1. Chapter 4 Memory Management Basic memory management Swapping
Virtual memory
Page replacement algorithms

2. Memory Management Ideally programmers want memory that is
large
fast
non volatile
Memory hierarchy
small amount of fast, expensive memory – cache
some medium-speed, medium price main memory
gigabytes of slow, cheap disk storage
Memory manager handles the memory hierarchy

3. Basic Memory Management Monoprogramming without Swapping or Paging
Three simple ways of organizing memory
- an operating system with one user process

4. Multiprogramming with Fixed Partitions
Disadvantage
Queue for small partition
may be full, but the queue
for a large partition is empty
Disadvantage
Wastage of space
If largest process is
selected, small process
Are deprived
Fixed memory partitions
separate input queues for each partition
single input queue

5. Relocation and Protection
Cannot be sure where program will be loaded in memory
suppose 1st instruction of a program jumps at absolute address 200 within the exe file and it is loaded into partition 1 (previous slide) started at 100K. Jump should be performed at 100K+200.
This problem is called relocation
Relocation Solution
As the program is loaded into memory, modify the instructions accordingly.
Address locations are added to the base location of the partition to map to the physical address.
Protection
Must keep a program out of other processes’ partitions
Relocation during loading may cause protection problem
Protection Solution
Address locations larger than limit value is an error

6. Swapping and Virtual Memory
Sometimes there is not enough main memory to hold all the currently active processes.
So excess processes must be kept on disk and brought into run dynamically
Two approach
Swapping:
Bring in each process entirely, running it for a while and then put it back on the disk
Virtual memory:
Brings each process partially, not entirely

7. Swapping (1) Memory allocation changes as
processes come into memory and leave memory
No concept of fixed partitioning of memory like previous. Number, location and size of partitions vary dynamically.
High Utilization of memory, but it complicates allocation and de-allocation of memory

8. Swapping (2) Allocating space for growing data segment
Allocating space for growing stack & data segment

9. Memory Management for Dynamic Allocation(3)
When memory is assigned dynamically, the OS must manage it. Two ways:
With bit maps
With linked list

10. Memory Management with Bit Maps and Linked List(4)
1 = Allocated block
0 = Free block
Linked List
P = Process allocation
H = Hole

11. Virtual Memory (1)
The basic idea behind virtual memory is that the combined size of the program may exceed the amount of physical memory available for it.
The OS keeps part of the program currently in use in main memory, and the rest on the disk.
For example
A 16MB program can run on a 4MB memory
Virtual memory also allows many programs in memory at once.
Most virtual memory systems use a technique called paging

12. Paging (2) The relation between virtual addresses and physical memory
addresses given by page
table

13. TLBs – Translation Lookaside Buffers (3)
A TLB to speed up paging

14. Page Replacement Algorithms
Page fault forces choice
which page must be removed
make room for incoming page
Modified page must first be saved
unmodified just overwritten
Better not to choose an often used page
will probably need to be brought back in soon

15. Optimal Page Replacement Algorithm
Replace page needed at the farthest point in future
Optimal but unrealizable
Estimate by …
logging page use on previous runs of process
although this is impractical

16. Not Recently Used Page Replacement Algorithm
Each page has Reference bit, Modified bit
bits are set when page is referenced, modified
Pages are classified
not referenced, not modified
not referenced, modified
referenced, not modified
referenced, modified
NRU removes page at random
from lowest numbered non empty class

17. FIFO Page Replacement Algorithm
Maintain a linked list of all pages
in order they came into memory
Page at beginning of list is replaced
Disadvantage
Highly used page may be a victim

18. Second Chance Page Replacement Algorithm
Operation of a second chance
pages sorted in FIFO order
Page list if fault occurs at time 20, A has R bit set (numbers above pages are loading times)

19. The Clock Page Replacement Algorithm

20. Least Recently Used (LRU)
Assume pages used recently will used again soon
throw out page that has been unused for longest time
Must keep a linked list of pages
most recently used at front, least at rear
update this list every memory reference !!
Alternatively keep counter in each page table entry
choose page with lowest value counter
periodically zero the counter

21. Page Size (1) Small page size Advantages Disadvantages
less internal fragmentation
less unused program in memory
Disadvantages
programs need many pages, larger page tables

22. internal fragmentation
Page Size (2)
Overhead due to page table and internal fragmentation
Where
s = average process size in bytes
p = page size in bytes
e = page entry
page table space
internal fragmentation
Optimized when

23. Page Fault Handling (1) 1. Hardware traps to kernel
2. OS determines which virtual page needed
3. Save registers
4. OS checks validity of address, seeks page frame
5. If selected frame is dirty, write it to disk

24. Page Fault Handling (2) 6. OS brings new page in memory from disk
7. Page tables updated
8. Faulting instruction backed up to when it began
9. Faulting process scheduled
10. Registers restored
11. Program continues