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. 31 30 - - - - - - - - - 12 11 - - - -- - - - - - - 0 Virtual Page No. Physical Page No. Page off set Page off set Translate 29 28 12 11 0 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