Memory Management II

Question 1

What is page demanding ?

Answer 1

When a new process starts, a page is loaded.
If an address is called that is outside the page it causes a page fault.
The new page is loaded, forming a resident set.

Question 2

What is a page table ?

Answer 2

Every processes has a page table. Each page entry has a:
- Present bit (indicates if the page is in real memory)
- Modified bit (shows if the page has been modified)

Question 3

What is virtual memory ?

Answer 3

Stores unused pages in swap space rather than on the physical memory. Page tables can be large and stored on secondary memory. Retrieving the pages is a lot slower than execution time.

Question 4

What is a working set of pages ?

Answer 4

A set of pages a process will need in a given time.

Question 5

What is thrashing ?

Answer 5

When there are too many processes the CPU spends time swapping pages instead of executing. This is fixed by balancing the number of processes and allocating memory wisely.

Question 6

What are the main page replacement algorithms ?

Answer 6

• Least recently used (LRU), replaces the page not used for the longest time
• Not recently used (NLU), replaces the page not being used in a given time frame
• First in first out (FIFO), replaces the oldest page\
• Second chance algorithm, uses a circular list and a used bit to give pages a second chance before replacing them

Question 7

Why are paging and segmentation useful ?

Answer 7

Paging allows efficient memory use and optimization
Segmentation allows logical structuring and dynamic memory allocation

Question 8

How does a paged segmented system work ?

Answer 8

Segment number points to a page table,
Page number points to page frame,
Displacement points to real memory address