What is OS?

• A program that acts as an intermediary between a user of a computer and the computer hardware. It’s an interface that host other programs.
• Top-down perspective: hardware abstraction layer, turn hardware into something that applications can use
  • e.g: App issue system calls to use OS abstractions
  • Easy to use, good for reusability and portable
• Bottom-up perspective: resource manager/coordinator, manage your computer’s resources
  • Computer has resources(CPU, memory, disk, device... etc.), and OS must manage them
  • Goof for sharing/multiplexing, protectino for apps from each other, and performance

OS abstraction: process

It’s a running program, stream of running instructions + process state

• A key OS abstraction: the applications you use are built of processes
  • Process are protected from each other, process = address space
  • Hide details of CPU, when & where to run

Unix process-related system calls

• int fork(void)
  • Create a copy of the invoking process
  • Return process ID of new process in “parent”
  • Return 0 in “child”
• int execv (const char* prog, const char* argv[])
  • Replace current process with a new one
  • prog: program to run
  • argv: arguments to pass to main()
• int wait(int* status)
  • wait for a child to exit

OS abstraction: file

• Array of bytes, persisetnt across reboot
  • Nice, clean way to read and write data
  • Hide the details of disk devices
• Related abstraction: directory, collection of file entries

OS Structure

• What goes into the kernel
  • Kernal: most interesting part of OS
    • Privileged: can do everything → so we must be careful
    • Manages other parts of OS
• Different structures lead to different
  • Perfirmance, functionality, ease of use, security, reliability,... etc
• Tradeoffs depend on technology and workload