Virtual memory is an address mapping technique that maps virtual addresses used by programs to physical addresses in memory. It allows programs to behave as if they have more memory than is physically installed on the system. The memory management unit (MMU) performs this mapping and handles memory protection and paging. When a program accesses memory using a virtual address, the MMU translates it to the corresponding physical address. If the page is not in memory, a page fault occurs and the OS loads the page from disk before resuming execution. The translation lookaside buffer (TLB) caches recent translations to improve performance.

1. Virtual Memory
Amirkabir
University of Technology
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2. What is Virtual Memory?
Virtual Memory is an address mapping
Source:Amirhosein Peyberah
• What is Virtual Memory?
•
Virtual Memory is an address mapping
● Maps virtual address space to
physical address space
– Maps virtual addresses to physical RAM
– Maps virtual addresses to hardware devices
o Separation of user logical memory from physical memory
o Only part of the program needs to be in memory for execution.
o Virtual Memory is an address mapping maps virtual address space
to
physical address space
– Maps virtual addresses to physical RAM
– Maps virtual addresses to hardware devices

3. Image source: Virtual Memory (Amirhosein Pyeberah(

4. Virtual Memory Features
• Each process has its own virtual address space
– Process invisible to each other
– Process cannot access another process memory
• MMU checks protection bits on memory access
– Pages can be protected from being written or being executed or even
being read
– System can distinguish different protection level

5. Address space
• Physical addresses
– Addresses as used by the hardware
- DMA, peripherals
• Virtual addresses
– Addresses as used by software
• MMU must map logical to physical.

6. Memory Managment

7. Memory-Management Unit (MMU)
• The Memory Management Unit (MMU) is a
hardware component which manages virtual
address mappings
• Sits between the CPU core and memory
• The MMU operates on basic units of memory called pages
• Transparently handles all memory accesses from
Load/Store instructions
– Maps accesses using virtual addresses to
system RAM
– Handles permissions
– Generates an exception (page fault) on an invalid access

8. Virtual Memory Sharing
• Pages of different processes mapped to one single frame on
physical memory
– Allows sharing of code
– Frame may be used to several processes until one writes to it
Image source: Virtual Memory (Amirhosein Pyeberah(

9. Paging

10. What is paging
Image source: google.com

11. Demand Paging
• Rather than swapping the entire process into memory, we
use a lazy swapper.
• I A lazy swapper never swaps a page into memory unless
that page will be needed.
Image source: Virtual Memory (Amirhosein Pyeberah(

12. Demand Paging
• Trap to the OS.
• Save the user registers and process state.
• Determine that the interrupt was a page fault.
• Check that the page reference was legal and determine the
location of the page on the disk.

13. Demand Paging (cont’d)
• Issue a read from the disk to a free frame:
– Wait in a queue for this device until the read request is serviced.
– Wait for the device seek and/or latency time.
– Begin the transfer of the page to a free frame.
• Issue a read from the disk to a free frame:
– Wait in a queue for this device until the read request is serviced.
– Wait for the device seek and/or latency time.
– Begin the transfer of the page to a free frame.

14. Page Fault

15. Page Fault
• A page fault is a CPU exception, generated
when software attempts to use an invalid virtual
address.
There are two cases:
– The virtual address is not mapped for the
process requesting it.
– The processes has insufficient
Image source: Virtual Memory (Amirhosein Pyeberah(

16. Handling Page Fault
• I Check an internal table for the process to determine
whether the reference was a valid or an invalid memory
access.
– I If the reference was invalid, we terminate the process.
– If it was valid but we have not yet brought in that page, we now
page it in.

17. Translation Lookaside Buffer

18. Translation Lookaside Buffer (TLB)
• Each virtual memory reference can cause two physical
memory access
– One to fetch the page table
– One to fetch the data
• To overcome this problem a high-speed cache is set up for
page table entries called TLB
• TLB is part of the MMU consulted by the MMU when the
CPU access a virtual address.
– Contains page table entries that have been most recently used
– Functions like a cache

19. Translation Look aside Buffer
• First checks if page is already in main memory
– If not a page fault is issued
• The TLB is updated to include the new page entry
• The TLB holds the entries of the mapping
– Virtual address
– Physical address
– permissions

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