Virtual Memory

Question 1

VM was created to allow running ____.

Answer 1

processes with memory requirements larger than available RAM to run in the computer.

Question 2

Currently, VM is used mainly to give each process its own address space.
True/False

Answer 2

True

Question 3

VM only keeps in RAM the memory that is currently in use.

The remaining memory is kept in disk in a special file called ___.

Answer 3

Swap space

Question 4

VM implementations:

Process swapping: The entire memory of the process is ___.

Answer 4

swapped in and out of memory

Question 5

VM implementations:
Segment swapping:
___ are swapped in and out of memory (libraries, text, data, bss etc).

Answer 5

Entire parts of the program (process)

Question 6

There are 3 VM implementations:
___ swapping
___ swapping
___

Answer 6

process; segment; paging

Question 7

Which of the following about paging is false?
A. Paging is used by modern operating.
B. The unit of memory that is swapping in and out is a page.
C. Paging divides the memory in pages of different sizes.
D. Usually the size of a page is 4KB in x86.

Answer 7

C.

Paging divides the memory in pages of fixed size.

Question 8

Paging:
The virtual memory system will keep in memory ___.
It will leave in disk ___.

Answer 8

the pages that are currently in use; the memory that is not in use

Question 9

Each page in the address space is backed by a file in disk, called ___.

Answer 9

backing-store

Question 10

Name the backing store for the following memory section:
Text
ROData
Data
BSS
Stack
Heap

Answer 10

Exe file; exe file; exe file (not modified), swap space (modified); swap space; swap space; swap space

Question 11

Name the permissions (RWX) for the following memory section:
Text
ROData
Data
BSS
Stack
Heap

Answer 11

RX; R; R, RW; RW; RW; RW

Question 12

Any section in memory that is not persistent and will go away when the process exits is stored in ___.

Answer 12

Swap space

Question 13

___ translates virtual memory addresses to physical memory addresses.

Answer 13

Memory Management Unit (MMU)

Question 14

___ memory addresses: the address that the CPU is using. Addresses used by programs are of this type.

Answer 14

Virtual

Question 15

___ memory addresses: The addresses of RAM pages. This is the hardware addresses.

Answer 15

Physical

Question 16

The MMU has a ___ that points to the current page table that will be used for the translation.

Answer 16

page table register

Question 17

Every process shares a page table.

True/False

Answer 17

False.

Each process has its own page table.

Question 18

The page table register is updated during ___.

The page table has the information of ___.

Answer 18

a context switch from one process to the other.

the memory ranges that are valid in a process.

Question 19

Consecutive pages in virtual memory are consecutive pages in physical memory.
True/False

Answer 19

False.

Consecutive pages in virtual memory may correspond to non-consecutive pages in physical memory.

Question 20

To prevent looking up the page table at every memory access, the most recent translations are stored in ___.

Answer 20

Translation Look-Aside buffer (TLB)

Question 21

The TLB ___ the translation from ___ to ___ memory addresses.

Answer 21

speeds up; virtual; physical

Question 22

A page fault is an interrupt generated by ___.

Answer 22

MMU

Question 23

The VM address is divided into 2 parts:

• ___ (higher 20 bits)
• offset (lower 12 bits)

Answer 23

Page number; offset

Question 24

Both the page number and the offset is translated.

True/False

Answer 24

False.

Only the page number is translated. The offset remains the same.

Question 25

Modern architectures use a ____ page table to reduce the space needed.

Answer 25

Multi-level

Question 26

In a two level page table, the page number is divided into 2 parts: ___ page number and ___ page number.

Answer 26

First level; second level

Question 27

Which of the following about two level page tables is false? A. Process always have a 1st level page table B. Process always have a 4KB 2nd level page table C. Both the 1st and 2nd level page tables use 4KB

Answer 27

B. | Second level page tables are allocated as needed.

Question 28

Page bits: The remaining __ bits are used to store the characteristics of the page.

Answer 28

12

Question 29

Describe the bit: Resident bit Modified bit Access bit

Answer 29

Page is resident in RAM instead of swap space/file. Page has been modified since the last time the bit was cleared. Set by MMU. Page has been read since the last time the bit was cleared. Set by MMU.`

Question 30

If a CPU operation exceeds the permissions of a page, the MMU ____. The interrupt may be translated into ___.

Answer 30

``` generate an interrupt (page fault). a signal (SEGV, SIGBUS) to the process. ```

Question 31

If a page is accesss and the page is not resident in RAM, the MMU ___. Then the kernel ___.

Answer 31

generates an interrupt to the kernel. | loads that page from disk.

Question 32

Types of page faults: ___ : page not in physical memory, it is in disk ___ : write or access permission violated

Answer 32

Page not resident; protection violation

Question 33

Sequence of processing a page fault:- | 1. A program tries to read/write a location in memory that is in a ___ page.

Answer 33

non-resident

Question 34

Sequence of processing a page fault:- | 2. The MMU tries to ___, and finds that ___. MMu generates ___.

Answer 34

look up the VM address; page is not resident using the resident bit; a page fault

Question 35

Sequence of processing a page fault:- | 3. The CPU save ____ and ___ in the stack.

Answer 35

return address; register

Question 36

Sequence of processing a page fault:- | 4. The CPU looks up the ___ that corresponds to the page fault in the ___ and jumps to this ___.

Answer 36

interrupt handler; interrupt vector; interrupt handler

Question 37

Sequence of processing a page fault:- 5. In the page fault handler, if the VM address corresponds to a page is not valid for this process, then generate ___. Otherwise, the page has to be ___.

Answer 37

a SEGV signal to the process. | Loaded from disk.

Question 38

The default behavior for SEGV is to ___.

Answer 38

kill the process and dump core

Question 39

Sequence of processing a page fault:- 6. Find ___ in physical memory. If there are no free pages, use ___.

Answer 39

free page; | One that is in use and write to disk if modified.

Question 40

Sequence of processing a page fault:- 7. Load the page from disk and ___. Clear ___ and ___ bits. Set ___ to 1.

Answer 40

update the page table with the address of the page replaced. modified and access; resident bit

Question 41

Which of the following is false? A. The page fault handler retries the offending instruction at the end of the page fault B. the program knows that the page fault occurred

Answer 41

B. The page fault is completely transparent to the program. The program will have no knowledge that the page fault occurred.

Question 42

The mmap() function established a mapping between ___ and ___.

Answer 42

process's address space; a file or shared memory object

Question 43

Mmap returns ___ and it will be always aligned to a ___.

Answer 43

address of the memory mapping; page size

Question 44

If NULL is passed in the address parameter in mmap(), the OS will choose the address of the mapping. True/False

Answer 44

True

Question 45

The mmaped file should have a length of smaller or the program gets a SEGV on access. True/False

Answer 45

False. | The mmaped file should have a length of larger or the program gets a SEGV on access.

Question 46

Flags in mmap: MAP_SHARED : changes in memory will be ___ MAP_PRIVATE : changes in memory will be ___ (__) MAP_FIXED : force to use ___ MAP_NORESERVE : do not ___ in advance MAP_ANON : do not use any ___

Answer 46

``` done in the file; kept private and not reflected in the file (copy on write); "addr" as is without changing; reserve swap space; fd ```

Question 47

Mmap parameters: | Offset has to be a multiple of ___.

Answer 47

a page size

Question 48

Which of the following about mmap is false? A. Writing in memory of a memory-mapped file will always update the file in the disk. B. Updating the disk will not happen immediately. C. The OS will cache the change until it is necessary to flush changes.

Answer 48

A. | Writing in memory of a memory-mapped file will update the file in the disk (only with MAP_SHARED).

Question 49

VM also speeds up the execution of programs:- 1. Mmap the ___ segment of an executable or shared library 2. Mmap the ___ segment of the program 3. Use of VM during ___ to copy memory of ___ into ___ 4. Allocate ___ and space for bss, stack and sbrk() 5. ___`

Answer 49

text; data; fork, parent , child; zero-initialized memory; shared memory

Question 50

Initialily, mmap does not ___. Any pages will be loaded on ___. Startup time is ___ because ___. It also saves ___ because only the portions of the program that are needed will be in ___.

Answer 50

read any pages; on demand when they are accessed; fast, only the pages needed will be loaded instead of the entire program; RAM, RAM

Question 51

Physical pages where the text segment is stored is shared by multiple instances of the same program. True/False

Answer 51

True

Question 52

During the oading of a program, the OS mmaps the data segment of the program. Multiple instances of the same program will share ___ where the data segment is mapped as long as ___. If a page is modified, the OS ___.

Answer 52

the same physical memory pages, the page is not modified; | will create a copy of the page and make the change in the copy

Question 53

After forking, the child gets a copy of ___. Both parent and child share the same ___ as long as they are not modified. When a page is modified by either parent or child, the OS ___.

Answer 53

memory of the parent; RAM pages (physical memory); creates a copy of the page in RAM and will do the modifications on the copy

Question 54

The copy on write in fork is accomplished by ___. The OS will catch the modifications. During the page fault, the OS will ___. Then it will ___.

Answer 54

Making the common pages read only; create a copy and update the page table of the writing process；retry the modify instruction

Question 55

Which of the following is false? A. VM is used to allocate space for bss, stack and sbrk() B. When allocating memory using sbrk or map with the MMAP_ANON falg, all the VM pages in this mapping will map to a single page in RAM that has 0s and is read only C. When a page is modified, the OS will directly write into the page D. Fast allocation; saves RAM

Answer 55

C. | When a page is modified, the OS creates a copy of the page (copy on write) and retries the modifying instruction.

Question 56

Processes may communicate using ___. Both processes share the same ___. A modification in one page by one process will be reflected by a change in ___.

Answer 56

shared memory; physical pages; the same page in the other process