Two main roles of OS

1. OS is an abstract machine
   • provides high-level abstractions
   • extends basic hardware with added functionality
   • it hides an details of the hardware
     • make application code portable
     • don't have to write code for specific hardware structure
2. OS is a resource manager
   • allocating resources to users and processes
   • must ensure
     • no starvation
     • progress
     • allocate via some desired policy
     • overall, the system is efficiently used

Operating System Kernel

• Part of the OS that runs in privileged mode
• contains fundamental functionality
• contains mode-frequently used functions

OS is privileged

• prevent applications from interfering with each other
• OS can enforce its resource allocation

Privilege-less OS

• Some embedded OS have no privileged mode
  • it can implement OS functionality, but cannot enforce it
    • All software runs together
    • No isolation
    • one fault might brings down entire system

System Libraries

• System libraries are just libraries of support functions
• Subset of Library functions are actually system calls
  • strcmp(), memcpy() are pure library functions
    • manipulating memory within application
  • open(), close(), read(), write() are system calls
    • crossing the user-kernel boundary (to read from disk)
    • implementation mainly focused on passing request to OS and returning result to application
    • man syscalls