1. CSCI206 - Computer Organization & Programming
Introduction to Virtual Memory
zyBook: 12.9

2. Review Memory Map Heap has largest address Code begins near the bottom
0x7FFF FFFF is the last byte in 32-bit MIPS
Code begins near the bottom
0x in MIPS
2 GB memory range

3. Problem We commonly run multiple programs on our computers (show top).
We don’t share data between programs!
How do separate programs access the same memory map without conflict?

4. Virtual Memory
A simulated address space is provided for each process by the operating system.
These addresses are called virtual addresses.
The operating system maintains a mapping between PID+virtual address to physical address.

5. Virtual Memory (aka logical memory)

6. VM - Basics
Each program is given the illusion of owning a large / flat memory space
Virtual memory supports a larger address space than is physically available
because of spatial / temporal locality we can (usually) move some data to disk (swap)
Transparent to the programmer
Efficient... with a bit extra little hardware

7. How VM Works Like cache, VM is allocated in chunks called pages
The Page offset is consistent between virtual and physical
Common page size is 4KB
Translations stored by the OS in the Page Table
In the example on the left, we have 4G VM, 1 G physical memory

8. Address range of VM
The range of address for VM is somewhat independent of amount of the physical memory
VM addresses for a process (program) are contiguous
Physical memory addresses do not have to be contiguous

9. What is a page fault?
There are more virtual addresses than physical addresses.
As a result, contents of some virtual addresses are located on disk
If a virtual address is needed by a program and it is currently on disk a page fault occurs
The OS allocates physical RAM and move data from disk to RAM (and possibly moves other data from RAM to disk)

10. Page Table (OS)
The OS stores the virtual to physical mapping in a table in memory.
indexed by virtual page number.
the contents are the physical page numbers.

11. Page Table Question
How many pages can there be in a 32-bit system with 4 KB pages?

12. Page Table Question
How many pages can there be in a 32-bit system with 4 KB pages?
4 KB nees 12 bits as offset
Which leaves 20 bits to count pages, which is 2^20 ~= 1 million pages

13. Page Table Question
How big is the page table for a 32-bit system with 4 KB pages?

14. Page Table Question
How big is the page table for a 32-bit system with 4 KB pages?
There are 2^20 entries, each with
19 bits (18 addr bits, 1 valid bit), a total of MB

15. Hardware Issues Program memory accesses use virtual addresses.
Hardware uses physical addresses to access data (RAM).
How does hardware (CPU) handle a memory access?

16. Cache with Virtual Addresses?
RAM
V to P ?
Offset
CACHE
Index
Tag
V to P ?
Does the cache use virtual addresses or physical addresses?

17. Virtual Memory Caching
All programs have their own virtual address space
Therefore, two programs will refer to the same addresses, but expect to find their own data!
As a result, we cannot cache using virtual addresses
There is no way to tell data from each process apart using only the virtual address
However, the virtual to physical translation is on the critical path for every cache access!
This needs to be fast, so we create a special cache, called the translation lookaside buffer (TLB), just for caching the page table!

18. TLB
Memory access begins with a TLB access to find the physical address, then access the cache.
The TLB is flushed on context switches, but cache contents are not!