Operating Systems 2015F Lecture 2
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:
- 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:
- system calls