Memory Management

Question 1

Define the following terms:
Page
Frame
Segment

Answer 1

Frame: A fixed length block of memory

Page: A fixed length block of data that resides in secondary memory (eg. disk)

Segment: A variable length block of data that resides in secondary memory.

Question 2

True or False
A page of data may be temporarily copied into a frame of main memory

Answer 2

True

Question 3

Define Segmentation

Answer 3

This process involves an entire segment being temporarily copied into main memory.

Question 4

-something to remember-

Answer 4

In a uniprogramming system, main memory is divided into two parts: one part
for the operating system (resident monitor, kernel) and other part for the program
currently being executed.

Question 5

-something to remember-

Answer 5

In a multiprogramming system, the “user” part of
memory must be further subdivided to accommodate multiple processes. The task
of subdivision is carried out dynamically by the operating system and is known as
memory management .

Question 6

What are the five requirements that memory management should satisfy?

Answer 6

1. Relocation
2. Sharing
3. Protection
4. Physical Organization
5. Logical Organization

Question 7

Describe the concept of relocation.

Answer 7

Swapping active processes in and out of memory to maximize utilization.
We cannot know ahead of time where a process may be in memory thus we must allow the possibility for processes to be swapped around for optimization.

Question 8

Describe the concept of protection.

Answer 8

Each process must be protected from the interference of other processes.

Process need permission to acquire a space in memory.

Question 9

Why is memory protection a hardware responsibility and not a software responsibility ?

Answer 9

This is because the OS cannot anticipate all of the memory references that a program will make. Even if such anticipation were possible, it would be prohibitively time consuming to screen each program in advance for possible memory-reference violations. Thus, it is only possible to assess the permissibility of a memory reference (data access or branch) at the time of execution of the instruction making the reference. To accomplish this, the processor hardware must have that capability.

Question 10

-something to remember-

Answer 10

In one sense, satisfaction of the relocation requirement increases the difficulty of satisfying the protection requirement

Question 11

True or False
Without special arrangement, a program in one process cannot access the data area of another process.

Answer 11

True

Question 12

Describe the concept of sharing

Answer 12

This deals with allowing each process to access the same copy of the program rather than have its own separate copy. This access should be controlled so it doesn’t violate the protection requirement

Question 13

What are three (3) benefits to using logical organization ( modules)

Answer 13

1. Different modules can have different protection measures (modifiable and non modifiable)
2. Modules can be shared among processes which makes it easier for users to specify the sharing that is desired
3. Modules can be run independently

Question 14

-something to remember-

Answer 14

Main memory is fast at a high cost and viol does not provide permanent storage.

Secondary memory is slower, cheaper and non vi

Question 15

Why is it impractical to make a programmer responsible for the flow to and from main memory and secondary memory.

Answer 15

1. The program and its data may be to large for main memory and in this case, the programmer will have to use a process known as overlaying to give one space in memory to multiple modules. Then has the responsibility of switching them in and out. This is very time consuming
2. The programmer cannot know how at the time of coding, how much memory will be available and where it is available

Question 16

Describe Fixed Partitions

Answer 16

Main memory is divided into static partitions at system generation time. The processes are loaded into partitions that are of equal size of greater.
adv: simple to implement
disadv: insufficient use of memory

Question 17

Describe Dynamic Partitioning

Answer 17

Partitions are created dynamically so that process are loaded into partitions that are exactly the same size.

adv: sufficient use of memory
disadv: external fragmentation

Question 18

Describe Simple Paging

Answer 18

Main memory is divided into equal sized frames then a process is divided into equal size pages to match the frames. The process is loaded in by loading in all of pages into available frames.
Adv: no internal fragmentation
disadv: small external fragmentation

Question 19

Describe Simple Segmentation

Answer 19

Each process is divided into segments. Then each segment is dynamically placed into a partitions

Question 20

Describe Virtual Memory paging

Answer 20

As with simple paging but not all pages have to be brought into main memory. Non resident pages can be brought in later, automatically.

Question 21

Describe Virtual Memory Segmentation

Answer 21

As with virtual memory segmentation but not all segments have to be loaded in. Non resident segments can be loaded in at a later time, automatically.

Question 22

What is the difference between internal and external fragmentation?

Answer 22

The difference between memory allocated and required space or memory is called Internal fragmentation (fixed)

The unused spaces formed between non-contiguous memory fragments are too small to serve a new process, which is called external fragmentation. (dynamic)

Question 23

List 2 disadvantages of fixed partitioning of equal sized

Answer 23

1. If there is a program that is two big for a partition, the program will need to overlay modules, laying modules over each other and swapping them in and out when needed.
2. Internal Fragmentation : Wasted space that occurs when programs are smaller that the memory space allocated.

Question 24

Explain the placement algorithm for equal partitions

Answer 24

employ a queue to each partition, as long as a partition is available, load a process into, if all partitions are unavailable.
some process must be swapped out.

Question 25

Explain the placement algorithm for unequal partitions.

Answer 25

Employ a single queue to all partitions, select the smallest partition that can hold the process. If no process is available then a swapping decision needs to be made.

Question 26

Disadvantages of unequal partitioning

Answer 26

• The number of partitions specified at system generation time limits the number of active (not suspended) processes in the system.
• Because partition sizes are preset at system generation time, small jobs will not utilize partition space efficiently.

Question 27

What technique is used by the IBM’s mainframe operating system?

Answer 27

Dynamic Partitioning

Question 28

Name and explain the technique used to solve external fragmentation.

Answer 28

Compaction: shifting the processes so that they are contiguous and so that all of the free memory is together in one block

Question 29

Differentiate between best fit, next fit and first fit

Answer 29

Best fit choose the block closest to the size of the request.

Next fit scans memory from the location of the last placement and choose the first available block that is large enough.

First fit scans from the beginning and chooses the first available block that is large enough

Question 30

What system is used to overcome the disadvantages of the fixed and dynamic partitioning?

Answer 30

the buddy system

Question 31

-something to remember-

Answer 31

When the fixed partition scheme is used, we can expect a process will always be assigned to the same partition Whichever partition is selected when a new process is loaded will always be used to swap that process back into memory after it has been swapped out

Question 32

-something to remember-

Answer 32

For equal and unequal partitioning with a single queue. When a process image is first created, it is loaded into some partition in main memory; Later, the process may be swapped out
When it is subsequently swapped back in, it may be assigned to a different partition than the last time
The same is true for dynamic partitioning

Question 33

What 2 steps of manipulation does the relative address go through by the processor ?

Answer 33

FIrst the value in the base register is added to relative address to create an absolute address
Then the result is checked with the bounds location, if within bounds, process and if not, stop.

Question 34

What is the page table?

Answer 34

The page table is a list that contains the frame number for each page

Question 35

What maintains the page table?

Answer 35

The Operating System

Question 36

Differentiate between logical, physical and relative addresses.

Answer 36

logical address is a reference to a memory location independent
of the current assignment of data to memory.

A relative address is a particular
example of logical address, in which the address is expressed as a location relative
to some known point, usually a value in a processor register.

A physical address, or
absolute address, is an actual location in main memory.