1. Virtual Memory So, how is this possible?
Modern Operating systems can run programs that require more memory than the system has
If your CPU is 32-bit, meaning that it has registers that are 32-bits, you can access up to 4G
Which means you would need 4Gb of RAM in order to take advantage of this
Although many systems are currently available with 512MB
Usually memory requirements of the programs you are running, reach far beyond the physical memory you have.
Usually we don't notice any performance problems.
So, how is this possible?
Virtual Memory

2. Virtual Memory
To solve this problem OS uses something called virtual memory
It is virtual because it can use more that you actually have.
In fact, with virtual memory you can use the whole 232 bytes.
Basically, what this means is that you can run more programs at once without the need for buying more memory.
E.g, in Linux OS if you have more data than physical memory, the system store it temporarily on the hard disk if not needed at the moment.
Process of moving data to and from the disk is called swapping.
Virtual Memory

3. Main Idea
Virtual Memory
(residing on disk)
Main Memory
System Cache
Is cached by
All memory transfers are only between consecutive levels (e.g. VM to main memory, main memory to cache).
Virtual Memory

4. Cache vs. VM
Concept behind VM is almost identical to concept behind cache.
But different terminology!
Cache: Block VM: Page
Cache: Cache Miss VM: Page Fault
Caches implemented completely in hardware.
VM implemented in software, with hardware support from CPU.
Cache speeds up main memory access, while main memory speeds up VM access.
Virtual Memory

5. Virtual Memory Design
Main Memory at Virtual Memory are both divided into fixed size pages.
Page size is typically about 16KB to 32KB.
Large page sizes are needed as these can be more efficiently transferred between main memory and virtual memory.
Size of physical page ALWAYS equal to size of virtual page.
Pages in main memory are given physical page numbers, while pages in virtual memory are given virtual page numbers.
I.e. First 32KB of main memory is physical page 0, 2nd 32KB is physical page 1 etc.
First 32KB of virtual memory is virtual page 0, etc.
Virtual Memory

6. This is extremely impractical for virtual memory!
Virtual Memory Design
In cache, we can search through all the blocks until we find the data for the address we want.
This is because the number of blocks is small.
This is extremely impractical for virtual memory!
The number of VM pages is in the tens of thousands!
Virtual Memory

7. Solution Use a look up table.
The addresses generated by the CPU is called the virtual address.
The virtual address is divided into a page offset and a virtual page number:
Virtual Page Number
Page Offset
The virtual page number indicates which page of virtual memory the data that the CPU needs is in.
Virtual Memory

8. Solution…cont
The data must also be in physical memory before it can be used by the CPU!
Need a way to translate between the virtual page number where the data is in VM, to the page number of the physical page where the data is in physical memory.
To do this, use Virtual Page Table.
Page Table resides in main memory.
One entry per virtual page. Can get VERY large as the number of virtual pages can be in the tens of thousands.
Virtual Memory

9. Virtual Page Table
Gives the physical page (frame) # of a virtual page, if that page is in memory.
Gives location on disk if virtual page is not yet in main memory.
frame0
frame1
page0
frame2
page1
frame3
page2
page3
Physical Memory
page4
page5
Virtual Memory Table
VM (on Disk Space)
Virtual Memory

10. Page Table Contents
The page table also contains a Valid Bit (V) to indicate if the virtual page is in main memory (V=1) or still on disk (V=0). 1
(2,1,7) 2
(7,2,9) 3
page0
page1
page2
page3
page4
page5
If a page is in physical memory (V=1), then the page table gives the Frame #.
Otherwise it gives the location of the page on disk , in the form (side#, track#, block#).
Virtual Memory

11. Accessing Data To retrieve data:
1. Extract the Virtual Page Number from the Virtual Address
Virtual Page Number (e.g. 02)
Page Offset
Virtual Memory

12. Accessing Data
2. Use the page to look up the page table. If V=1, get the frame from the page table: 1
(2,1,7) 2
(7,2,9) 3
page0
page1
page2
page3
page4
page5
page = 2
frame=0
Here virtual page#2 mapped to frame#0.
Virtual Memory

13. Accessing Data
3. Combine the frame found with the page offset to form the physical memory address:
Physical Page Number 0
Page Offset
Phyiscal Page Number 0
Physical Address
Virtual Memory

14. Accessing Data 4. Access main memory using the physical address.
A page consists of many bytes (e.g. 32KB)
The page offset tells us exactly which byte of these 32KB we are accessing.
Similar to the idea of block offset and byte offset in caches
Virtual Memory

15. Page Fault What if the page we want is not in main memory yet?
1. In this case, V=0, and the page table contains the disk address of the page (e.g. page1 in the previous example is still at side 2, track 1, block 7 (2,1,7) of the disk.
2. Find a free physical page
- if none are available, apply a replacement policy (e.g. LRU) to find one.
3. Load the virtual page into the physical page.
- Set the V flag, and update the page table to show which physical page the virtual page has gone to.
Virtual Memory

16. Behavior of Page Replacement Algorithms
Virtual Memory

17. C-Miss P-Fault load 00100 0 00 load 32 load 00101 0 00 C-Miss P- Hit!
Example: 2 blocks cache, 4bytes/block VA space=8 pages &32 Byte/page VA= 3bits(page#) + 5-bit(offset)
C-Miss
P-Fault
load
load 32
load
C-Miss
P- Hit!
load 40
Index
Valid
Tag
Data N 1 Y
00100
Memory[ ] Y
00101
Memory[ ]
Virtual Page Number
Page offset
001
00000
Memory
001
01000
Page Index
Valid
Frame number N 1
...
Disk Y
111
Virtual Memory

18. 4 Megabytes just for page tables!! Too Big
Example
Suppose we have 32 bit virtual address (232 bytes) bytes per page (212 bytes) 4 bytes per page table entry (22 bytes) What is the total page table size?
4 Megabytes just for page tables!! Too Big
Virtual Memory

19. Writing to VM
Writes to Virtual Memory is always done on a write-back basis.
To support write-back, the page-table must be augmented with a dirty-bit (D).
This bit is set if the page is updated in physical memory.
Virtual Memory

20. Writing to VM Here virtual page#2 was updated in physical frame#0.1
(2,1,7) 2
(7,2,9) 3
page0
page1
page2
page3
page4
page5
frame or disk location V D
Here virtual page#2 was updated in physical frame#0.
If frame#0 is ever replaced, its contents must be written back to disk to update page#2.
Virtual Memory

21. Translation Look-aside Buffer
An access to virtual memory requires 2 main memory accesses at best.
One access to read the page table, another to read the data.
Remember from the Cache section that main memory is slow
Fortunately, page table accesses themselves tend to display both temporal and spatial locality!
Temporal Locality: Accesses to the different words in the same page will cause access to same entry in page table!
Spatial Locality: Sequential access of data from one virtual page into the next will cause consecutive accesses to page table entries.
Initially I am at page0, and I access Page Table entry for page0. As I move into page1, I will access Page Table entry for page1, which is next to page table entry for page0!
Virtual Memory

22. Translation Look-aside Buffer
Solution:
Implement a cache for the page table! This cache is called the translation look-aside buffer, or TLB.
The TLB is separate from the caches we were looking at earlier.
Those caches cached data from main memory.
The TLB caches page table entries! Different!
TLB is small (about 8 to 10 blocks), and is implemented as a fully associative cache.
Virtual Memory

23. Translation Look-aside Buffer
Fully Associative
New page table entries go into the next free TLB block, or a block is replaced if there are none.
Note that only page table entries with V=1 are written to the TLB!
The page table entries already in the TLB are not usually updated, so no need to consider write-through or write-back
Exceptional cases: page aliasing, where more than 1 page can refer to the same Physical Page.
Virtual Memory

24. Translation Look-aside Buffer
The tags used in the TLB is the virtual page number of a virtual address.
All TLB blocks are searched for the page. If found, we have a TLB hit and the physical page number is read from the TLB. This is joined with the page offset to form the physical address.
If not found, we have a TLB miss. Then we must go to the page table in main memory to get the page table entry there. Write this entry to TLB.
Virtual Memory

25. Translation Look-aside Buffer
Complication
If we have a TLB miss and go to main memory to get the page table entry, it is possible that this entry has a V of 0 - page fault.
In this case we must remedy the page fault first, update the page table entry in main memory, and then copy the page table entry into TLB. The tag portion of TLB is updated to the page of the virtual address.
Note that the TLB must also have a valid bit V to indicate if the TLB entry is valid (see cache section for more details on the V bit.)
Virtual Memory

26. Integration Cache, Main Memory and Virtual Memory
Suppose a Virtual Address V is generated by the CPU (either from PC for instructions, or from ALU for lw and sw instructions).
1. Perform address translation from Virtual Address to Physical Address
(a) Look up TLB or page table (see previous slides). Remedy page fault if necessary (again, see previous slides).
2. Use the physical address to access the cache (see cache notes).
3. If cache hit, read the data (or instruction) from the cache.
4. If cache miss, read the data from main memory.
Virtual Memory

27. Use of a Translation Lookaside Buffer
Virtual Memory

28. Integration Cache, Main Memory and Virtual Memory
Note that a page-fault in VM will necessarily cause a cache miss later on (since the data wasn’t in physical memory, it cannot possibly be in cache!)
Can optimize algorithm in event of page fault:
1. Remedy the page fault.
2. Copy the data being accessed directly to cache.
3. Restart previous algorithm at step 3.
This optimization eliminates 1 unnecessary cache access that would definitely miss.
Virtual Memory

29. Page Table Size
A Virtual Memory System was implemented for a MIPS workstation with 128MB of main memory. The Virtual Memory size is 1GB, and each page is 32KB. Calculate the size of the page table.
Virtual Memory

30. Page Table Size Previous calculation shows that page tables are huge!
These are sitting in precious main memory space.
Solutions:
Use inverted page tables
Instead of indexing virtual pages, index physical pages.
Page table will provide virtual page numbers instead.
Search page table for the page of address virtual address V. If the page is found in entry 25, then the data can be found in physical page 25.
Have portions of page table in virtual memory.
Slow, complex
Virtual Memory

31. Finer Points of VM VM is a collaboration between hardware and OS
TLB
Page Table Register
Indicates where the page table is in main memory
Memory Protection
Certain virtual pages are allocated to processes running in memory.
If one process tries to access the virtual page of another process without permission, hardware will generate exception.
This gives the famous “General Protection Fault” of windoze and the “Segmentation Fault” of Unix.
Virtual Memory

32. Finer Points of VM Hardware Operating System
Does address translations etc.
Operating System
Actually implements the virtual memory system.
Does reads and writes to/from disk
Creates the page table in memory, sets the Page Table Register to point to the start of the page table.
Remedies page faults,updates the page table.
Remedies VM violations
Windows: Pops up blue screen of death, dies messily. Sometimes thrashes your hard-disk.
Unix: Gives “Segmentation Fault”. Kills offending process and continues working.
Virtual Memory

33. Finer Points of VM Where is the Virtual Memory located on disk?
Virtual memory is normally implemented as a very large file, created by the OS. E.g. in Windows NT, the virtual memory file is called swapfile.sys
Insecure. Sometimes sensitive info gets written to swapfile.sys, and you can later retrieve the sensitive info.
In Unix, implemented as a partition on the disk that cannot be read except by the OS. Unix good. Windows bad.
Whenever virtual memory is read or written to, the OS actually reads or writes from/to this file.
Virtual Memory is NOT the other files on your disk (e.g. your JAVA assignment)
Virtual Memory

34. Page Tables
Virtual Memory

35. Making Address Translation Fast
A cache for address translations: translation lookaside buffer
Virtual Memory

36. TLBs and caches
Virtual Memory