1. Lecture 28: Virtual Memory-Address Translation

2. Review: Memory management for different systems
Processes are known in advance and can fit in memory
Embedded systems, e.g., radios, washing machines, and microwaves
Processes are unknown in advance, memory might not be large enough
Swapping
Virtual memory

3. Review: Swapping Each individual process can fit in memory
But memory cannot fit all processes
Memory allocation/de-allocation
Bitmap
Linked list
Dynamic address translation
Base register in CPU
Process isolation
Limit register in CPU

4. Virtual memory What if a single process is larger than memory?
Not all parts of the large process are equally possible to be used
Put the parts that are less used in disk
The process can still run
Load a part from disk to memory when needed

5. Paging: Key idea of virtual memory
Divide a program into fixed-size small parts
Called virtual pages or pages
Divide the memory into fixed-size small blocks
Called page frames
Load some pages into page frames

6. An example of paging 64K Virtual Address Space 32K Physical Memory
Divided into 16 pages
4K for each page
32K Physical Memory
8 pages of 4K each

7. Problems in paging Address translation Page fault
When a page that the process is trying to use is not in memory, hardware issues page fault
Load in the page into a page frame
Which page frame goes to disk?
This is called page replacement problem

8. Address translation Process generates a “virtual address”
Virtual address is translated into a physical address
Physical address goes onto the bus

9. Address translation: Memory Management Unit (MMU)

10. Address translation via Page Table
Page table is a data structure
Each page has an entry in this page table
The index of the page is used to find the entry in the page table
Each entry stores various information about the page

11. Address translation via Page Table

12. Typical Page Table Entry
Present/absent: 1 if the page is in memory
Protection: what operations to the page are allowed
Modified: 1 if the page is modified since it is loaded. Also called
“dirty” bit
Referenced: 1 if the page is read or written
Caching disabled: 1 if disabling caching this page

13. Page Fault What if required page not in memory?
MMU reports a page fault to CPU
CPU gives control to the OS
OS fetches it from the disk
Needs to evict an existing page from memory (page replacement policy)
Instruction is restarted

14. Performance The address translation is done on every memory reference
The translation better be fast!

15. Question
Where is the page table stored?
Registers?
Memory?

16. Store in registers
Need many registers
Context switching is slow

17. Store in memory
One register to store the start address of the page table
Context switching is fast
May do paging for the page table itself.
Store some page entries in disk if the page table is too large.