection by associating different permissions with each segment. However, segmentation can lead to external fragmentation since segments are variable-sized and may not fit perfectly into available memory holes[1, p. 19].
Key Points:
Logical division of memory into segments.
Each segment can grow or shrink independently.
Facilitates sharing and protection.
May cause external fragmentation.
Q77
78. Explain demand paging with suitable examples. (4 marks)
Demand paging is a memory management technique where pages are loaded into memory only when they are needed, rather than loading the entire process at once. When a program tries to access a page that is not in memory, a page fault occurs, and the operating system loads the required page from secondary storage into memory[1, p. 28].
Example:
Suppose a process requires five pages but only the first two are loaded into memory initially. When the process tries to access the third page, a page fault occurs. The OS then loads the third page into memory, possibly replacing another page if memory is full. This approach saves memory and reduces initial load time, but frequent page faults can degrade performance (a condition known as thrashing)[1, p. 28; internet sources].
Advantages:
Efficient memory usage.
Faster program startup.
Disadvantages:
Page faults can cause delays.
Excessive page faults lead to thrashing.
80. Write short notes on:
a) Translation Lookaside Buffer (TLB)
b) Page replacement algorithms
(4 marks)
a) Translation Lookaside Buffer (TLB):
A TLB is a special, fast cache memory that stores recent translations of virtual page numbers to physical frame numbers. When the CPU generates a virtual address, the TLB is checked first. If the page number is found (TLB hit), the corresponding frame number is retrieved quickly. If not (TLB miss), the page table in main memory is accessed, and the TLB is updated[1, p. 25]. The TLB significantly speeds up address translation and reduces the average memory access time.
b) Page Replacement Algorithms:
Page replacement algorithms determine which memory page to replace when a new page needs to be loaded, and memory is full. Common algorithms include:
FIFO (First-In, First-Out): Replaces the oldest page in memory.
LRU (Least Recently Used): Replaces the page that has not been used for the longest time.
Optimal: Replaces the page that will not be used for the longest period in the future (theoretical, used for comparison).
These algorithms aim to minimize page faults and optimize system performance[1, p. 29; internet sources].
References:
-1 OS-M5-memory-management.pdf (Page numbers as indicated above)
Additional explanations and examples are supplemented from standard operating system literature and online resources as explicitly stated.
82. What is the difference between simple paging and virtual memory paging? (4 marks)
Simple paging is a memory management technique where both primary memory and the process address space are divided into equal-sized frames and pages. In simple paging, all pages of a process must be loaded into main memory for the process to run. This means the entire process must reside in memory, and only then can it be executed. There is no concept of bringing in pages on demand; the process is loaded as a whole (Reference not found in attached document, answer generated from internet source 2).
Virtual memory paging, on the other hand, allows only the currently needed pages of a process to reside in main memory. This is usually implemented using demand paging, where pages are loaded into memory only when they are required during execution. Not all pages need to be in memory at once, which allows processes larger than physical memory to execute and improves memory utilization. If a required page is not in memory, a page fault occurs, and the operating system loads the required page from secondary storage. Thus, virtual memory paging enables efficient use of memory and supports multitasking by allowing more processes to be loaded into memory at the same time2.
Key Differences:
In simple paging, all pages of a process must be in memory to run; in virtual memory paging, only the required pages are loaded as needed.
Virtual memory paging supports demand paging and page replacement policies, while simple paging does not.
Virtual memory allows processes larger than physical memory to execute, which is not possible in simple paging.
(Reference: Not found in attached document; answer generated from internet source2.)
83. What is paging and explain address translation in a paging system with suitable diagrams. (4 marks)
Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory and thus avoids external fragmentation. In paging, the logical memory is divided into fixed-size blocks called pages, and the physical memory is divided into blocks of the same size called frames. The operating system maintains a page table for each process, which keeps track of the mapping between pages and frames.
Address Translation in Paging:
The logical (virtual) address generated by the CPU is divided into two parts: a page number (p) and a page offset (d).
The page number is used as an index into the page table to find the corresponding frame number.
The frame number is then combined with the offset to form the physical address.
Diagram:
text Logical Address (p, d) | v +---------------------+ | Page Table | +---------------------+ | v Physical Frame Number + Offset (d) | v Physical Address
Explanation:
For example, if the CPU generates a logical address (p, d), the paging hardware uses the page number (p) to look up the frame number in the page table. The physical address is then constructed by concatenating the frame number with the offset (d).
(Reference: Not found in attached document; answer generated from standard OS concepts and internet sources.)
84. What is the difference between internal and external fragmentation? (4 marks)
Internal fragmentation occurs when memory is divided into fixed-sized blocks (such as in paging or fixed partitioning), and the memory allocated to a process is slightly larger than the memory requested. The unused space within the allocated block is wasted and is called internal fragmentation. For example, if a process needs 18 KB and is allocated a 20 KB block, the 2 KB is internally fragmented3.
External fragmentation occurs when memory is allocated in variable-sized blocks (such as in segmentation or dynamic partitioning). Over time, as processes are loaded and removed, free memory is split into small, non-contiguous holes. Even if the total free memory is enough to satisfy a request, it may not be contiguous, and thus cannot be allocated. This unused scattered memory is known as external fragmentation3.
Internal Fragmentation
External Fragmentation
Occurs with fixed-sized partitions
Occurs with variable-sized partitions
Wasted space is inside allocated block
Wasted space is outside allocated block
Solution: Best-fit block allocation
Solution: Compaction, paging, segmentation
(Reference: Not found in attached document; answer generated from internet source3.)
86. What is Page table and Inverted page table? (4 marks)
Page Table:
A page table is a data structure used by the operating system to store the mapping between virtual page numbers and physical frame numbers. Each process has its own page table, and it is used during address translation to locate the physical address corresponding to a virtual address. The page table contains entries for each page, including the frame number and additional information like valid/invalid bits4.
Inverted Page Table:
An inverted page table has one entry for each physical frame in memory, rather than for each virtual page. It stores information about which virtual page (and which process) is stored in each physical frame. There is only one inverted page table for the entire system, which helps to reduce memory overhead, especially when there are many processes. However, address translation can be slower because the system may need to search the table to find the mapping4.
Page Table
Inverted Page Table
One per process
One for the entire system
Maps virtual pages to frames
Maps frames to virtual pages/processes
Can be large for big address space
More memory-efficient for large systems
87. What are the distinctions among logical, relative and physical addresses? (4 marks)
Logical Address:
A logical address (also called virtual address) is generated by the CPU during program execution. It is the address used by a process to access memory. The logical address is mapped to a physical address by the memory management unit (MMU)5.
Physical Address:
A physical address is the actual location in main memory (RAM) where data or instructions are stored. The physical address is what the memory hardware uses to access data5.
Relative Address:
A relative address is an address specified relative to a known point, such as the start of a program or a segment. It represents an offset from a base address. The relative address is converted to a logical address by adding it to the base address.
Summary Table:
Address Type
Definition
Who uses it?
Example
Logical Address
Generated by CPU, used by processes
User/program
0x00001234
Relative Address
Offset from a known base (e.g., base + 200)
Program/Compiler
base + 200
Physical Address
Actual address in RAM where data is stored
Memory hardware
0xFF001234
88. What is the difference between a page and a frame?
What is the difference between a page and a segment?
(Un)
Difference between a Page and a Frame:
A page is a fixed-size block of virtual memory used by processes. It is the basic unit of data for memory management in the virtual address space. Pages are created by dividing the logical address space of a process into equal-sized chunks.
A frame is a fixed-size block of physical memory (RAM). Physical memory is divided into frames, each of which can hold one page. The operating system maps pages to frames using a page table, allowing the virtual address space to be mapped onto physical memory.
Pages exist in the virtual (logical) address space, while frames exist in the physical address space.
The size of a page is always equal to the size of a frame, ensuring a one-to-one mapping between them 2 6.
Aspect
Page (Virtual)
Frame (Physical)
Location
Virtual/Logical memory
Physical memory (RAM)
Purpose
Divides process address space
Divides main memory
Managed by
OS (via page tables)
OS (via frame allocation)
Difference between a Page and a Segment:
A page is a fixed-size block of memory, both in the logical and physical address space. Paging divides the process address space into equal-sized pages, and main memory into equal-sized frames.
A segment is a variable-sized logical unit of memory, such as a function, array, or data structure. Segmentation divides the process address space into segments of different lengths, which correspond to logical divisions in the program (e.g., code, data, stack).
Pages are fixed in size; segments are variable in size.
Paging is managed purely by the operating system, while segmentation is often influenced by the structure of the program (compiler/user).
Paging can cause internal fragmentation; segmentation can cause external fragmentation.
Aspect
Page
Segment
Size
Fixed
Variable
Division basis
OS divides memory into pages
Program divides memory into segments
Fragmentation
Internal fragmentation possible
External fragmentation possible
Address
Page number + offset
Segment number + offset
(References: 2 3 6)
89. Explain with neat diagram hardware architecture of “Inverted Paging”
(Un)
Inverted Page Table Architecture:
An Inverted Page Table (IPT) is a system-wide data structure used in virtual memory systems to map physical memory frames to virtual pages. Unlike traditional page tables (which have one page table per process and an entry for every virtual page), the IPT has one entry per physical frame and is shared by all processes. Each entry contains:
The virtual page number
The process ID (PID)
Control bits (valid, dirty, etc.)
Chained pointer (for shared memory or hash collisions)
Working:
When the CPU generates a virtual address, the MMU (Memory Management Unit) uses the combination of PID and virtual page number to search the IPT. If a match is found, the corresponding physical frame is used; otherwise, a page fault occurs.
Diagram:
text +-------------------+ +---------------------+ | Virtual Address | | Inverted Page Table | | (PID, Page No.) | ----> |---------------------| +-------------------+ | PID | Page | Frame | |-----|------|--------| | ... | ... | ... | +---------------------+ | v +-------------------+ | Physical Address | +-------------------+
Advantages:
Reduces memory overhead for page tables, especially in systems with large address spaces or many processes.
Only one IPT for the entire system.
Disadvantages:
Lookup can be slower, often requiring associative search or hashing.
More complex to implement shared memory4.
(Reference: 4)
90. Explain with neat diagram hardware architecture of “Segmented Paging”
(Un)
Segmented Paging Architecture:
Segmented Paging combines both segmentation and paging to leverage the benefits of both techniques. Here, the logical address space is divided into segments (as in segmentation), and each segment is further divided into pages (as in paging).
Address Structure:
Logical address: (Segment Number, Page Number, Offset)
Each process has a segment table. Each entry in the segment table points to a page table for that segment.
The page table maps the segment’s pages to physical frames.
Translation Steps:
1.
CPU generates a logical address (segment no., page no., offset).
2.
Segment number indexes into the segment table to find the base address of the page table for that segment.
3.
Page number indexes into the page table to get the frame number.
4.
Frame number combined with offset gives the physical address.
Diagram:
text +-------------------+ +------------------+ +-------------------+ | Logical Address | | Segment Table | | Page Table | | (Seg, Pg, Offset) |-----> | Seg# | PT Base |----> | Pg# | Frame # | +-------------------+ +------------------+ +-------------------+ | v +-------------------+ | Physical Address | +-------------------+
Advantages:
Supports logical program structure (segmentation) and efficient memory use (paging).
Reduces external fragmentation compared to pure segmentation.
Disadvantages:
More complex hardware and address translation.
Both segment and page tables must be managed5.
(Reference: 5)
91. What is the difference between a field and a record?
In the context of data storage and management, fields and records are fundamental concepts that define the structure of data organization. A field is the smallest unit of named data that has meaning in a record. Think of it as an attribute or a single piece of information about an item. For example, in a database of students, fields might include "StudentID," "Name," "Major," and "GPA." Each field is designed to hold a specific type of data, such as text, numbers, or dates.
A record, on the other hand, is a collection of related fields that are treated as a single unit. A record represents a complete set of information about a particular entity or item. In our student database example, a record would consist of all the fields (StudentID, Name, Major, GPA) for one particular student. So, a record provides a holistic view of all attributes of a single entity.
In essence, fields are the columns in a table, while records are the rows. The key difference is that a field holds a single piece of information, while a record contains all the related pieces of information about a single instance (Reference: general database knowledge).
92. What is the difference between a file and a database?
A file is a self-contained collection of data, typically stored as a single unit on a storage device. Files are managed by the operating system's file system. They can contain various types of data, such as text, images, or executable code. Files are a basic way to store information, but they often lack advanced organizational features.
A database is a structured collection of data that is organized and managed to allow efficient storage, retrieval, and manipulation of data. Databases are managed by a Database Management System (DBMS), which provides a set of tools and features for defining data structures, enforcing data integrity, and managing access control.
The main differences lie in structure, management, and functionality. Files are simple, unstructured, and managed by the operating system. Databases are structured, managed by a DBMS, and offer advanced features like indexing, querying, and transaction management. Databases are designed for complex data relationships and efficient data handling, while files are best suited for simple data storage needs (Reference: general database knowledge).
93. What is a file management system? Explain with a neat diagram elements of file management.
A File Management System (FMS) is a software system (or part of an operating system) that manages files and directories on a storage device. It provides a structured way to organize, store, retrieve, and manipulate files. The FMS is responsible for maintaining metadata about files, such as their names, sizes, locations, creation dates, and permissions.
Key elements of a file management system include:
Files: The basic unit of storage, containing data.
Directories (Folders): Containers for organizing files into a hierarchical structure.
Metadata: Information about files and directories, used for management and retrieval.
File Attributes: Properties associated with each file, such as read-only, hidden, etc.
Storage Allocation: Management of disk space to store files efficiently.
Access Control: Mechanisms to control who can access or modify files.
Here's a simple textual representation of a file management system's hierarchical structure:
text Root Directory (/) ├── Directory A │ ├── File 1.txt │ └── File 2.jpg ├── Directory B │ ├── Subdirectory C │ │ └── File 3.pdf │ └── File 4.docx └── File 5.exe
The FMS allows users and applications to interact with files and directories in a structured and organized manner, providing essential functions like creating, deleting, renaming, and moving files (Reference: general operating system knowledge).
94. What criteria are important in choosing a file organization?
When selecting a file organization method, several criteria must be considered to ensure efficiency and effectiveness:
Access Speed: The speed at which records can be accessed and retrieved. Different file organizations offer varying access speeds, depending on the access pattern (sequential, random, etc.).
Storage Efficiency: How efficiently the file organization utilizes storage space. Some methods may result in wasted space due to internal or external fragmentation.
Ease of Update: How easily records can be inserted, deleted, or modified. Some organizations may require significant reorganization overhead for updates.
Data Redundancy: The extent to which data is duplicated within the file. Minimizing redundancy is important for data consistency and storage efficiency.
Cost: The overhead associated with implementing and maintaining the file organization, including hardware and software requirements.
Application Requirements: The specific needs of the application, such as the type of data, access patterns, and performance requirements.
The choice of file organization should align with these criteria to optimize performance and resource utilization for the given application (Reference: general database knowledge).
95. List and briefly define five file organizations?
Five common file organizations include:
1.
Sequential File Organization: Records are stored in a linear sequence, typically sorted by a key field. Access is usually sequential, meaning records are processed in the order they are stored. Best suited for applications that require processing all records in a file.
2.
Indexed Sequential File Organization: Records are stored sequentially, but an index is also maintained to allow faster access to specific records. The index provides a mapping between key values and record locations. Combines the benefits of sequential access and indexed access.
3.
Direct or Random File Organization: Records are stored in a non-sequential manner, and each record can be accessed directly using a key or record number. Requires a mechanism to map keys to record locations, such as hashing. Suitable for applications that require fast random access to records.
4.
Hashed File Organization: Records are stored based on a hash function applied to a key field. The hash function maps keys to storage locations. Provides fast direct access to records, but collisions (multiple keys mapping to the same location) must be handled.
5.
B-Tree File Organization: A tree-based structure used for indexing and organizing records. B-trees are self-balancing, ensuring that access times remain relatively consistent even as the file grows. Commonly used in database management systems for efficient indexing.
Each file organization has its strengths and weaknesses, making them suitable for different types of applications and data access patterns (Reference: general database knowledge).
96. Why is the average search time to find a record in a file less for an indexed sequential file than for a sequential file.
The average search time in an indexed sequential file is generally less than in a sequential file due to the presence of an index. In a sequential file, to find a specific record, you might have to read through a large portion of the file, especially if the record is located towards the end. This results in a linear search time complexity, O(n), where n is the number of records.
In an indexed sequential file, an index is maintained, which maps key values to the corresponding record locations within the file. This index allows the system to quickly locate the approximate position of a record. The search process involves first searching the index to find the location of the desired record, and then directly accessing the record at that location.
The index search time is typically much faster than a linear search through the entire file. Once the index identifies the approximate location, a short sequential search may be needed within a smaller section of the file to locate the exact record. This combination of indexed and sequential access reduces the average search time, making indexed sequential files more efficient for applications that require both sequential and random access patterns (Reference: general database knowledge).
97. What are typical operations that may be performed on a directory?
Directories are essential components of a file system, and several operations can be performed on them to manage files and organize data effectively. Typical operations include:
Create: Creating a new directory allows users to organize files into logical groups and establish a hierarchical structure.
Delete: Deleting a directory removes it from the file system, along with all its contents (files and subdirectories).
List: Listing the contents of a directory displays the files and subdirectories it contains, providing an overview of the directory's structure.
Rename: Renaming a directory changes its name, allowing users to update directory names for better organization or clarity.
Move: Moving a directory changes its location in the file system hierarchy, allowing users to reorganize directories as needed.
Search: Searching for a file or subdirectory within a directory helps users quickly locate specific items without manually browsing the directory structure.
Set Permissions: Setting permissions on a directory controls who can access and modify its contents, ensuring data security and access control.
These operations provide users with the necessary tools to manage and organize their files and directories effectively within the file system (Reference: general operating system knowledge).
98. What is the relationship between a pathname and a working directory?
The pathname specifies the location of a file or directory in a file system. A pathname can be either absolute or relative.
An absolute pathname provides the complete path to a file or directory, starting from the root directory of the file system. It uniquely identifies the location of the item, regardless of the current working directory.
A relative pathname specifies the location of a file or directory relative to the current working directory. It is a shorthand way of specifying a path without having to type out the entire absolute path.
The working directory (also known as the current directory) is the directory in which the user or a process is currently operating. It serves as the starting point for resolving relative pathnames. When a relative pathname is used, the operating system interprets it with respect to the working directory to determine the actual location of the file or directory.
The relationship is that the working directory provides the context for interpreting relative pathnames. If the working directory is /home/user/documents, and you specify a relative pathname of report.txt, the operating system will interpret it as /home/user/documents/report.txt. This makes it easier to access files and directories within the current working area without having to specify the full absolute path each time (Reference: general operating system knowledge).
99. What are typical access rights that may be granted or denied to a particular user for a particular file?
Typical access rights specify what actions a user can perform on a particular file. These rights are fundamental for security and data protection. The main access rights include:
Read: This right allows the user to view the contents of the file. Without read access, the user cannot open or see the data within the file.
Write: This permits the user to modify the file's content. This includes adding, deleting, or altering any part of the file.
Execute: Applicable primarily to executable files or scripts, this right allows the user to run the file. Without execute permission, the file cannot be executed as a program.
Delete: This right enables the user to remove the file from the file system. It's a powerful permission that, when granted, allows the user to permanently erase the file.
Change Permissions: This advanced right allows a user to modify the access rights of the file for other users. It essentially grants the ability to control who can access the file and what they can do with it.
These rights can be granted or denied individually or in combinations. For example, a user might have read and execute permissions but not write or delete, providing them with the ability to use the file but not alter or remove it (Reference: Not found in attached document; generated from internet knowledge).
100. List and briefly define three techniques for performing I/O.
I/O techniques are methods by which data is transferred between the CPU/memory and external devices. Here are three key techniques:
1.
Programmed I/O (PIO): In Programmed I/O, the CPU directly controls the I/O operation. The CPU issues commands to the I/O device and then actively waits for the device to complete the operation. The CPU directly transfers data between memory and the I/O device. This method requires the CPU to be heavily involved in the I/O process, constantly monitoring the device's status. This can be inefficient as the CPU remains occupied during the entire I/O operation, unable to perform other tasks.
2.
Interrupt-Driven I/O: This technique aims to improve CPU utilization. The CPU initiates an I/O operation but, instead of waiting, continues with other tasks. When the I/O device completes the operation, it sends an interrupt signal to the CPU. Upon receiving the interrupt, the CPU suspends its current task, handles the data transfer, and then resumes the interrupted task. This allows the CPU to perform other computations while the I/O device is working, leading to better overall system efficiency.
3.
Direct Memory Access (DMA): DMA is a more advanced technique designed to offload I/O operations entirely from the CPU. A dedicated DMA controller is used to transfer data directly between the I/O device and memory, without involving the CPU. The CPU initiates the transfer by providing the DMA controller with the necessary information (source/destination addresses, data size, etc.). The DMA controller then takes over, managing the entire data transfer process. Once the transfer is complete, the DMA controller notifies the CPU via an interrupt. This method significantly reduces the CPU's involvement in I/O operations, allowing it to focus on other tasks and greatly improving system performance (Reference: Module-6-IO-Management.pdf, page not specified).
101. What is the difference between logical I/O and device I/O?
Logical I/O and Device I/O represent different levels of abstraction in the I/O process:
Logical I/O: This is the abstract interface that the operating system provides to applications for performing I/O operations. It deals with data in terms of logical units, such as records, blocks, or streams, without needing to know the specific details of the underlying hardware. Applications make requests based on logical concepts (e.g., "read record X from file Y"), and the OS handles the translation to the appropriate device-specific commands. Logical I/O provides a level of abstraction that simplifies programming and allows applications to work with different types of devices in a uniform way.
Device I/O: This involves the actual physical operations performed by the I/O device to transfer data. It deals with the specific hardware commands, device addresses, and data formats required by the particular I/O device. This layer is concerned with the low-level details of how data is physically read from or written to the device. The operating system's device drivers are responsible for translating logical I/O requests from applications into device I/O commands.
In essence, logical I/O is what applications see and use, while device I/O is how the operating system interacts with the hardware to fulfill those requests (Reference: Not found in attached document; generated from internet knowledge).
102. Write short note on DMA
Direct Memory Access (DMA) is a crucial hardware technique for efficient data transfer in computer systems. It allows certain hardware subsystems (e.g., disk controllers, network cards) to access system memory independently of the CPU.
The key idea behind DMA is to offload the task of data transfer from the CPU to a dedicated DMA controller. The process begins with the CPU initiating the transfer by providing the DMA controller with essential information: the source address (where the data is coming from), the destination address (where the data is going to), the amount of data to be transferred, and the direction of the transfer (read or write). Once configured, the DMA controller takes over and directly transfers data between the I/O device and memory, without further CPU intervention.
After the transfer is complete, the DMA controller notifies the CPU via an interrupt signal. This allows the CPU to continue executing other tasks while the data transfer is in progress, significantly improving system performance. DMA is particularly beneficial for high-speed I/O operations involving large amounts of data (Reference: Not found in attached document; generated from internet knowledge).
103. What is the difference between block-oriented devices and stream-oriented devices? Give a few examples of each.
Block-oriented and stream-oriented devices differ significantly in how they organize and access data:
Block-Oriented Devices: These devices store data in fixed-size blocks. Data transfers occur one block at a time. A key characteristic is that they support random access, meaning any block can be accessed directly without needing to read through preceding blocks. Examples of block-oriented devices include hard disk drives (HDDs), solid-state drives (SSDs), and USB flash drives. Block-oriented devices are suitable for applications that require random access to data, such as databases and file systems.
Stream-Oriented Devices: These devices transfer data as a continuous stream of bytes, without a fixed block structure. They typically only support sequential access, meaning data must be accessed in a specific order. Examples of stream-oriented devices include magnetic tapes, network sockets, and serial ports. Stream-oriented devices are often used for applications that involve continuous data flow, such as audio/video streaming and data logging.
The fundamental difference lies in the organization and access method: block-oriented devices use fixed-size blocks and allow random access, while stream-oriented devices use continuous streams and primarily support sequential access (Reference: Not found in attached document; generated from internet knowledge).
104. Why would you expect improved performance using a double buffer rather than a single buffer for I/O?
Double buffering improves I/O performance by enabling concurrent operations between the CPU and I/O devices.
Single Buffer: With a single buffer, the CPU or I/O device must wait for the other to complete its operation before proceeding. For example, the CPU might have to wait for the I/O device to fill the buffer with data before it can begin processing that data. Similarly, the I/O device might have to wait for the CPU to empty the buffer before it can write more data. This leads to idle time for both the CPU and the I/O device.
Double Buffer: With double buffering, two buffers are used. While one buffer is being filled or emptied by the I/O device, the CPU can simultaneously process the data in the other buffer. This overlapping of I/O operations with CPU processing significantly reduces idle time and increases overall system throughput. For instance, while the I/O device is writing data into one buffer, the CPU can be reading data from the other buffer.
Double buffering is effective in reducing the overhead associated with I/O operations, leading to improved performance, especially in scenarios where the CPU and I/O device have different processing speeds (Reference: Not found in attached document; generated from internet knowledge).
105. What delay elements are involved in a disk read or write?
Several delay elements contribute to the overall time taken for a disk read or write operation:
1.
Seek Time: This is the time required for the disk arm to move the read/write head to the correct cylinder (the track containing the desired data). Seek time is one of the most significant factors affecting disk access time, especially for random access patterns.
2.
Rotational Latency: Once the head is positioned over the correct cylinder, rotational latency is the time it takes for the desired sector to rotate under the read/write head. On average, the rotational latency is half the time it takes for a full rotation of the disk.
3.
Transfer Time: This is the actual time spent transferring the data between the disk and memory (or vice versa). The transfer time depends on the amount of data being transferred and the disk's transfer rate.
4.
Controller Overhead: This includes the time taken by the disk controller to process the I/O request, manage the data transfer, and perform any necessary error checking or correction.
Minimizing these delay elements is crucial for improving disk I/O performance. Techniques such as disk scheduling algorithms, caching, and defragmentation aim to reduce seek times, rotational latency, and overall overhead (Reference: Module-6-IO-Management.pdf, page 468).
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