W7 - Virtual Memory (1)

Question 1

Define virtual address space.

Answer 1

Virtual storage assigned to a process

Question 2

Distinguish difference between real memory and virtual memory.

Answer 2

Real memory: main memory, actual RAM

Virtual memory: memory on disk, multiprogramming

Question 3

What are the 2 characteristics fundamental to memory management?

Answer 3

1) all memory references that are logical addresses are dynamically translated into physical addresses at runtime
2) a process may be broken up into a number of pieces that don’t need to be continuously located in main memory during execution

Question 4

True or False:

An interrupt is generated when an address is needed that is not in main memory.

Answer 4

True

Question 5

Name some support functions required for virtual memory.

Answer 5

1) hardware must support paging and segmentation
2) operating system must include software for managing the movement of pages and/or segments between secondary memory and main memory

Question 6

Paging

Answer 6

Each process has its own page table. Each page table entry contains the frame number of the corresponding page in main memory

Question 7

What do the 2 extra bits need to indicate in paging?

Answer 7

1) whether the page is in main memory or not

2) whether the contents of the page has been altered since it was last loaded

Question 8

What is an alternative to page table problem?

Answer 8

A problem is that the type of page tables, is that their size if proportional to that of the virtual address space. Our solution is inverted page table.

Question 9

How are inverted page table efficient?

Answer 9

1) Use hashmap to map out tables
2) fixed proportion of real memory is required for the table regardless of number of processes
3) indexes page table entries by frame number than virtual page number

Question 10

What information does an inverted page table include?

Answer 10

Page number
Process identifier
Control bits
Chain pointer

Question 11

What is the purpose of Translation Lookaside Buffers (TLB)?

Answer 11

To overcome the problem of storing 2 physical memory accesses for page table and its data. It contains page table entries that have been most recently used

Question 12

Briefly describe the TLB operation.

Answer 12

1) given virtual address
2) if page table entry present (TLB hit)
3) if page table entry absent (TLB miss)
4) check if page already in main memory
5) TLB updated to include new page entry

Question 13

Describe pro and cons of different page sizes.

Answer 13

The smaller page size, the more pages required per process

Larger page tables means large portion of page tables in virtual memory