The address generated by the CPU is called a logical address (or virtual address), whereas the address seen by the memory unit is the physical address.
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2The run-time mapping from virtual to physical addresses is done by a hardware device called the:
A.Loader
B.Device Driver
C.Memory Management Unit (MMU)
D.CPU Scheduler
Correct Answer: Memory Management Unit (MMU)
Explanation:
The MMU is the hardware component responsible for translating logical addresses into physical addresses at run-time.
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3In the context of memory management, what is the backing store?
A.The fast disk large enough to accommodate copies of all memory images for all users
B.The Registry
C.The L1 Cache
D.The main memory (RAM)
Correct Answer: The fast disk large enough to accommodate copies of all memory images for all users
Explanation:
The backing store is usually a hard disk or SSD used in swapping and paging to store memory pages that are not currently in main memory.
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4Which memory allocation policy allocates the smallest hole that is big enough to hold a process?
A.Worst fit
B.Next fit
C.Best fit
D.First fit
Correct Answer: Best fit
Explanation:
Best fit searches the entire list (unless ordered by size) and produces the smallest leftover hole.
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5External fragmentation occurs when:
A.Allocated memory may be slightly larger than requested memory
B.Pages are not modified in memory
C.A process is loaded into memory but never executed
D.Total memory space exists to satisfy a request, but it is not contiguous
Correct Answer: Total memory space exists to satisfy a request, but it is not contiguous
Explanation:
External fragmentation happens when there is enough total free memory to satisfy a request, but the available spaces are scattered in small blocks.
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6What is the solution to external fragmentation?
A.Swapping
B.Segmentation
C.Compaction
D.Paging
Correct Answer: Compaction
Explanation:
Compaction shuffles memory contents to place all free memory together in one large block, though Paging also solves it by allowing non-contiguous allocation.
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7In paging, physical memory is broken into fixed-sized blocks called:
A.Frames
B.Segments
C.Sectors
D.Pages
Correct Answer: Frames
Explanation:
Physical memory is divided into frames, while logical memory is divided into blocks of the same size called pages.
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8If the page size is 4 KB (2^12), how many bits are required for the page offset?
A.32 bits
B.10 bits
C.12 bits
D.20 bits
Correct Answer: 12 bits
Explanation:
The offset requires bits to address every byte within the page. Since 4 KB = 2^12 bytes, 12 bits are needed for the offset.
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9What data structure is used to map logical pages to physical frames?
A.Segment table
B.Page table
C.Frame table
D.Process Control Block
Correct Answer: Page table
Explanation:
The page table contains the base address of each page in physical memory.
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10Paging suffers from which type of fragmentation?
A.Neither
B.External fragmentation
C.Both internal and external
D.Internal fragmentation
Correct Answer: Internal fragmentation
Explanation:
Paging eliminates external fragmentation, but the last frame allocated to a process may not be completely full, resulting in internal fragmentation.
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11The Translation Look-aside Buffer (TLB) is used to:
A.Store the entire page table
B.Speed up the translation of logical addresses to physical addresses
C.Handle page faults
D.Perform segmentation
Correct Answer: Speed up the translation of logical addresses to physical addresses
Explanation:
The TLB is a small, fast-lookup hardware cache that stores recent translations from virtual memory to physical memory.
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12In segmentation, the logical address consists of:
A.Frame number and offset
B.Base and Limit
C.Segment number and offset
D.Page number and offset
Correct Answer: Segment number and offset
Explanation:
A logical address in segmentation is a two-dimensional entity represented by a tuple: <segment-number, offset>.
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13Which register is used in segmentation to store the starting physical address of a segment?
A.Instruction register
B.Index register
C.Base register
D.Limit register
Correct Answer: Base register
Explanation:
The base register contains the starting physical address where the segment resides in memory.
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14What technique allows a process to be larger than the physical memory allocated to it?
A.Overlays
B.Contiguous allocation
C.Segmentation
D.Virtual Memory
Correct Answer: Virtual Memory
Explanation:
Virtual memory separates logical memory from physical memory, allowing the execution of processes that are not completely in memory.
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15Overlays are used when:
A.The process is larger than the amount of memory allocated to it
B.Paging is disabled
C.The process is smaller than physical memory
D.The OS supports virtual memory
Correct Answer: The process is larger than the amount of memory allocated to it
Explanation:
Overlays allow a process to be larger than the physical memory by keeping in memory only those instructions and data that are needed at any given time.
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16In demand paging, a page is brought into memory only when:
A.The time quantum expires
B.It is needed/referenced during execution
C.It is modified
D.The process starts
Correct Answer: It is needed/referenced during execution
Explanation:
Demand paging loads pages only when they are demanded by the executing process.
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17What happens when a process tries to access a page that is not currently in memory?
A.Segmentation fault
B.Page fault
C.System crash
D.Deadlock
Correct Answer: Page fault
Explanation:
Accessing a page marked invalid (not in memory) causes a page-fault trap to the operating system.
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18Which bit in the page table indicates whether a page is in memory or on the disk?
A.Valid-invalid bit
B.Reference bit
C.Dirty bit
D.Protection bit
Correct Answer: Valid-invalid bit
Explanation:
The valid-invalid bit is set to 'valid' if the page is in memory, and 'invalid' if the page is not in the logical address space or is currently on the disk.
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19What is the main advantage of multi-level paging?
A.Simpler hardware implementation
B.Reducing the size of the page table in memory
C.Elimination of internal fragmentation
D.Faster memory access
Correct Answer: Reducing the size of the page table in memory
Explanation:
Multi-level paging allows the page table itself to be paged, meaning the entire page table does not need to reside contiguously in main memory.
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20In the context of page replacement, Belady's Anomaly states that:
A.For some algorithms, the page-fault rate may increase as the number of allocated frames increases
B.Thrashing occurs when CPU utilization is low
C.Optimal replacement is impossible to implement
D.LRU is always better than FIFO
Correct Answer: For some algorithms, the page-fault rate may increase as the number of allocated frames increases
Explanation:
Belady's Anomaly is a phenomenon in which increasing the number of page frames results in an increase in the number of page faults for certain algorithms like FIFO.
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21Which page replacement algorithm has the lowest possible page-fault rate for a fixed number of frames?
A.Optimal
B.Counting-based
C.LRU
D.FIFO
Correct Answer: Optimal
Explanation:
The Optimal algorithm replaces the page that will not be used for the longest period of time, guaranteeing the lowest fault rate, but it requires future knowledge.
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22Which page replacement algorithm replaces the page that has not been used for the longest period of time?
A.Optimal
B.MFU (Most Frequently Used)
C.LRU (Least Recently Used)
D.FIFO
Correct Answer: LRU (Least Recently Used)
Explanation:
LRU looks backward in time and replaces the page that hasn't been accessed for the longest duration.
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23What is 'Thrashing' in an operating system?
A.A high rate of I/O operations
B.A deadlock situation
C.A process spending more time paging than executing
D.When the hard disk is full
Correct Answer: A process spending more time paging than executing
Explanation:
Thrashing occurs when a system spends a majority of its time swapping pages in and out of memory rather than executing instructions, causing performance to degrade.
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24The 'Working Set Model' is used to prevent:
A.Thrashing
B.Page faults
C.fragmentation
D.Deadlocks
Correct Answer: Thrashing
Explanation:
The Working Set Model estimates the number of frames a process needs (its locality) to prevent thrashing by ensuring the process has enough frames.
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25In Segmentation with Paging:
A.There are no page tables
B.External fragmentation is high
C.The segment is divided into pages
D.The page is divided into segments
Correct Answer: The segment is divided into pages
Explanation:
To combine the benefits of both, the logical address space is divided into segments, and each segment is further divided into pages to eliminate external fragmentation.
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26Which bit is used to indicate if a page has been modified while in memory?
A.Reference bit
B.Dirty (or Modify) bit
C.Valid bit
D.Present bit
Correct Answer: Dirty (or Modify) bit
Explanation:
The dirty bit is set when a page is written to. If set, the page must be written back to the disk upon replacement.
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27Pure demand paging implies:
A.Loading all pages before execution
B.Starting a process with no pages in memory
C.Never swapping pages out
D.Using segmentation only
Correct Answer: Starting a process with no pages in memory
Explanation:
In pure demand paging, the process starts with zero pages in memory, and every initial instruction causes a page fault until the working set is loaded.
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28The effective access time in a paged memory system depends heavily on:
A.The Page Fault Rate
B.The size of the hard disk
C.The clock speed
D.The number of processes
Correct Answer: The Page Fault Rate
Explanation:
Since disk access is extremely slow compared to memory access, even a small increase in the page fault rate drastically increases the effective access time.
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29Which allocation algorithm allocates the first hole that is big enough?
A.Worst fit
B.First fit
C.Best fit
D.Quick fit
Correct Answer: First fit
Explanation:
First fit scans the list of free blocks and allocates the first one that is large enough, which is generally faster than Best fit.
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30Logical address space is seen by:
A.The Bus
B.The RAM
C.The User / Program
D.The Memory Management Unit
Correct Answer: The User / Program
Explanation:
The user program deals with logical addresses, unaware of where the data physically resides in memory.
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31The limit register in segmentation is used to:
A.Specify the length of the segment
B.Count the number of segments
C.Store the starting address
D.Point to the segment table
Correct Answer: Specify the length of the segment
Explanation:
The limit register specifies the length of the segment. The offset is compared against this limit to prevent illegal access.
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32If a page fault occurs and there are no free frames, what must the OS do?
A.Execute a page replacement algorithm
B.Increase the page size
C.Terminate the process
D.Ignore the instruction
Correct Answer: Execute a page replacement algorithm
Explanation:
The OS must select a victim page to swap out to disk to free up a frame for the new page.
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33Which of the following is a disadvantage of the FIFO page replacement algorithm?
A.It is hard to implement
B.It requires hardware support
C.It suffers from Belady's Anomaly
D.It uses a stack
Correct Answer: It suffers from Belady's Anomaly
Explanation:
FIFO is simple but can perform poorly and paradoxically increase faults when more memory is added (Belady's Anomaly).
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34Copy-on-Write (COW) allows parent and child processes to initially share the same:
A.Segments
B.Stack
C.File descriptors
D.Pages
Correct Answer: Pages
Explanation:
COW allows processes to share pages in memory. A copy is only made when one of the processes modifies a shared page.
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35The Second Chance (or Clock) algorithm is an approximation of:
A.Optimal
B.FIFO
C.MRU
D.LRU
Correct Answer: LRU
Explanation:
Second Chance uses a reference bit to give pages a 'second chance' before replacement, approximating LRU behavior without the high hardware cost.
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36Locality of reference refers to:
A.The physical location of memory sticks
B.The tendency of a program to access the same set of memory locations frequently over a short period
C.Global variables only
D.The distance between the CPU and RAM
Correct Answer: The tendency of a program to access the same set of memory locations frequently over a short period
Explanation:
This principle explains why demand paging and caching work efficiently, as programs tend to stay within a specific region of memory for a time.
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37Which scheme allows the user to view memory as a collection of variable-sized logical units?
A.Paging
B.Segmentation
C.Contiguous Allocation
D.Hashing
Correct Answer: Segmentation
Explanation:
Segmentation supports the user view of memory (e.g., main, stack, symbol table) where segments vary in size.
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38In a system using paging, if the logical address is 32 bits and the page size is 4KB (2^12 bytes), what is the size of the page number?
A.20 bits
B.12 bits
C.32 bits
D.10 bits
Correct Answer: 20 bits
Explanation:
Logical Address (32) = Page Number (p) + Page Offset (d). Since d = 12, p = 32 - 12 = 20 bits.
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39Internal fragmentation is NOT possible in:
A.Paging
B.Demand Paging
C.Fixed Partitioning
D.Segmentation
Correct Answer: Segmentation
Explanation:
Segmentation allocates exactly the memory required for a segment (variable size), so there is no unused space inside the allocated block (no internal fragmentation).
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40What is the purpose of the 'fence register' in early memory management?
A.To clock the CPU
B.To store the stack pointer
C.To prevent a user program from modifying the OS part of memory
D.To count pages
Correct Answer: To prevent a user program from modifying the OS part of memory
Explanation:
A fence register separates the OS memory partition from the user memory partition to provide protection.
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41In inverted page tables:
A.There is one entry for each real physical frame
B.There is one entry for each logical page
C.It increases memory usage significantly
D.The table is stored on the disk
Correct Answer: There is one entry for each real physical frame
Explanation:
Inverted page tables map physical frames to logical pages (PID + page number) to save memory when the logical address space is very large.
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42Which allocation strategy creates the largest leftover hole?
A.Next fit
B.Best fit
C.First fit
D.Worst fit
Correct Answer: Worst fit
Explanation:
Worst fit allocates the largest available hole, leaving the largest possible remaining fragment, which might be useful for future requests.
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43Swapping requires the backing store to be:
A.Tape storage
B.Volatile
C.Accessible only by the kernel
D.Large and fast
Correct Answer: Large and fast
Explanation:
The backing store must be large enough to hold memory images and fast enough to allow efficient swapping (usually a disk).
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44A 'Global' page replacement policy allows:
A.The OS to replace the kernel
B.A process to only replace its own frames
C.Pages to be shared globally
D.A process to select a replacement frame from the set of all frames in the system
Correct Answer: A process to select a replacement frame from the set of all frames in the system
Explanation:
Global replacement allows a process to 'steal' a frame from another process, which can improve system throughput but affect the predictability of individual process performance.
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45If the TLB hit ratio is high, the effective memory access time is:
A.Doubled
B.Halved
C.Tripled
D.Close to the memory access time
Correct Answer: Close to the memory access time
Explanation:
With a high TLB hit ratio, the system rarely needs to access the page table in RAM (which would require an extra cycle), keeping access time low.
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46Pre-paging is a technique used to:
A. Compress pages
B.Clear memory before use
C.Avoid the large number of initial page faults
D.Encrypt pages
Correct Answer: Avoid the large number of initial page faults
Explanation:
Pre-paging attempts to bring in all the pages a process will need at once (or the working set) to prevent the performance hit of many small page faults at startup.
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47Which fragmentation is solved by 'Compaction'?
A.External
B.Internal
C.Logical
D.Virtual
Correct Answer: External
Explanation:
Compaction consolidates free memory blocks to solve external fragmentation.
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48The mapping of a logical address to a physical address is done at 'Load Time' if:
A.The process can be moved during execution
B.We know where the process will reside in memory at compile time
C.Virtual memory is used
D.Memory location is not known until execution
Correct Answer: We know where the process will reside in memory at compile time
Explanation:
If the memory location is known at compile time, absolute code can be generated. If it is relocatable, binding happens at load time.
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49In a Multi-level paging scheme with a two-level page table, the logical address is divided into: