ntuck@neu

Computer Systems

Introducing Virtual Memory

  • Draw the simple address space diagram.
  • Complications:
    • Two programs may want to use the same numerical address.
    • When we fork, the programs are guaranteed to have the same addresses.
    • So the addresses can't be physical locations on the RAM chips.
    • Instead, the CPU and OS conspire to give every process its own virtual address space.
  • Discuss swapping
    • Everything is backed by something on disk.
    • For text and rodata, that's the binary.
    • For stack and heap, that's swap.
    • For rodata, that's the binary COW to swap.
  • Granularity: 4k pages
  • On fork, we COW
    • Not true without MMU - then we have to copy.
    • Discuss fork vs. vfork
  • We can cram other stuff in here.
  • Stalker on 32-bit Windows story.
    • Stalker used about 1.6 GB of RAM
    • By default, windows assigns 2gb to user, 2gb to kernel
    • 512 meg video card, mapped into userspace
    • 3gb flag
    • Draw a diagram.
  • Mmmaped files
    • Map a file into memory, backed by itself.
    • Can do I/O directly with memory accesses.
    • Kernel handles reading.
    • Can be more efficient than normal read/write, sometimes.
    • Definitely easier for manipulating structs on disk, as long as you don't care about files being portable across systems.