Memory Management (PPT 5 - 6)

Question 1

Can we extend main memory using a hard disk?

Answer 1

Yes, this is virtual memory. However, it is still slow.

Question 2

What governs memory address and register usage?

Answer 2

The program code being run.

Question 3

Why use memory management?

Answer 3

The Operating System needs to allocate memory as efficiently as possible and keep track of those sector of memory which are free and which are in use (And what process is using each section)

Question 4

What is memory management?

Answer 4

Management is needed to handle memory resources of a computer as efficiently as possible. It is partly static and partly dynamic and is governed by the OS, with hardware support from the processors.

Question 5

What are the aims of memory management?

Answer 5

Primary function is to bring processes and data from secondary storage to main memory ready for execution. The following aims are desirable:
Protection
Sharing
Relocation

Question 6

What is Sharing?

Answer 6

In a multi user and multi program environment, the same data can be used multiple times. It is therefore memory efficient if only one copy is stored and shared between processes.

Question 7

What is Protection?

Answer 7

We need to prevent unauthorised programs accessing areas of memory allocated to other processes. This means the OS must protect processes (“Fence off” memory areas)

Question 8

What is Relocation?

Answer 8

A process must be capable of being loaded into any part of the address space. This means data pointers and branch pointers within the program can be accessed in a relative way, using their base address (start address of the program), and a logical way, which is relative to the base address. This is done by the compiler

Question 9

How is memory allocated?

Answer 9

When a process is loaded, it must be placed into memory as efficiently as possible. If insufficient free memory is available, we must remove another process to free memory.

Question 10

What are the four partitioning schemes?

Answer 10

• Fixed Size Partition
• Variable Size Partition
• Segmentation
• Paging

Question 11

What are Fixed Size Partitions?

Answer 11

When a process is loaded, it is placed into the smallest free partition which can fit all of it. If no segments are free, a process is then unloaded

Question 12

What are the key points of Fixed Sized Partitions?

Answer 12

• There is always memory wasted at the end of a segment
• When a process is unloaded, the whole segment is freed
• Data structures can grow but there is a limit

Question 13

What are Variable Size Partitions?

Answer 13

When a process is loaded in, it is placed in the first available partition which can fit all of it. If no segments are free, memory is unloaded.

Question 14

What are the key points of Variable Size Partitions?

Answer 14

• No memory is wasted when loading processes

- As time goes on, memory becomes fragmented. Possible to defragment but this takes time.

Question 15

What is Segmentation?

Answer 15

When a process is loaded in, it is itself segmented in order to fit into all available memory spaces

Question 16

What are the key points of Segmentation?

Answer 16

• Fragmentation is eliminated but the process itself becomes fragmented (not an issue if we keep track of fragments)
• Data structures that grow and shrink can be handled more easily
• Different protection properties can be given to each segment
• Hardware support in current CPUs

Question 17

What is Paging?

Answer 17

When a process is loaded in, it is divided into relatively small fixed-size pages. Memory is divided into frames of the same size. Process pages are loaded into the minimum required frames

Question 18

What are the key points of Paging?

Answer 18

• At most, a fraction of one frame is wasted when a process is loaded
• When a process is unloaded the space is freed in page sized sections, so is readily re used by the next process
• The frames used by a process need be contiguous in memory, so relocation is easy
• There is some fragmentation but it does not get worse over time.

Question 19

What is a major benefit of using Pages?

Answer 19

We only need to load the page actually being processed into memory. This means that:

• We can start running a process as soon as the first page is loaded.
• We can have pages from many processes inn main memory, maximising the multi processing capabilities
• We can run processes that are actually larger than main memory

Question 20

Is there a clear better Partitioning scheme?

Answer 20

No, each partitioning scheme has its advantages and disadvantages. Operating systems generally only use one scheme.

Question 21

What is MMU?

Answer 21

Memory Management Unit
There must be a map between the address requested by the CPU and the actual page which stores that data. The MMU uses a page translation table to information about memory contents, such as which frames are used and which pages they contain.

Question 22

What does the mapping process do?

Answer 22

The mapping process converts the logical address from the CPU into a physical address by looking up the page translation table

Question 23

What is the difference between large and small pages?

Answer 23

Large pages contain more addresses within them but there is usually less of them. While smaller pages contain fewer addresses but there are more of them

Question 24

How do you find the physical address of a page?

Answer 24

You first take the address offset which is where the memory address is in relation to the start of the page. You then take the page number and replace this with the frame which it is contained within. these two parts create the physical address

Question 25

What is the Physical Address of the Logical Address $00015F08 when it is contained within frame 004E and the page size is 64K?

Answer 25

Physical Address is $004E5F08

Question 26

How are Translation Tables stored?

Answer 26

Tables are stored in main memory. Most CPUs use a Address Translation Cache which caches translated addresses so they can be supplied faster

Question 27

What is a Single Level Translation Table?

Answer 27

The entire table is stored in main memory , including invalid entries which cause page faults. These tables can be very large unless large pages are used.

Question 28

How can we reduce the size of Translation Tables?

Answer 28

We can reduce the size by using multi level translation tables. This means having multiple tables which only contain a few frames within them. The logical address will contain level zero page and level one page along with the offset

Question 29

What are the advantages and disadvantages of Multi level Translation?

Answer 29

Adv: Only a complete level 0 table has to be in main memory(level 1 tables can be virtual memory), so tables take up less main memory space (despite bing larger than single level tables)

Disadv:Longer table walks required by MMU as it has to search down two levels to find final page address (This will be rare due to ATC)

Question 30

What is virtual memory?

Answer 30

Virtual memory is when pages which are inactive are temporarily put on a hard disk. In theory, it will be 50:50 real to virtual memory. In practice, however, the code actually being used must be in the real part.

Question 31

What happens when a requested page’s physical address shows it is in virtual memory?

Answer 31

A page or hard fault occurs meaning the page must be brought from disk, likely causing a swap, then a read cycle can be performed. This is demand paging

Question 32

Why do we want to avoid page faults?

Answer 32

Page faults take a long time to process as the hard disk is slow and if the program keeps jumping from one page to another with only one page in memory, then we can get an I/O “thrashing” situation. Locality of reference means that this problem won’t happen often

Question 33

What are the benefits of virtual memory?

Answer 33

If an I/O operation is blocking the progress of a process, the OS can give other processes more time on the CPU by sending the process to virtual memory/

Question 34

What are the ways we can deal with replacing a page if there are no free pages?

Answer 34

Similar to cache, we can use First in First out, Least Recently Used or Least Frequently Used. The same principles apply.