Address Translation

• What is the simplest way to translate from virtual to physical addresses?
  • Simply have virtual addresses start at 0, then have a base register.
• This adds an add to the CPUs critical path. Why doesn’t this have a noticeable effect on performance?
  • Pipelining.
• What services must OS provide to make this work?
  • It needs to set the base register when changing processes.
  • It needs to choose where processes “live” in physical memory.
• What is present/missing among efficiency, transparency, protection?

• Simplifying assumptions: Processes use a single, contiguous, compact memory space. (e.g., only as much room as they need. Forget MIPS address conventions for now.)

• How can we add protection as efficiently as possible?
  • Limit register.
• What should happen if code tries to access address that goes beyond the limit register? (Or address is negative)
  • An exception (a kind of interrupt).
• Note that the instructions to set the base and limit registers must be privileged.
• What problems are we going to have in “real life” using this system?
  • Using physical memory efficiently — assigning processes to a section of memory.
  • What are some of the challenges? What could go wrong?
• Is it possible to move an address space once a program begins running?