1. 4.3 Virtual Memory

2. Virtual memory
Want to run programs (code+stack+data) larger than available memory.
Overlays
programmer divides program into pieces
OS move pieces in and out from disk
Virtual memory – OS automates overlays
Virtual addresses vs. physical addresses

3. CPU MMU translates virtual addresses to physical addresses

4. Memory unit = page

5. Mapping virtual to physical addresses
Done in hardware for speed.
Virtual address = page number and offset within page
Each process has its own page table.
Can be large
Physical address = pageTable[pageNumber] + offset
Must be fast
Page fault occurs when a page is not in memory and is references.

6. Ex. 32 bit virtual address space
Given 32 bit virtual address space.
Given 212=4096 byte pages.
How many pages?
1 million!

7. Ex. Page table
Page table size = 24
Page size = 212

8. Multilevel page tables

9. Example page table entry
Caching disabled bit
For memory mapped I/O
(use device, not cache)
Referenced = used bit
Modified = “dirty” bit
Protection = R W X flags
Present = in memory flag

10. TLBs – translation lookaside buffers
a.k.a. associative memory
Lookup/search occurs in parallel
8 to 64 entries
“cache” of frequently used page table entries

11. Inverted page tables Using virtual to physical page tables: Solution:
Over 1 million entries for 4K pages on a 32 bit system.
Over 30 million gigabytes on a 64 bit system
Solution:
Use one entry per physical page (rather then one entry per virtual page)
Search for virtual to physical translation becomes hard (slow)
Use TLB and hash table

12. Page table vs. inverted page table