Operating Systems 2015F Lecture 2

Notes

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