Operating System (QUESTION BANK) (CSE)
Question and answer
UNIT-4
[NOTE: may be some answers of the above question are not contain proper syllabus topics ]
1. List and explain activities involved in Memory Management.
Memory management is a crucial aspect of operating systems that involves various activities to efficiently allocate and deallocate memory resources. The main activities involved in memory management are:
a. Memory Allocation: This activity involves allocating memory to processes or programs based on their memory requirements. The operating system keeps track of free memory blocks and assigns suitable blocks to processes when requested.
b. Memory Deallocation: Once a process no longer needs a memory block, the memory should be deallocated and made available for other processes to use. Memory deallocation involves releasing the allocated memory back to the system.
c. Memory Mapping: Memory mapping involves establishing a correspondence between logical addresses used by a process and physical addresses in the physical memory. This enables processes to access memory in a transparent manner, without being aware of the underlying physical memory organization.
d. Memory Protection: Memory protection ensures that each process can only access the memory assigned to it. It prevents unauthorized access and maintains data integrity and security.
e. Memory Sharing: In some cases, it is desirable for multiple processes to share memory regions. Memory management facilitates sharing of memory segments between processes, allowing them to communicate and cooperate efficiently.
f. Memory Compaction: As processes are allocated and deallocated memory, free memory blocks become scattered, leading to fragmentation. Memory compaction rearranges the allocated and free memory blocks to reduce fragmentation and improve memory utilization.
g. Memory Paging and Swapping: Paging and swapping are techniques used to manage memory when the available physical memory is insufficient to hold all active processes. Paging involves dividing the logical memory into fixed-size pages, while swapping moves entire processes or parts of them between main memory and secondary storage.
2. Explain the working of demand paging.
Demand paging is a memory management technique used in operating systems to optimize memory usage and reduce the need for extensive upfront memory allocation. In demand paging, instead of loading an entire program into memory, only the required pages are loaded when needed. The basic working of demand paging involves the following steps:
1. Initially, when a program starts execution, only a small portion, typically a few pages, is loaded into memory, known as the initial or resident set.
2. As the program executes, it generates memory references. When a memory reference is made to a page that is not present in memory, a page fault occurs.
3. When a page fault occurs, the operating system identifies the missing page and fetches it from secondary storage, such as a hard disk. The page is then loaded into an available page frame in the physical memory.
4. After the missing page is loaded, the program's execution is resumed, and the instruction that caused the page fault is re-executed.
5. The process continues, and pages are brought into memory on demand as long as there are available page frames. If the physical memory becomes full, the operating system selects a page to evict, typically using a page replacement algorithm, and replaces it with the required page.
Demand paging allows for efficient memory usage as only the necessary pages are loaded, reducing the initial memory requirements. It also allows programs to utilize more memory than available physical memory by utilizing secondary storage as an extension of the main memory. However, it introduces additional overhead due to page faults and the need to fetch pages from secondary storage.
3. Explain virtual memory management in detail.
Virtual memory management is a technique used by operating systems to provide each process with its own isolated and contiguous address space, known as the virtual address space. It allows programs to operate on a larger address space than the physical memory available by utilizing secondary storage, such as hard disks, as an extension of the main memory. The key components and concepts involved in virtual memory management are:
a. Virtual Address Space: Each process is provided with a virtual address space, which represents the range of addresses that the process can use. The virtual address space is divided into fixed-size units called pages.
b. Physical Memory: The physical memory consists of actual RAM available in the system and is divided into fixed-size units called page frames, which correspond to the size of the pages in the virtual address space.
c. Page Table: To establish the correspondence between virtual addresses and physical addresses, each process has a page table. The page table is a data structure maintained by the operating system that maps virtual pages to physical page frames. Each entry in the page table contains the physical address of the corresponding page frame.
d. Page Fault: When a process accesses a virtual address that is not currently in physical memory, a page fault occurs. The operating system handles the page fault by bringing the required page into memory from secondary storage and updating the page table to reflect the new mapping.
e. Page Replacement: If there are no free page frames available in physical memory when a page fault occurs, the operating system needs to select a page to evict and make room for the incoming page. Page replacement algorithms, such as Least Recently Used (LRU) or Optimal, are used to determine which page to evict based on past and predicted future usage.
f. Page Swapping: In situations where the available physical memory is insufficient to hold all active processes, the operating system can swap entire pages or processes between main memory and secondary storage. Swapping involves moving pages out of memory to free up space for other pages.
g. Demand Paging: Virtual memory systems often incorporate demand paging, which is a technique where pages are loaded into memory only when they are accessed by the process. This reduces the initial memory requirements and allows for more efficient memory usage.
Virtual memory management provides several benefits, including increased address space for processes, efficient memory utilization, and improved multitasking by allowing more processes to run simultaneously. However, it also introduces additional overhead due to page faults, page table management, and disk I/O operations. Effective page replacement algorithms and efficient handling of page faults are crucial for optimizing the performance of virtual memory systems.
4. Explain memory protection by using relocation and limit registers as strategies used to solve the dynamic storage allocation problem (First fit, best fit, worst fit).
Memory protection is an essential aspect of memory management in operating systems. It ensures that each process can only access the memory assigned to it and prevents unauthorized access or modification of other processes' memory. Relocation and limit registers are strategies used to implement memory protection and solve the dynamic storage allocation problem. The main components and strategies involved are:
a. Relocation Registers: Relocation registers are used to implement the relocation strategy. Each process is assigned a base register that contains the starting physical address of its allocated memory block. When a process generates a memory reference, the base register is added to the virtual address to obtain the corresponding physical address. This ensures that the process can only access its allocated memory region.
b. Limit Registers: Limit registers work in conjunction with relocation registers to enforce memory protection. Each process is assigned a limit register that specifies the size of the allocated memory block. When a process generates a memory reference, the limit register is checked to ensure that the virtual address falls within the allocated memory range. If the address exceeds the limit, a hardware exception or interrupt is generated, indicating an illegal memory access.
c. Dynamic Storage Allocation: The dynamic storage allocation problem refers to the challenge of efficiently allocating and deallocating memory blocks for processes at runtime. Several strategies, such as first fit, best fit, and worst fit, are used to address this problem.
- First Fit: The first fit strategy searches the memory space for the first available block that is large enough to accommodate the process. It allocates the process to that block and splits the block into two parts if necessary. This strategy is relatively fast but may lead to fragmentation.
- Best Fit: The best fit strategy searches the memory space for the smallest available block that is large enough to accommodate the process. It selects the block that minimizes internal fragmentation. This strategy can lead to more efficient memory utilization but requires additional time for searching.
- Worst Fit: The worst fit strategy searches the memory space for the largest available block. It allocates the process to that block, leaving behind the largest possible fragment. This strategy can result in significant external fragmentation.
By combining relocation and limit registers with dynamic storage allocation strategies, memory protection is enforced, and processes are allocated and deallocated memory blocks efficiently at runtime. These strategies help maintain data integrity, prevent unauthorized access, and optimize memory utilization based on the specific requirements of the system and processes.
Note: The terms "first fit," "best fit," and "worst fit" mentioned here are associated with dynamic storage allocation rather than memory protection.
5. Describe paging using a translation look-aside buffer.
Paging is a memory management technique that divides the logical address space of a process into fixed-size units called pages. The physical memory is also divided into fixed-size units called page frames. The translation look-aside buffer (TLB) is a hardware cache used to accelerate the translation process between logical addresses and physical addresses in paging systems.
The process of paging using a TLB can be described as follows:
1. Page Table: Each process has a page table that maps the logical pages to physical page frames. The page table resides in the main memory.
2. TLB Structure: The TLB is a small, high-speed cache that stores a subset of the page table entries. It contains the most recently accessed page-to-frame mappings, allowing for faster address translation.
3. Address Translation: When a process generates a logical address, the memory management unit (MMU) receives the address and checks the TLB for a matching entry.
4. TLB Hit: If the TLB contains the required page-to-frame mapping, a TLB hit occurs. The MMU retrieves the corresponding physical frame number from the TLB and combines it with the page offset to generate the physical address.
5. TLB Miss: If the TLB does not contain the required mapping, a TLB miss occurs. The MMU consults the page table in the main memory to find the appropriate page-to-frame mapping.
6. Page Table Lookup: The MMU uses the page number from the logical address to access the page table in the main memory. It retrieves the corresponding physical frame number and updates the TLB with this mapping.
7. TLB Update: After the page table lookup, the MMU updates the TLB with the new page-to-frame mapping to improve future address translations.
8. Address Translation (Continued): Once the TLB is updated with the new mapping, the MMU performs the address translation again. This time, it finds the required mapping in the TLB and generates the physical address.
9. Memory Access: The MMU provides the physical address to the memory system, allowing the process to access the desired memory location.
The TLB acts as a cache for frequently accessed page-to-frame mappings, reducing the need for accessing the page table in the main memory for every address translation. This speeds up the translation process and improves overall system performance. However, if a required mapping is not present in the TLB (TLB miss), additional time is required to access the page table, resulting in a slightly longer translation time.
The TLB is typically implemented as a content-addressable memory (CAM) or associative memory, allowing for efficient parallel searches and fast access times. Its size and organization can vary depending on the specific hardware implementation and system requirements.
6. Explain LRU and Optimal page replacement algorithms.
LRU (Least Recently Used) and Optimal are page replacement algorithms used in virtual memory management to decide which page to evict from the memory when a page fault occurs. Let's understand each algorithm:
1. LRU (Least Recently Used) Algorithm:
The LRU algorithm selects the page for replacement based on the least recently used criterion. It assumes that the pages that have not been accessed for the longest duration are the least likely to be used in the near future.
Working of LRU algorithm:
- Each page in memory is associated with a timestamp or a counter that keeps track of the time of the last access.
- When a page fault occurs and a new page needs to be brought into memory, the algorithm selects the page with the oldest timestamp for replacement.
- The timestamp of a page is updated every time the page is accessed, keeping it up to date.
- This algorithm aims to minimize the number of page faults by evicting the page that has been accessed the least recently.
2. Optimal Page Replacement Algorithm:
The Optimal algorithm, also known as the clairvoyant algorithm, makes the best possible decision for page replacement by assuming that it knows the future references of pages. It selects the page that will not be used for the longest duration in the future.
Working of Optimal algorithm:
- The Optimal algorithm requires knowledge of the future page references, which is not practically feasible.
- It is used as a theoretical benchmark to evaluate the efficiency of other page replacement algorithms.
- The algorithm determines the page that will be referenced farthest into the future and replaces that page.
- Since it requires future information, it cannot be implemented directly in real systems but serves as a reference for comparison.
The LRU algorithm is commonly used in practical systems as it provides a reasonable approximation to the optimal algorithm. Although LRU requires additional bookkeeping overhead to maintain the access timestamps, it is effective in minimizing the number of page faults by evicting the least recently used page. However, implementing a true optimal algorithm is not feasible in real-world scenarios due to the inability to predict future page references accurately.
7. What is locality of reference? How is this principle used in virtual memory? Calculate logical and physical address bits given a logical address space of 32 pages of 1024 words per page mapped to physical memory of 16 frames.
Locality of reference is a principle in computer systems where memory accesses tend to cluster around certain locations or regions. It refers to the phenomenon that programs often access a relatively small portion of their address space at any given time, exhibiting temporal and spatial locality.
In virtual memory, the principle of locality of reference is leveraged to optimize memory access. The system exploits the fact that a process is likely to access nearby memory locations in the near future. By using paging or segmentation techniques, only a portion of the address space that is actively being used by the process needs to be loaded into physical memory, while the rest can reside in secondary storage.
Calculating logical and physical address bits:
Given:
- Logical address space: 32 pages of 1024 words per page
- Physical memory: 16 frames
To calculate the number of bits required for addressing:
1. Logical Address Space:
The logical address space consists of 32 pages. Since there are 1024 words per page, we can calculate the total number of logical addresses as follows:
Total number of logical addresses = Number of pages × Number of words per page
= 32 × 1024
= 32,768
To represent 32,768 addresses, we need log2(32,768) bits:
Number of bits for logical address = log2(32,768)
= log2(2^15)
= 15 bits
2. Physical Memory:
The physical memory consists of 16 frames. Each frame has the same size as a page, which is 1024 words. Thus, the total number of physical addresses is:
Total number of physical addresses = Number of frames × Number of words per frame
= 16 × 1024
= 16,384
To represent 16,384 addresses, we need log2(16,384) bits:
Number of bits for physical address = log2(16,384)
= log2(2^14)
= 14 bits
Therefore, the logical address requires 15 bits, and the physical address requires 14 bits in the given scenario.
The utilization of the locality of reference principle in virtual memory allows for efficient memory management by dynamically swapping pages between physical memory and secondary storage based on the active working set of a process. This approach minimizes the need to keep the entire address space in physical memory, reducing the memory requirements and allowing more processes to run concurrently without excessive memory usage.
a. Relocation and compaction.
b. Paging and demand paging.
c. Swapping and thrashing.
d. Paging and segmentation.
e. Logical and physical address.
a. Relocation and compaction:
- Relocation: Relocation refers to the process of moving a program or process from one area of memory to another. It is performed to allow the program to execute correctly regardless of the actual memory location it is loaded into. Relocation involves updating the program's references and addressing modes to reflect the new memory location.
- Compaction: Compaction is the process of rearranging the memory to reduce fragmentation. It involves moving allocated memory blocks closer together, filling in gaps left by deallocated memory. Compaction helps to maximize memory utilization by reducing external fragmentation.
b. Paging and demand paging:
- Paging: Paging is a memory management scheme that divides the logical address space and physical memory into fixed-size blocks called pages and page frames, respectively. It allows for non-contiguous allocation of memory and enables efficient virtual memory management.
- Demand Paging: Demand paging is a technique used in virtual memory systems where pages are brought into physical memory only when they are required. Instead of loading the entire program into memory at once, demand paging loads the pages on-demand as the program accesses them. This helps in conserving memory resources and improving overall system performance.
c. Swapping and thrashing:
- Swapping: Swapping is the process of moving an entire process from main memory to secondary storage (such as disk) and bringing it back when needed. It is used when the system needs to free up memory space for other processes or when a process is not actively executing. Swapping allows for efficient memory management and enables the execution of larger processes than the available physical memory.
- Thrashing: Thrashing refers to a situation where a system spends a significant amount of time and resources continuously swapping pages in and out of memory. It occurs when the system is heavily overloaded, and the processor spends more time swapping pages than executing useful work. Thrashing leads to a severe decline in system performance and is often caused by an insufficient amount of physical memory for the workload.
d. Paging and segmentation:
- Paging: Paging is a memory management technique that divides the logical address space and physical memory into fixed-size blocks called pages and page frames, respectively. It enables efficient memory allocation and address translation by mapping logical pages to physical frames. Paging allows for non-contiguous allocation of memory and helps in implementing virtual memory.
- Segmentation: Segmentation is a memory management technique where the logical address space of a process is divided into variable-sized segments. Each segment represents a logical unit such as a code segment, data segment, or stack segment. Segmentation provides a flexible memory model that allows processes to grow or shrink dynamically. It simplifies memory allocation but requires additional hardware support for address translation.
e. Logical and physical address:
- Logical Address: A logical address is an address generated by a program or process in its own address space. It represents a location in the logical address space, which may be larger than the actual physical memory available. Logical addresses need to be translated into physical addresses for actual memory access.
- Physical Address: A physical address refers to the actual location in the physical memory or RAM. It represents the location where data is stored. Physical addresses are used by the memory management unit (MMU) to access the corresponding memory location and retrieve or store data.
In summary, relocation involves moving a program or process to a different memory location, while compaction rearranges memory to reduce fragmentation. Paging divides the address space into fixed-size blocks, while demand paging loads pages on-demand. Swapping moves entire processes between memory and secondary storage, while thrashing is excessive swapping that leads
to poor performance. Paging enables non-contiguous memory allocation, while segmentation divides the address space into variable-sized segments. Logical addresses are program-generated addresses in the logical address space, while physical addresses represent actual memory locations in physical memory.
9. What is thrashing? What are the causes of thrashing? How can the effects of thrashing be limited?
Thrashing refers to a situation in virtual memory systems where the system spends a significant amount of time and resources continuously swapping pages in and out of memory, resulting in low overall system performance. It occurs when the system is overloaded and unable to provide enough physical memory to meet the demands of the running processes.
Causes of thrashing:
1. Insufficient Physical Memory: When the system does not have enough physical memory to accommodate the working set of active processes, it leads to excessive page swapping between memory and disk.
2. High Degree of Multiprogramming: Running too many processes concurrently, especially those with large memory footprints, can exceed the available physical memory capacity and result in frequent page faults and excessive swapping.
3. Inadequate Page Replacement Policies: Inefficient page replacement algorithms that fail to identify and evict nonessential pages from memory can contribute to thrashing. For example, a poorly implemented page replacement algorithm may repeatedly bring back pages into memory that are not actually needed.
4. Improper Process Scheduling: Unequal allocation of CPU time and resources among processes can exacerbate thrashing. If some processes are given excessive resources, they may keep swapping pages unnecessarily, starving other processes and leading to overall system thrashing.
Effects of thrashing can include:
1. Severe Degradation in Performance: Thrashing consumes significant CPU and disk I/O resources, resulting in poor system responsiveness and slower execution of processes. The system becomes unresponsive, and the throughput decreases.
2. Increased Response Time: The excessive swapping of pages leads to longer page fault resolution times, causing delays in accessing required data. This results in slower response times for processes.
3. Poor CPU Utilization: As the system spends more time swapping pages instead of executing useful work, the CPU utilization decreases, and system efficiency is compromised.
To limit the effects of thrashing, the following measures can be taken:
1. Increasing Physical Memory: Adding more physical memory to the system can alleviate thrashing by providing sufficient space to accommodate the working set of active processes. This reduces the frequency of page faults and the need for excessive swapping.
2. Optimizing Page Replacement Policies: Using efficient page replacement algorithms, such as the LRU (Least Recently Used) algorithm, can help identify and evict the least recently used pages, ensuring that the most relevant and necessary pages remain in memory.
3. Tuning Process Scheduling: Balancing the allocation of CPU time and resources among processes can prevent certain processes from monopolizing resources and causing excessive page swapping. Implementing fair process scheduling algorithms can help distribute resources more evenly.
4. Working Set Model: Implementing the working set model helps in identifying the set of pages that a process actively requires during its execution. By keeping the working set in memory, the frequency of page faults and swapping can be reduced.
5. Efficient I/O Handling: Improving disk I/O performance through techniques like disk scheduling algorithms and disk caching can minimize the impact of page swapping on overall system performance.
By addressing the causes of thrashing and implementing appropriate strategies, the effects of thrashing can be limited, leading to improved system performance and responsiveness.
10. Given memory partitions of 100 KB, 500 KB, 200 KB, 300 KB, and 600 KB, how would each of the first fit, best fit, and worst fit algorithms place processors of sizes 417 KB, 112 KB, and 426 KB (in order)? Which algorithm makes efficient use of memory?
To determine how each algorithm (first fit, best fit, and worst fit) would place processors of sizes 417 KB, 112 KB, and 426 KB into the given memory partitions, let's go through each algorithm:
1. First Fit:
The first fit algorithm allocates the first available partition that is large enough to accommodate the process.
- For the processor size of 417 KB:
- First fit would allocate it to the 500 KB partition, leaving 83 KB of unused space in that partition.
- For the processor size of 112 KB:
- First fit would allocate it to the 200 KB partition, leaving 88 KB of unused space in that partition.
- For the processor size of 426 KB:
- First fit would allocate it to the 600 KB partition, leaving 174 KB of unused space in that partition.
2. Best Fit:
The best fit algorithm allocates the smallest available partition that is large enough to accommodate the process.
- For the processor size of 417 KB:
- Best fit would allocate it to the 500 KB partition, leaving 83 KB of unused space in that partition.
- For the processor size of 112 KB:
- Best fit would allocate it to the 200 KB partition, leaving 88 KB of unused space in that partition.
- For the processor size of 426 KB:
- Best fit would allocate it to the 500 KB partition, leaving 74 KB of unused space in that partition.
3. Worst Fit:
The worst fit algorithm allocates the largest available partition to the process, leaving the most unused space.
- For the processor size of 417 KB:
- Worst fit would allocate it to the 600 KB partition, leaving 183 KB of unused space in that partition.
- For the processor size of 112 KB:
- Worst fit would allocate it to the 200 KB partition, leaving 88 KB of unused space in that partition.
- For the processor size of 426 KB:
- Worst fit would allocate it to the 600 KB partition, leaving 174 KB of unused space in that partition.
Efficient use of memory:
In terms of efficient use of memory, the best fit algorithm tends to utilize the memory most effectively. It selects the smallest available partition that can accommodate the process, leaving less wasted space compared to first fit and worst fit. However, it may result in more fragmentation over time as smaller partitions get filled up.
11. What is segmentation? Explain briefly the segmentation hardware.
Segmentation is a memory management technique that divides the logical address space of a process into variable-sized segments. Each segment represents a logical unit or a functional unit of the program, such as a code segment, data segment, stack segment, or heap segment. Segmentation allows for a flexible memory model where segments can dynamically grow or shrink based on the needs of the process.
Segmentation hardware refers to the hardware components and mechanisms involved in implementing the segmentation memory management scheme. These components work together to facilitate the mapping of logical addresses to physical addresses.
The segmentation hardware consists of the following key components:
1. Segment Table: The segment table is a data structure maintained by the operating system that stores information about each segment of a process. It typically includes entries for each segment, containing the base address and the length or limit of the segment. The segment table is used during address translation to map logical addresses to physical addresses.
2. Segment Descriptor: A segment descriptor is associated with each segment in the segment table. It contains additional information about the segment, such as access permissions (read, write, execute), segment type, and other attributes. The segment descriptor helps enforce memory protection by controlling the access rights of processes to specific segments.
3. Segment Selector: The segment selector is an identifier used by the processor to identify the segment to which a memory access belongs. It is typically an index or a pointer to a segment descriptor in the segment table. The segment selector is part of the logical address and is used during the address translation process.
4. Segment Register: The segment register is a processor register that holds the segment selector value for the current executing process. The processor uses the segment register to fetch the corresponding segment descriptor from the segment table during address translation.
5. Segment Translation: When a program generates a logical address, the segmentation hardware performs the address translation by using the segment register and the segment table. The hardware retrieves the segment descriptor based on the segment selector in the segment register and obtains the base address and limit of the segment. The logical address is then combined with the segment base address to generate the corresponding physical address.
6. Protection Mechanism: Segmentation hardware provides memory protection by enforcing access permissions specified in the segment descriptor. It checks the access rights of a process before allowing memory operations on a specific segment. This helps ensure that processes can only access memory areas they are authorized to, preventing unauthorized access and enhancing system security.
Overall, segmentation provides a flexible memory management scheme by dividing the logical address space into segments. The segmentation hardware, including the segment table, segment descriptors, segment selectors, and segment registers, facilitates the translation of logical addresses to physical addresses and enforces memory protection mechanisms.
14. What are the actions taken by the operating system if a page fault occurs in the system?
When a page fault occurs in the system, the operating system takes the following actions:
1. Page Fault Interrupt: The occurrence of a page fault triggers a page fault interrupt, which transfers control to the operating system.
2. Interrupt Handler: The operating system's interrupt handler receives the page fault interrupt and begins executing.
3. Page Fault Handler: The page fault handler is a part of the operating system's memory management subsystem. It is responsible for handling page faults and resolving them.
4. Check Page Table: The page fault handler checks the page table to determine the location of the required page in the secondary storage (such as the hard disk).
5. Fetch Required Page: If the required page is not present in the physical memory (RAM), the page fault handler initiates a page replacement algorithm to select a victim page to be replaced. It then schedules a disk I/O operation to fetch the required page from the secondary storage into an available page frame in the physical memory.
6. Update Page Table: Once the required page is brought into the physical memory, the page fault handler updates the page table to reflect the new mapping between the logical page and the corresponding physical page frame.
7. Resume Process Execution: After handling the page fault, the page fault handler updates the process control block (PCB) of the interrupted process to indicate that the required page is now in the physical memory. The interrupted process is then allowed to resume execution from the point where it was interrupted.
8. Repeat Instruction: The instruction that caused the page fault is re-executed to ensure that the process can proceed without any interruption due to the same page fault.
By following these steps, the operating system manages page faults by bringing required pages into the physical memory when they are not present, ensuring efficient memory utilization and uninterrupted process execution.
15. How is logical to physical address translation done in paging? Explain with an example.
In paging, logical to physical address translation is done using a page table. The page table is a data structure maintained by the operating system that maps logical page numbers to physical page frames in the memory. The page table enables the translation of a logical address to a physical address.
Here's an example to illustrate the process of logical to physical address translation in paging:
Assume we have a system with a 32-bit address space and a page size of 4 KB (2^12 bytes). This means each page contains 2^12 bytes of data.
1. Logical Address Format:
- The 32-bit logical address consists of two parts: the page number and the page offset.
- Let's assume the page number occupies the most significant 20 bits, and the page offset occupies the least significant 12 bits.
2. Page Table:
- The page table is maintained by the operating system and resides in the main memory.
- It contains entries that map logical page numbers to physical page frame numbers.
- Each entry typically consists of a page number and a corresponding physical page frame number.
3. Translation Process:
- When a process generates a logical address, it is divided into the page number and the page offset.
- The page number is used to index the page table, retrieving the corresponding entry.
- The entry contains the physical page frame number.
- The page offset remains unchanged.
4. Example:
- Let's say we have a logical address of 0x3A7D8, which is 0011 1010 0111 1101 1000 in binary.
- The page number is the most significant 20 bits: 0011 1010 0111 1101 10.
- The page offset is the least significant 12 bits: 00 0111 1101 1000.
- Using the page number, we look up the corresponding entry in the page table.
- Suppose the page table entry for this page number contains the physical page frame number 0x7B.
- The physical address is formed by combining the physical page frame number with the page offset:
- Physical address: 0x7B << 12 | 0x7D8 = 0x7B7D8.
In this example, the logical address 0x3A7D8 is translated to the physical address 0x7B7D8 using paging and the page table. The page number determines the page table entry, and the physical page frame number combined with the page offset forms the physical address.
The process of logical to physical address translation allows the operating system to provide virtual memory to processes, enabling them to access a larger address space than the physical memory can accommodate. Paging provides a flexible and efficient mechanism for address translation and memory management.
16. Describe the terms:
a. Internal and external fragmentation.
Internal Fragmentation:
Internal fragmentation occurs when a process or allocation has allocated more memory than it actually needs. It happens when the allocated memory space is larger than the requested size, resulting in wasted memory within a process or a memory allocation. Internal fragmentation is common in fixed-size allocation schemes or when memory is allocated in units larger than the size requested by the process. This wasted memory cannot be used by other processes, leading to inefficient memory utilization.
External Fragmentation:
External fragmentation occurs when free memory blocks, although available in total, are scattered throughout the memory space in a non-contiguous manner. It happens when the free memory is divided into small, non-contiguous blocks, making it impossible to allocate a larger contiguous block of memory to satisfy a process's request. External fragmentation is commonly observed in variable-sized allocation schemes or when processes are dynamically loaded and unloaded from memory. It can result in inefficient memory utilization and can cause difficulties in allocating contiguous blocks of memory to satisfy larger memory requests.
b. Dirty Page and Clean Page.
Dirty Page:
A dirty page refers to a page in the main memory (RAM) that has been modified (written to) since it was last brought in from secondary storage (disk). It means that the contents of the page have been altered or updated by the executing process, and the modified data has not been synchronized with the corresponding page on the disk. Dirty pages occur when a process writes to its allocated memory space, resulting in a mismatch between the data in memory and the disk. To ensure data consistency, dirty pages need to be periodically written back to the disk to update the persistent storage.
Clean Page:
A clean page, on the other hand, refers to a page in the main memory that has not been modified since it was brought in from secondary storage. It means that the contents of the page in memory are identical to the corresponding page on the disk. Clean pages occur when a process reads data from its allocated memory space without modifying it or when the modified data has already been written back to the disk. Clean pages do not require any synchronization with the disk since their content is up-to-date.
The distinction between dirty pages and clean pages is important for memory management and caching mechanisms. It allows the system to prioritize the writing back of modified data to disk for dirty pages while avoiding unnecessary disk I/O operations for clean pages, thus optimizing the efficiency of data synchronization and storage management.
17. Consider the following segment table:
What are the physical addresses for the following logical addresses?
a. Logical Address: 0430
- Segment: 0
- Offset: 430
- Physical Address = Base of Segment 0 + Offset = 219 + 430 = 649
b. Logical Address: 110
- Segment: 2
- Offset: 110
- Physical Address = Base of Segment 2 + Offset = 90 + 110 = 200
c. Logical Address: 2500
- Segment: 1
- Offset: 2500
- Physical Address = Base of Segment 1 + Offset = 2300 + 2500 = 4800
d. Logical Address: 3400
- Segment: 3
- Offset: 3400
- Physical Address = Base of Segment 3 + Offset = 1327 + 3400 = 4727
e. Logical Address: 4112
- Segment: 4
- Offset: 4112
- Physical Address = Base of Segment 4 + Offset = 1952 + 4112 = 6064
The physical addresses for the given logical addresses can be calculated by adding the offset to the base address of the corresponding segment. Each logical address is composed of a segment number and an offset within that segment. By using the segment table, we can determine the base address of the segment and then calculate the physical address by adding the offset.
(Note: this answers may be wrong so don't depend on that refer any other material for numeracies )
