Kernel Vs User Mode

What:

It would be incredibly dangerous to allow users to willy-nilly handle memory. Thus, the CPU is split into 2 modes: Kernel and User, trusted and untrusted respectively. The Operating System (OS) is in charge of splitting between the two.

Modes:

• Kernel: A software layer directly on top of the hardware that controls / restricts the secure sharing of low-level resources between users/applications.
• Execution Modes:
  • User Mode: Can’t actually directly call resources, so install invokes syscall, which temporarily switches to kernel mode.
  • Kernel Mode: Direct access to computer resources
    • Privileged instructions (accessing I/O devices, handling page table, etc) only executable in Kernel mode.
    • Only enterable through syscalls (eg read() in C, OS).
    • eret instruction sets mode back to previous mode.

Advantages:

• Ensures programs don’t interfere with each other - only have access to the the resources for which they have permission