Operating Systems 2015F Lecture 2

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The video for this lecture is now available. Note that this lecture was pre-recorded.


Very abstract view of a computer:


Flow of control for a system call:


Key Questions:

  • What is a system call, really?
  • How are programs separated from each other?
  • What is this "scheduler" anyway?

System calls are the interface between a process and the operating system kernel. They are like function calls, but they are not function calls (although they may be wrapped in library function calls).

Classic system calls:

  • files: open, read, write, close
  • process control: fork, execve, exit
  • memory: sbrk
  • networking (socket calls)

Key idea: CPU privilege levels

  • applications run in "user mode", lowest level of privileges
    • no direct access to I/O devices
    • no ability to change amount of RAM
    • cannot control how often it has the CPU
  • Operating system runs at higher privilege
    • gets privileges because it runs first (at boot)
    • specifically, the OS kernel runs in "supervisor mode"
    • can control CPU, RAM, and I/O devices
    • kernel maintains control by setting interrupt registers (will explain more later)

So why isn't a system call a function call?

  • function calls involve references to specific memory locations
  • processes cannot directly see kernel memory

So what does an application do instead to make a system call?

  • executes a special CPU instruction (software interrupt)
  • cannot be done outside of assembly language without special support from the compiler

Concepts that I think are unclear:

  • mounting
  • strace
  • system calls