1. Lecture 27: Virtual Memory

2. Review: goals of memory management
Memory allocation
A new process comes into memory
Memory de-allocation
A process leaves memory
Address translation
Virtual address to physical address
Process isolation
One process cannot access the memory of other processes

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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