6.828 Lecture 1 - Introduction & Examples

  • OS Purposes
    • Abstract the hardware: filesystems over direct I/O, for example
    • Multiplex many threads of execution/control onto the hardware
    • Isolation/sharing/security
    • OS overhead should be minimal; application perf should be as close to 100% as possible
  • 6.828 focuses on the kernel + userspace’s interface to the kernel
  • Does xv6 have a libc implementation (or equivalent)?
  • Why is this difficult?
    • Programming environment in the kernel is unforgiving
    • Design tradeoffs
      • Efficiency/perf vs. high-level abstractions
      • Powerful OS services vs. low overhead / simple interfaces
      • Flexible interfaces vs. secure interfaces
  • xv6
    • read all the source code + the book during the course
    • RISC-V
    • QEMU
  • System calls are implemented by invoking a RISC-V instruction (brk? no, ecall) that calls into kernel code
  • exec preserves the file descriptor table of the “parent”
  • wait receives the exit code of the child that last exited (and its pid)
  • Arrays are terminated by null pointers
  • xv6 doesn’t have copy-on-write “yet” - implemented later in the course
  • No easy way for a child to wait on its parent
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