Why virtual memory?

• With a 32-bit address you can access 4 GB of physical memory (you will never get the full memory though)
  • Seems enough for most day-to-day applications
  • But there are important applications that have much bigger memory footprint: databases, scientific apps operating on large matrices etc.
  • Even if your application fits entirely in physical memory it seems unfair to load the full image at startup
  • Just takes away memory from other processes, but probably doesn’t need the full image at any point of time during execution: hurts multiprogramming
• Need to provide an illusion of bigger memory: Virtual Memory (VM)

Virtual memory

• Need an address to access virtual memory
  • Virtual Address (VA)
• Assume a 32-bit VA
  • Every process sees a 4 GB of virtual memory
  • This is much better than a 4 GB physical memory shared between multiprogrammed processes
  • The size of VA is really fixed by the processor data path width
  • 64-bit processors (Alpha 21264, 21364; Sun UltraSPARC; AMD Athlon64, Opteron; IBM POWER4, POWER5; MIPS R10000 onwards; Intel Itanium etc., and recently Intel Pentium4) provide bigger virtual memory to each process
  • Large virtual and physical memory is very important in commercial server market: need to run large databases

Addressing VM

• There are primarily three ways to address VM
  • Paging, Segmentation, Segmented paging
  • We will focus on flat paging only
• Paged
  • 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