1. Chapter 3.2 : Virtual Memory
What is virtual memory?
Virtual memory management schemes
Paging
Segmentation
Segmentation with paging
Page table management
Ceng Operating Systems

2. Problems with Memory Management Techniques so far
1. Unused (wasted) memory due to fragmentation
2. Memory may contain parts of program which are not used during a run (ie., some routines may not be accessed in that particular run)
3. Process size is limited with the size of physical memory
Ceng Operating Systems

3. Virtual memory of process on disk Ceng 334 - Operating Systems
Virtual Memory (VM)
Virtual memory of process on disk
Map (translate)
virtual address to real
Real memory of system
Ceng Operating Systems

4. Ceng 334 - Operating Systems
Virtual Memory (VM)
VM is conceptual
It is constructed on disk
Size of VM is not limited (usually larger than real memory)
All process addresses refer to the VM image
When the process executes all VM addresses are mapped on to real memory
Ceng Operating Systems

5. Did We Solve the “Problems”?
1. Unused (wasted) memory due to fragmentation (We’ll see!)
2. Memory may contain parts of program which are not used during a run
YES! Virtual memory contents are loaded into memory on demand
3. Process size is limited with the size of physical memory
YES! Process size can be larger than real memory
Ceng Operating Systems

6. Ceng 334 - Operating Systems
(Pure) Paging
Virtual and real memory are divided into fixed sized pages
Programs are divided into pages
A process address (both in virtual & real memory) has two components
Page #
Offset within page
Ceng Operating Systems

7. Interpretation of an Address
16 bits for addressing means that the memory addressed is 64K bytes (0000 -FFFF)
Suppose page size is 4K bytes (12 bits)
The virtual memory has 16 pages (4 bits)
Real memory can have at the most 16 pages
Example :
address : F B C
Page #
Offset
Ceng Operating Systems

8. The relation between virtual addresses and physical memory addresses
64K Virtual Memory
32K Real Memory
Ceng Operating Systems

9. Ceng 334 - Operating Systems
Paging (Cont.)
When process pages are transferred from VM to real memory, page numbers must be mapped from virtual to real memory addresses
This mapping is done by software & hardware
When the process is started only the first page (main) is loaded. Other pages are loaded on demand
Ceng Operating Systems

10. Virtual Memory – Memory Management Unit
The position and function of the MMU
Ceng Operating Systems

11. Ceng 334 - Operating Systems
Internal operation of MMU with 16 4 KB pages
Note that virtual address is 16 bits (64K virtual memory) but the physical address is 15 bits (32K real memory)
Ceng Operating Systems

12. Ceng 334 - Operating Systems
Page Tables
Page Table
Page frame #
Page protection
Reference bit
Modification bit
Validity bit
Page Table Entry 1 8 2 3 4 5 6 7
Index of page table is the virtual page #
Ceng Operating Systems

13. Page Table Entry Fields
Validity bit is set when the page is in memory
Reference bit is set set by the hardware whenever the page is referred
Modified bit is set whenever the page is modified
Page-protection bits set the access rights (eg., read, write restrictions)
Ceng Operating Systems

14. Address Mapping in Paging
Page tables of processes
PTn
PT 1
PT2
. .
Page #
Offset within page
Page Table Register +
Catenate
Virtual memory address
Real memory address
Ceng Operating Systems

15. Address Mapping in Paging (Cont.)
During the execution every page reference is checked against the page map table entry
If the validity bit is set (ie., page is in memory) execution continues
If the page is not in memory a page fault (interrupt - trap) occurs and the page is fetched into memory
If the memory is full, pages are written back using a page replacement algorithm
Ceng Operating Systems

16. Memory Management Problems : Re-visit due to paging
1. Unused (wasted) memory due to fragmentation
ONLY on the last page (Page Break)
2. Memory may contain parts of program which are not used during a run
Virtual memory contents are loaded into memory on demand
3. Process size is limited with the size of physical memory
Process size can be larger than real memory
Furthermore, program does not occupy contiguous locations in memory (virtual pages are scattered in real memory)
Ceng Operating Systems

17. Ceng 334 - Operating Systems
Segmentation
Pages are fixed in size, segments are variable sized
A segment can be a logical entity such as
Main program
Some routines
Data of program
File
Stack
Ceng Operating Systems

18. Ceng 334 - Operating Systems
Segmentation (Cont.)
Process addresses are now in the form
Segment #
Offset within segment
Segment Map Table has one entry for each segment and each entry consist of
Segment number
Physical segment starting address
Segment length
Ceng Operating Systems

19. Ceng 334 - Operating Systems
Segmentation (1)
One-dimensional address space with growing tables
One table may bump into another
Ceng Operating Systems

20. Ceng 334 - Operating Systems
Segmentation (2)
Allows each table to grow or shrink, independently
Ceng Operating Systems

21. Ceng 334 - Operating Systems
Segmentation (3)
Comparison of paging and segmentation
Ceng Operating Systems

22. Implementation of Pure Segmentation
(a)-(d) Development of fragmentation
(e) Removal of the fragmentation by compaction
Ceng Operating Systems

23. Problems with Segmentation
Similar problems in dynamic partitioning
Fragmentation in real memory
Relocation is necessary for compaction
Ceng Operating Systems

24. Segmentation with Paging
Segmentation in virtual memory, paging in real memory
A segment is composed of pages
An address has three components
Page #
Offset within page
Segment #
The real memory contains only the demanded pages of a segment, not the full segment
Ceng Operating Systems

25. Addressing in Segmentation with Paging
Segment Table
Page Tables
Pages
Page #
Offset within page
Segment #
Ceng Operating Systems

26. Segmentation with Paging: MULTICS (1)
Descriptor segment points to page tables
Segment descriptor – numbers are field lengths
Ceng Operating Systems

27. Segmentation with Paging: MULTICS (2)
A 34-bit MULTICS virtual address
Ceng Operating Systems

28. Segmentation with Paging: MULTICS (3)
Conversion of a 2-part MULTICS address into a main memory address
Ceng Operating Systems

29. Segmentation with Paging: MULTICS (4)
Simplified version of the MULTICS TLB
Existence of 2 page sizes makes actual TLB more complicated
Ceng Operating Systems

30. Segmentation with Paging: Pentium (1)
A Pentium selector
Ceng Operating Systems

31. Segmentation with Paging: Pentium (2)
Pentium code segment descriptor
Data segments differ slightly
Ceng Operating Systems

32. Segmentation with Paging: Pentium (3)
Conversion of a (selector, offset) pair to a linear address
Ceng Operating Systems

33. Segmentation with Paging: Pentium (4)
Mapping of a linear address onto a physical address
Ceng Operating Systems

34. Segmentation with Paging: Pentium (5)
Level
Protection on the Pentium
Ceng Operating Systems

35. Ceng 334 - Operating Systems
How Big is a Page Table?
Consider a full 2 32 byte (4GB) address space
Assume 4096 byte (2 12 byte) pages
4 bytes per page table entry
The page table has 2 32/2 12 (= 2 20 ) entries (one for each page)
Page table size would be 2 22 bytes (or 4 megabytes)
Ceng Operating Systems

36. Problems with Direct Mapping?
Although a page table is of variable length depending on the size of process, we can not keep them in registers
Page table must be in memory for fast access
Since a page table can be very large (4MB), page tables are stored in virtual memory and be subjected to paging like process pages
Ceng Operating Systems

37. Ceng 334 - Operating Systems
How to Solve?
Two-level Lookup
Inverted Page Tables
Translation Lookaside Buffers
Ceng Operating Systems

38. Virtual Address - 32 bits (4 GB)
Two-Level Lookup
12 bits Byte pages
10 bits pages
10 bits directories
Virtual Address - 32 bits (4 GB)
Directory
Page
Offset
Ceng Operating Systems

39. Two-Level Lookup (Cont.)
Dir
Page
Offset +
Physical Address
Page Directory
Page Table
Ceng Operating Systems

40. Two-Level Lookup (Cont.)
A process is represented by one or more entries of the page directory (ie., several page tables)
Typically a page table size is set as one page which makes swapping easier
This is one of the addressing schemes used by Intel family of chips (Intel chips can use paging, segmentation or a combination of both)
Ceng Operating Systems

41. Two-Level Lookup (Cont.) Ceng 334 - Operating Systems
Second-level page tables
Top-level
page table
32 bit address with 2 page table fields
Two-level page tables
Ceng Operating Systems

42. Ceng 334 - Operating Systems
Inverted Page Tables
Page #
PID
Page #
Offset within page
Virtual Address 21 17
hash
Hash Table 76 23
Page # 101
Process # 17
Hash # 2 3
213 32
Page Frame Table
Ceng Operating Systems

43. Inverted Page Tables (Cont.)
The virtual page number is hashed to point to a hash table
The hash table contains a pointer to the inverted page table
Inverted page table contains page table entries (one for each real memory page)
Entries having the same hash codes are chained
Ceng Operating Systems

44. Inverted Page Tables (Cont.)
A fixed portion of memory is used for mapping regardless of the number of processes or virtual pages
This approach is used by IBM’s AS/400 and RISC System/6000 computers
Ceng Operating Systems

45. Translation Lookaside Buffer
Frame #
Offset
TLB Hit
TLB Miss
TLB
Page
Table
Page #
Virtual Address
Real Address
Page Fault
Ceng Operating Systems

46. Translation Lookaside Buffer (Cont.)
Translation lookaside buffer (TLB) is a cache for page table entries
TLB contains page table entries that have been most recently used
Whenever the page table is referred (TLB miss), the page table entry is also copied to the TLB
TLB is usually an associative memory (content addressable memory)
Ceng Operating Systems

47. TLBs – Translation Lookaside Buffers
A TLB to speed up paging
Ceng Operating Systems