Modul 7- Swapping

Question 1

Virtual memory goals?

Answer 1

1: Allowing two or more processes to use main memory, given them an illusion of private memory.
2: Provide the illusion of a much larger address space than provided by the main memory.

Pages can be temporarily stored in secondary memory i.e. on disk.

Question 2

What does swapping help us with?

Answer 2

Swapping let’s us have some pages of processes virtual memory on a disk (hard-drive).

Question 3

Problems with swapping?

Answer 3

• Memory management must detect that a page is currently not in memory.
• If it is not in memory, how do we find it?
• If the memory is full, which page do we throw out?
• When we throw out a page, do we have to copy it to disk?
• Who should do all this, hardware or operating system?

Question 4

When a page is asked for what happens?

Answer 4

1. Check if page is present in memory. (TLB)
   
   2. 1 If present check if the access is allowed (Hit)
   3. 1 If it is allowed translate address, else segmentation fault
   4. 2 If page isn’t present, check if page is allocated on disk (Miss)
2. 2 If it’s on the disk, swap in page, else check is segment is allowed.
3. If the segment is allowed, allocate page else segmentation fault.

(Check slide lecture about swapping page 8).

Question 5

What is the swap space?

Answer 5

• Reserved space in the disk for moving pages back and forth.

Question 6

What does present bits in a page table entry tell us?

Answer 6

• It tells us whether a page is present in physical memory (present bit = 1) or if it’s on the disk (present bit = 0)

Question 7

What is a page fault?

Answer 7

• The act of accessing a page that is not in physical memory.
• Upon page fault the OS is invoked to service the page fault, known as page-fault handler. Thus arranges for the transfer of the desired page from disk to memory.