1. Paging

2. Memory Partitioning Troubles
Fragmentation
Need for compaction/swapping
A process size is limited by the available physical memory
Dynamic growth of partition is troublesome
No winning policy on allocation/deallocation P2 P3

3. The Basic Problem A process needs a contiguous partition(s) But
Contiguity is difficult to manage
Contiguity mandates the use of physical memory addressing (so far)

4. The Basic Solution Give illusion of a contiguous address space
The actual allocation need not be contiguous
Use a Memory Management Unit (MMU) to translate from the illusion to reality

5. A solution: Virtual Addresses
Use n-bit to represent virtual or logical addresses
A process perceives an address space extending from address 0 to 2n-1
MMU translates from virtual addresses to real ones
Processes no longer see real or physical addresses

6. Paged Memory
Subdivide the address space (both virtual and physical) to “pages” of equal size
Use MMU to map from virtual pages to physical ones
Physical pages are called frames

7. Paging: Example
Virtual
Physical
Virtual
Process 1
Process 0

8. Key Facts
Virtual address spaces of different processes are independent
Two or more may have the same address range
Yet the mappings differentiate between them
A virtual page has no storage of its own
It must be backed by a physical frame (real page) that provides the actual storage
A contiguous virtual space need not be physically contiguous

9. Key Facts
Physical address space is independent of virtual address spaces
They can have different sizes
Allows process size to be independent of available physical memory size
Page size is always a power of 2, to simplify hardware addressing

10. Page Tables
Virtual
Page Table

11. Page Table Structure Indexed by virtual page number
Contains frame number (if any)
Contains protection bits
Contains reference bit
Frame No. v w r x f m
Frame No. v w r x f m
Frame No. v w r x f m
v: valid bit
w: write bit
r: read bit
x: execute bit (rare)
f: reference bit
m: modified bit

12. Mapping Virtual to Real Addresses
n bits
Virtual
address
virtual page number
offset
s bits
index into
page table
s bits
frame no.
offset
p bits
s: log (page size)
Physical address

13. Example: PDP-11 Page size: 8K Up to 4M mem 16 bits Virtual address vpn
offset
13 bits
13 bits
frame no.
offset
8-entry
page
table
22 bits
Physical address
(in hardware)

14. Weird Stuff: Free Page Management
Virtual
Physical
Virtual
Process 0
Process 1
Key fact:
A memory frame
cannot be accessed
unless mapped
Free space

15. Fun Stuff: Sharing
Virtual
Physical
Virtual
Process 1
Process 0

16. Sharing
Processes can share pages by mapping their virtual pages to the same memory frame
In UNIX and Windows, code segments of processes running the same program to share the pages containing executables
saves a lot of memory
saves loading time for frequently run programs
Fine tuning using protection bits (rwx)

17. Fork Revisited: Copy-on-Write
Virtual
Physical
Virtual W W W W W W
Father
Child

18. Copy-on-Write Fork Initially no memory copying
Efficient
If an exec follows, no much harm
Page tables set the protection to disallow writes
If either father or child attempts to write, a page fault occurs

19. Copy-on-Write
Virtual
Physical
Virtual W W W W W W
Father
Child

20. Requirements for Sharing
Page frames must have a reference count
Cannot be deallocated unless unmapped
Adds complexity to memory manager
Protection bits must be set properly
Must externalize protection bits to user programs (mprotect())
Protection bits meaning could be overloaded (bad)