Virtual Memory

With 3 frames, RS = {0 1 2 1 3} where the first reference modifies the page 0, and the pointer at page 0, the first page fault will replace page

1

When a page with the bits 00 is modified, the bits are set to_

11

Aging page replacement algorithm

Does not maintain pages sorted in the exact LRU order, but groups together pages referenced during a period of d consecutive references.

The size of the working set grows linearly with d.

False

VM cannot be implemented without the modified bit.

False

modified-bit (m-bit)

Is a binary flag in each page table entry that indicates whether the corresponding page has been modified during execution.

Referenced bit (r-bit)

Is a bit associated with a page and is set automatically by the hardware whenever the page is referenced by any instruction.

Virtual memory

Is a collection of one or more logical address spaces, each of which may exceed the size of physical memory. A logical address is then referred to as a virtual address.

working set (WS)

Is a process at time t is the set of pages referenced during the past d memory operations preceding t.

Page fault

Is an interrupt that occurs when a program attempts to reference a non-resident page.The interrupt triggers the operating system to find the page on disk, copy the page into a frame, and set the present bit to 1.

Page Replacement

Is the act of overwriting a page in memory with a different page loaded from the disk when needed.

Effective access time

Is the average time to access memory in the presence of page faults.

Page fault rate

Is the number of page faults, f, occurring during a number of memory references, t. The page fault rate can be expressed as P = f/t, where 0 ≤ P ≤ 1.

Demand paging

Is the principle of loading a page into memory only when the page is needed, rather than at the start of the execution.

Reference string

Is the sequence of page numbers referenced by an executing program during a given time interval.

When using an aging register, and a page fault occurs:

Select the page with the smallest aging register value for replacement. & If multiple pages have the same smallest value then select one at random.

FIFO page replacement algorithm

Selects the page that has been resident in memory for the longest time.

Least-recently-used page replacement algorithm (LRU)

Selects the page that has not been referenced for the longest time.

second-chance page replacement algorithm

The algorithm uses the r-bit to divide all pages into only two categories: recently referenced and not recently referenced

VM cannot be implemented without demand paging.

True

VM cannot be implemented without page replacement.

True

Third-chance page replacement algorithm, is also called "The not-recently-used page replacement algorithm"

Which divides pages into 4 categories based on the 4 possible combination of the r- bit and the m-bit.

Present bit

a binary flag in each page table entry that indicates whether the corresponding page is currently resident in memory. If a page is resident, then the entry points to the frame that holds the page.

The page fault rate _____ as d increases

decreases

The aging registers have 4 bits. The parameter d is 100. Page p1 has not been accessed for more than 400 references and page p2 has not been accessed for more than 600 references. When a page fault occurs, the aging algorithm will replace _____.

either page chosen at random

At a page fault, the resident set size can increase by more than 1

false

Internal fragmentation _____ as page size increases.

grows

Page fault rate _____ as page size increases.

grows

The memory space covered by the TLB _____ as page size increases.

grows

When p = 1 then the corresponding page resides

in both physcial memory and on disk. (p = 1 means the page is resident in physical memory but a copy still exists on disk.)

optimal working set

is a process is the set of resident pages that will still be needed in the immediate future and thus should remain resident.

Aging register

is associated with a page and is shifted periodically to the right by 1 bit. Unless the most significant bit is set to 1, the page is aging in the sense that the associated register value is steadily decreasing.

When the number of processes is very large, CPU utilization is generally

low

When the number of processes is very small, CPU utilization is generally

low

When p = 0 then the corresponding page resides

on disk only. (p = 0 means the page is not resident in physical memory, only a copy on disk exists.)

A page with the r/m-bits set to 01 has been

referenced and modified

When CPU utilization is high then the number of processes should

remain the same

Optimal page replacement algorithm

selects the page that will not be referenced for the longest time in the future.

Page table size _____ as page size increases.

shrinks

The average cost of disk I/O _____ as page size increases

shrinks

page-fault-frequency replacement algorithm

takes a direct approach to controlling the page fault rate by adjusting the current resident set based on how frequently consecutive page faults occur.

A page fault is triggered when

the page is currently not resident in memory and has not been modified

A page must be written back to disk when

the page is currently resident in memory and the page has been modified.

Immediately after a page replacement, the bits in the page table are

the page is currently resident in memory and the page has not been modified

At a page fault, the resident set size can decrease by more than 1

true

At a page fault, the resident set size can increase by 1

true

At a page fault, the resident set size can remain the same

true

working set page replacement algorithm

uses a trailing window of size d superimposed on the RS to determine the size and composition of the working set at time t.