Addressing VM

• There are primarily three ways to address VM
  • Paging, Segmentation, Segmented paging
  • We will focus on flat paging only
• Paged VM
  • The entire VM is divided into small units called pages
  • Virtual pages are loaded into physical page frames as and when needed ( demand paging )
  • Thus the physical memory is also divided into equal sized page frames
  • The processor generates virtual addresses
  • But memory is physically addressed: need a VA to PA translation

VA to PA Translation

• The VA generated by the processor is divided into two parts:
  • Page offset and Virtual page number (VPN)
  • Assume a 4 KB page: within a 32-bit VA, lower 12 bits will be page offset (offset within a page) and the remaining 20 bits are VPN (hence 1 M virtual pages total)
  • The page offset remains unchanged in the translation
  • Need to translate VPN to a physical page frame number (PPFN)
  • This translation is held in a page table resident in memory: so first we need to access this page table
  • How to get the address of the page table?