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Module 7 Final Quiz: Virtual Memory MCQs
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1
What is Demand Paging?
A.
Loading every page of a program into physical RAM before execution begins
B.
Bringing a page into physical memory only when an execution reference is made to that page (lazy swapper)
C.
Demanding that users pay for additional RAM
D.
Swapping out the operating system kernel to disk
Explanation & Key
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2
What sequence of events occurs when the CPU accesses a page whose Valid-Invalid bit is set to 'Invalid' (0)?
A.
The instruction completes normally using dummy data
B.
The MMU triggers a Page Fault trap -> OS catches trap -> OS checks if reference is valid -> OS finds a free physical frame -> OS reads page from disk into frame -> OS updates page table to valid -> OS restarts instruction
C.
The process is immediately terminated for security violation
D.
The entire system reboots
Explanation & Key
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3
What is Belady's Anomaly in page replacement algorithms?
A.
LRU replacing the most recently used page
B.
The counter-intuitive phenomenon where increasing the number of physical page frames results in an INCREASE in the number of page faults
C.
The CPU overheating during memory compaction
D.
A page table exceeding total system memory
Explanation & Key
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4
Which page replacement algorithm is theoretically optimal but cannot be implemented in general-purpose operating systems?
A.
FIFO (First-In, First-Out)
B.
Optimal Page Replacement (OPT / MIN / Belady's Algorithm)
C.
LRU (Least Recently Used)
D.
Second-Chance (Clock) Algorithm
Explanation & Key
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5
How does the Second-Chance (Clock) page replacement algorithm approximate LRU at low hardware overhead?
A.
By sorting all pages with quicksort on every tick
B.
By inspecting a 1-bit Reference Bit in a circular buffer: if bit=1, clear to 0 and advance pointer; if bit=0, select page for replacement
C.
By keeping high-precision 64-bit timestamps in every page table entry
D.
By maintaining a doubly linked list in kernel heap
Explanation & Key
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6
What is the Dirty Bit (Modify Bit) in a page table entry used for?
A.
To detect computer viruses in RAM
B.
To indicate whether a page has been written to (modified) since being loaded from disk, so the OS knows if it must be written back to disk upon eviction
C.
To mark a page as read-only
D.
To count the number of page faults
Explanation & Key
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7
What is Thrashing in virtual memory systems?
A.
When a process spends more time paging (swapping pages in and out of disk) than executing actual instructions
B.
When a hard drive head crashes physically into the platter
C.
When the TLB cache becomes corrupted
D.
When CPU temperature exceeds thermal thresholds
Explanation & Key
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8
How does the Working Set Model (Peter Denning) prevent thrashing?
A.
By allocating all RAM to a single process
B.
By defining the working set as the set of pages referenced in the most recent $\Delta$ time units, and ensuring a process is only scheduled if all its working set pages fit in RAM
C.
By disabling virtual memory when RAM is 90% full
D.
By terminating background processes at random
Explanation & Key
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9
Given page reference string: 7, 0, 1, 2, 0, 3, 0, 4 with 3 empty frames under FIFO. How many page faults occur?
A.
5 page faults
B.
6 page faults
C.
7 page faults
D.
8 page faults
Explanation & Key
Correct Answer:
10
Why does the Page Fault Frequency (PFF) strategy provide a dynamic mechanism against thrashing?
A.
It raises the CPU clock frequency when page faults rise
B.
It establishes upper and lower bounds on page fault rates: if fault rate exceeds upper bound, process is allocated more frames; if below lower bound, frames are reclaimed
C.
It clears the TLB on every instruction
D.
It limits the maximum file size on disk
Explanation & Key
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