Virtual Memory (Sec. 5.7)

1
Virtual Memory (Sec. 5.7)

• Two Major Motivations
• Allow efficient and safe sharing of memory among
  multiple programs
• Allow a single program to exceed the size of main
  memory
• Some Terminology
• A virtual memory block is called a page
• A miss is called a page fault
• The CPU produces a virtual address, which is
  translated to a physical address
• A physical address is used to access the main
  memory

2
Mapping Virtual Address to Physical Address
Virtual Address Physical Address Page size
4 KB Virtual address space 4 GB Physical
address space 1 GB
3
Page Table

• Penalty for a page fault is very high
• So, we optimize page placement to reduce fault
  rate
• Hence, motivation to use fully associative
  placement
• Pages are located using a page table
• Page table is indexed with the page number of
  Virtual Addresses and contains the page numbers
  of Physical Addresses as data
• Each program has its own page table

4
Fast Address Translation Translation
Look-aside Buffer (TLB)

• A special cache is used for address translation,
  called Translation Look-aside Buffer (TLB)
• Each tag entry of TLB holds portion of virtual
  page number
• Each data entry of TLB holds a physical page
  number
• On every reference, the TLB is searched to find
  the desired virtual page number
• If it is a hit, continue
• If it is a miss, check the Page Table
• If it is a page fault, go to exception handling
• If no page fault, load the TLB with the desired
  entry