1. Memory Map

2. Paging
• In protected mode linear address space is
mapped directly into a large physical memory
• Or indirectly (using paging) into a smaller
physical memory & disk storage.
• This second method of mapping the linear
address space is referred as virtual memory or
demand paged virtual memory

3. Paging
• Paging is another type of memory
management useful for virtual memory
multitasking operating systems
• Freed space of an outgoing task always same
as that needed for an incoming task

4. Paging
• Linear address space is divided into fixed size pages
• Pages can directly mapped into physical memory
and/or disk storage
• A page most likely corresponds to only a portion of
a module or data structure

5. Paging
• Page –
– Fixed Size
– May include Code/Data/Stack
– Linear address is used to generate address of a
page
• A program on HDD is looked at as collection of
same sized pages

6. Paging
• Fixed size
• Data structures and
program modules can be
partly in memory and
partly in disk storage
• Pages bear no direct
relation to the logical
structure of a program
Segmentation
• Size same as the code or
data structures they hold
• Data structures and
program modules will
have all its parts in
memory
• While segment selectors
can be considered the
logical ``name'' of a
program module or data
structure,

7. Advantages
• A uniform size of 4KB for all of the elements
simplifies memory allocation and reallocation
schemes, since there is no problem with
memory fragmentation.

8. Important Bits

9. Components of
the paging mechanism
• The page directory
• The page tables
• The page itself (page frame)

10. HDD
P2.1
P1.0
P2.0
P3.0
P1.1
P1.2
P2.2
P2.3
FREE Page
P1.0
P1.1
P1.2
P2.0
P2.1
P2.2
P2.3
P3.0
0
1
2
3
4
5
6
7
8
Page
No.
Base
address
0 0
1 2
2 5
Page
No.
Base
address
0 3
Page No. Base
address
0 1
1 4
2 6
3 7
PMT : P3 process
PMT : P1 process
PMT : P2 process
HDD

11. Page Map Table, Page Directory
• Pages of a unique program are tabulated in a
table – page Map table
• Base addresses of All page tables are
tabulated in a directory
• Every application loaded has a unique page
table

12. Page Directory
• Tabulates information about all page tables in
memory
• Every record is called a Page Directory Entry
(PDE)
• Processor provides a special register viz CR3 to
hold base address of this directory.
• CR3 is known as Page Directory Base Register
(PDBR)

13. Paging Exceptions

14. Page Directory Entry
Page Table Base Address OS
Reserved
(User)
0 0 D
(U)
A 0 0 U/S R/
W
P
31 12 11 10 9 8 7 6 5 4 3 2 1 0
Pg Size –
‘0’  4KB
Whether a page table
-
A – Accessed
P – Present
Access Rights
U/S –
‘0’ – table is assigned supervisor privilege level
‘1’ – table is assigned user level privilege level
R/W -
Read Write privileges for a group of pages i.e. table

15. PAGE TABLES
• Each Page Table is 4K bytes and holds up to
1024 Page Table Entries.
• Page Table Entries contain the starting address
of the page frame and statistical information
about the page
• Page tables can be shared between tasks and
swapped to disks.

16. Page Table Entry
PAGE FRAME ADDRESS
31..12
OS
Reserved
(User)
0 0 D A 0 0 U/S R/
W
P
31 12 11 10 9 8 7 6 5 4 3 2 1 0
Whether a page
D - Dirty
A – Accessed
P – Present
Access Rights
U/S –
‘0’ – table is assigned supervisor privilege level
‘1’ – table is assigned user level privilege level
R/W -
Read Write privileges for a group of pages i.e. table

17. Non Present Page directory or Page Table entry
31 1 P
Available to operating system for use 0

18. LINEAR ADDRESS TRANSLATION (4-KBYTE PAGES)
To select the various table entries, the linear address is
divided into three sections:
• Page-directory entry—Bits 22 through 31 provide an offset
to an entry in the page directory. The selected entry provides
the physical base address of a page table.
• Page-table entry—Bits 12 through 21 of the linear address
provide an offset to an entry in the selected page table. This
entry provides the physical base address of a page in physical
memory.
• Page offset—Bits 0 through 11 provides an offset to a
physical address in the page.

19. Linear Address Translation - (4 KB Pages)
Directory Entry Page Table Entry
4KB page
PDBR
Index to Directory Index to Table Offset in Page
31 22 21 12 11 0
Page Directory Page Table
Page
CR3

20. Virtual address 801C3400H
Directory Entry
000E4000
Page Table Entry
00028000
PDBR
Index to Directory Index to Table Offset in Page
1000000000 0111000011 010000000000
Page Directory Page Table
CR3
200H
1C3H
400H
+
Physical address
00028400H