"Memory Management & Virtual Memory" — Practice

Question 1

A system has 16 KB pages and 36-bit virtual addresses. How many bits are the offset and the page number?

Accepted Answer

16 KB is 2 to the 14, so the offset is 14 bits and the page number is 36 minus 14, which is 22 bits.

Question 2

Does paging suffer internal fragmentation, external fragmentation, or both?

Accepted Answer

Only internal, averaging half a page per process. External fragmentation is eliminated because any free frame can hold any page.

Question 3

Memory access is 80 nanoseconds and the TLB adds 10 nanoseconds. With a single-level page table and a 95 percent hit ratio, compute effective access time.

Accepted Answer

A hit costs 10 plus 80, which is 90. A miss costs 10 plus 80 plus 80, which is 170. EAT is 0.95 times 90 plus 0.05 times 170, which is 85.5 plus 8.5, or 94 nanoseconds.

Question 4

A 32-bit system has 4 KB pages and 4-byte page table entries. Design a two-level split so each inner table occupies exactly one page, and justify it.

Accepted Answer

A page holds 4096 divided by 4, which is 1024 entries, needing 10 index bits. The offset is 12 bits, leaving 20 for the page number, split 10 and 10. So the scheme is 10 outer, 10 inner, 12 offset.

Question 5

For the string 7,0,1,2,0,3,0,4,2,3,0,3,2 with three frames, count faults under optimal replacement.

Accepted Answer

Seven faults. 7, 0, 1 fill the frames; 2 evicts 7; 3 evicts 1; 4 evicts 0; and 0 evicts 4, with all remaining references hitting.

Question 6

Memory access is 200 nanoseconds and a page fault costs 8 milliseconds. What fault rate keeps effective access time below 220 nanoseconds?

Accepted Answer

Set 220 equal to (1 minus p) times 200 plus p times 8000000. That gives 20 equal to 7999800 p, so p is about 2.5 times 10 to the minus 6.

Question 7

Why does an inverted page table complicate page sharing?

Accepted Answer

It has one entry per physical frame recording a single process ID and virtual page number, so a frame mapped by two processes at different virtual addresses cannot be represented by a single entry without extra chaining.

Question 8

Show Belady's anomaly for FIFO on 1,2,3,4,1,2,5,1,2,3,4,5 by counting faults with three and with four frames.

Accepted Answer

With three frames FIFO takes 9 faults; with four frames it takes 10. More memory produced more faults because FIFO's eviction order ignores usage, so the resident set with four frames is not a superset of the set with three.

Question 9

A system reports 4 percent CPU utilisation and 98 percent paging disk utilisation. Diagnose it and evaluate: raise the degree of multiprogramming, install a faster CPU, add memory.

Accepted Answer

This is thrashing. Raising multiprogramming worsens it by cutting frames per process. A faster CPU achieves nothing since the CPU is already idle and the disk is the bottleneck. Adding memory works, as does suspending processes, because both raise frames per process toward the working set size.

Question 10

Explain why LRU cannot suffer Belady's anomaly, referring to the stack property.

Accepted Answer

With n frames LRU holds exactly the n most recently used pages, which is always a subset of the n plus 1 most recently used pages held with one more frame. Any reference that hits with n frames therefore also hits with n plus 1, so faults cannot increase.

Practice — "Memory Management & Virtual Memory"

Sign in to save attempts to your dashboard and track your accuracy over time

Find a verified tutor

Join a live course

Browse coaching centres

About

Categories

Support

Documentation

Get Study Tips & Updates

Download Our App

Download for iOS

Download for Android

Popular Cities