1. Virtual Memory Hakim Weatherspoon CS 3410 Computer Science
Cornell University
[Weatherspoon, Bala, Bracy, McKee, and Sirer]

2. Where are we now and where are we going?
How many programs do you run at once?
a) 1
b) 2
c) 3-5
d) 6-10
e) 11+

3. Big Picture: Multiple Processes
How to run multiple processes?
Time-multiplex a single CPU core (multi-tasking)
Web browser, skype, office, … all must co-exist
Many cores per processor (multi-core) or many processors (multi-processor)
Multiple programs run simultaneously

4. Processor & Memory CPU address/data bus... … routed through caches
… to main memory
Simple, fast, but…
CPU
0xfff…f
0x7ff…f
Stack $$
Heap
Data
Text
0x000…0
Memory

5. Multiple Processes
Q: What happens when another program is executed concurrently on another processor?
CPU
0xfff…f
0x7ff…f
Stack
Stack $$ $$
Heap
Heap
CPU
Data
Data
Text
Text
0x000…0
Memory

6. Multiple Processes Q: Can we relocate second program? $$ $$ CPU CPU
0xfff…f
0x7ff…f
Stack
Stack $$ $$
Heap
Heap
CPU
Data
Data
Text
Text
0x000…0
Memory

7. Big Picture: (Virtual) Memory
Process 1 3 2 1 A B C
Give each process an illusion that it has exclusive access to entire main memory D
Process 2 3 2 1 E F G H

8. But In Reality… Process 1 Process 2 14 D 13 12 E 11 10 C 9 8 7 B G 6 H5
Process 2 4 A 3 2 F 1
Physical Memory

9. How do we create the illusion?14 D 13 12
Process 1 3 2 1 A E 11 B 10 C C 9 D 8 7 B G 6 H 5
Process 2 3 2 1 E 4 F A 3 G 2 H F 1
Physical Memory

10. How do we create the illusion?14 D 13 12
Process 1 3 2 1 A E 11 B 10 C
All problems in computer science can be solved by another level of indirection.
– David Wheeler C 9 D 8 7 B G 6 H 5
Process 2 3 2 1 E 4 F A 3 G 2 H F 1
Physical Memory

11. How do we create the illusion?14 D 13 12
Process 1 3 2 1 A E 11 B
Map virtual address to physical address 10 C C 9 D 8
Virtual address
Physical address 7 B
Memory management unit (MMU) takes care of the mapping G 6 H 5
Process 2 3 2 1 E 4 F A 3 G 2 H F 1
Virtual Memory
(just a concept; does not exist physically)
Physical Memory

12. How do we create the illusion?14
Process 1 wants to access data C D 13 12
Process 1
Process 1 thinks it is stored at addr 1 3 2 1 A E 11 B
So CPU generates addr 1 10 C C 9 D
This addr is intercepted by MMU 8
Virtual address B
Physical address 7 G 6
MMU knows this is a virtual addr H 5
Process 2 3 2 1 E
MMU looks at the mapping 4 F A 3 G
Virtual addr 1 -> Physical addr 9 2 H
Data at Physical addr 9 is sent to CPU F 1
Virtual Memory
(just a concept; does not exist physically)
And that data is indeed C!!!
Physical Memory

13. How do we create the illusion?14 D 13 12
Process 1 3 2 1 A E 11 B
Map virtual address to physical address 10 C C 9 D 8 7 B
Memory management unit (MMU) takes care of the mapping G 6 H 5
Process 2 3 2 1 E 4 F 3 G 2 H 1
Virtual Memory A F
Physical Memory
Disk

14. Big Picture: (Virtual) Memory
From a process’s perspective –
Process only sees the virtual memory
Contiguous memory
Process 1 3 2 1 A B
Hidden from Process C C D
Physical Memory
Virtual Memory

15. Big Picture: (Virtual) Memory
From a process’s perspective –
Process only sees the virtual memory
Contiguous memory
No need to recompile - only mappings need to be updated
Process 1 3 2 1 A B
Hidden from Process C C D
Physical Memory
Virtual Memory

16. Big Picture: (Virtual) Memory
From a process’s perspective –
Process only sees the virtual memory
Contiguous memory
No need to recompile - only mappings need to be updated C
Process 1 3 2 1 A B
Hidden from Process C D
Physical Memory
Virtual Memory

17. Big Picture: (Virtual) Memory
From a process’s perspective –
Process only sees the virtual memory
Contiguous memory
No need to recompile - only mappings need to be updated
When run out of memory, MMU maps data on disk in a transparent manner
Process 1 3 2 1 A B
Hidden from Process C D
Physical Memory
Virtual Memory C
Disk

18. Next Goal How does Virtual Memory work?
i.e. How do we create the “map” that maps a virtual address generated by the CPU to a physical address used by main memory?

19. Next Goal (after spring break!)
How does Virtual Memory work?
i.e. How do we create the “map” that maps a virtual address generated by the CPU to a physical address used by main memory?

20. Virtual Memory Agenda What is Virtual Memory?
How does Virtual memory Work?
Address Translation
Overhead
Paging
Performance