1. Vivek Seshadri 15740/18740 Computer Architecture
Virtual Memory
Vivek Seshadri
15740/18740 Computer Architecture

2. Readings
Jacob & Mudge, “Virtual Memory in Contemporary Microprocessors,” IEEE Micro, 1998
Hennessy and Patterson, Appendix C.4-C.5

3. Why Virtual Memory? X
CPU
Load X
Store Y Y
Main Memory

4. Why Virtual Memory? Small available physical memory
Contiguous address space
Portability
Protection
Code & data
OS data and application data
One application’s data from another
Sharing

5. Segmentation Segment: Base address + Size + Permissions
Segments are re-locatable
Can be copied from one location to another
Each process is associated with a list of segments it can access

6. Virtual Memory All problems in computer science can be
solved by another level of indirection
- David Wheeler
For an application to make progress, it is sufficient that the next instruction and the data required to execute the next instruction are present in main memory.

7. Virtual Memory CPU Load X Store Y Main Memory Virtual Address Space X

8. Virtual Memory Solves It All!
Small available physical memory
Large virtual address space (VAS)!
Contiguous address space
Data should be contiguous only in VAS
Protection
Different process -> Different VAS
Sharing
Map VAS to same location in physical memory

9. Implementation Pages Page tables Inverted page tables
Large (to reduce mapping overhead)
Page tables
One entry per virtual page
Hierarchical (to reduce size of page tables)
Inverted page tables
Speeding up translations
Translation Look-aside Buffers (TLB)

10. Physical Memory Management
Allocating pages
Replacing pages (page fault)
Swap space in disk
Replacement policy
LRU?

11. Virtual Memory & Caching
Virtually-Indexed Virtually-Tagged Caches
Virtual Page Number
Offset
Cacheline Tag
Index
Two processes can have the same virtual address

12. Solution - 1 Physically-Indexed Physically-tagged Cache
Virtual Page Number
Offset
PhysicalPage Number
Offset
Cacheline Tag
Index

13. Solution - 2 Virtually-Indexed Physically-tagged Cache
Virtual Page Number
Offset
Physical Page Number
Offset
Cacheline Tag
Index

14. Two virtual pages can map to the same physical page
Synonyms
Two virtual pages can map to the same physical page
Virtual Page Number
Offset
Cacheline Tag
Index