Virtual Memory

• We have talked about virtualizing the CPU.
• Now we need to virtualize memory: We want each process to be able to believe that it is the only process using RAM.
• Key questions
  • How to minimize overhead (i.e., maximize efficiency)
  • Provide protection

Address Space

• Early on, only one process loaded on machine.
  • Process had the entire address space
  • OS was effectively just a library.
• Soon wanted time sharing
  • Run customer B’s process while customer A’s process blocked on I/O.
  • Not practical (or necessary) to “save” entire address space.
• Stepped up even more when we wanted interactivity
  • Also need protection at this point.
  • (Imagine two competitors paying for time on the same mainframe)
• Having two programs in memory at once requires careful thought.
  • Protection, and
  • How to manage addresses in code
• When you compile a program, ideally, we’d like instructions like j, beq lw and sw to use actual addresses (i.e., avoid extra “work”)
• But, process shouldn’t have to be explicitly aware of where it is loaded.
• Besides, how would you change the addresses depending on where process was placed in memory?
• We need a system where the addresses a program “sees” can be different from the addresses the hardware “sees”.
• Key features of a solution:
  • Transparency (unaware)
  • Efficiency
    • For example emulation is transparent, but not efficient
  • Protection