6.033 | Spring 2018 | Undergraduate

Computer System Engineering

Week 2: Operating Systems Part II

Lecture 3 Outline

  1. Previously
    • Modularity reduces complexity.
    • Naming is necessary for modularity.
  2. Operating Systems
    • Job: Enforce modularity on a single machine.
      • Also: Multiplexing, isolation, cooperation, portability, performance, …
    • To enforce modularity on a single machine, need to:
      • Protect programs’ memory from each other.
      • Allow programs to communicate.
      • Allow programs to share a single CPU.
    • Virtualization is how we do that.
    • Today: Virtualize memory. Assume one CPU per program and that programs don’t need to communicate.
  3. Virtual Memory
    • Two components: Main memory, CPU.
    • CPU holds instruction pointer (EIP).
    • Naive method: Two programs can just point to each other’s memory (bad).
    • Another method: Force programs to only use particular blocks of memory by having them address only part of the space. Complicated.
    • Virtual memory addressing: Let each program address the full 32-bit space. MMU translates virtual to physical addresses.
  4. Page Tables
    • Idea 1: Store physical addresses, use virtual addresses as an index into that table.
    • Problem: Table is too big.
    • Solution: Virtual address = page number + offset. MMU maps virtual page numbers to physical page numbers. Keeps offset the same.
    • Page table entries contain other stuff. Among that stuff:
      • Present bit
        • This bit lets us know if a page resides in RAM or storage. That’s how the OS deals with not actually having 2^32* (number of programs) physical addresses in RAM: Pages can live on disk when necessary.
      • R/W bit
      • U/S bit
      • These bits let the OS know when to trigger page faults.
  5. Hierarchical Page Tables
    • “Normal” page tables (described above) still use a lot of space.
    • Page tables have to be allocated all at once or not at all.
    • Hierarchical page tables solve this by creating a hierarchy of page tables and allocating each table only when it’s needed.
      • Virtual addresses get divided into multiple parts, one part per level in the hierarchy + an offset.
    • Downside? Speed. Multiple lookups instead of one. More page faults.
  6. Kernel
    • Virtualized memory doesn’t protect the page table.
    • Kernel mode vs. user mode does this.
    • Switch between user and kernel modes via interrupts.
  7. Abstraction
    • Some things can’t be virtualized (disk, network, …).
    • OS abstractions (system calls) make these things portable.
    • System calls are implemented as interrupts.
  8. Virtual Memory as Naming
    • Virtual memory is just a naming scheme.
    • Gives us hiding, controlled sharing, indirection.

Next lectures: Get rid of our initial assumptions (one CPU per program, etc.).

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As Taught In
Spring 2018
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