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OS - three piece

  • virtualization - virtual machine
  • concurrency - resource allocator
  • persistence - control program

What exactly? Device drivers, memory management, file system, networking, security(Authentication), Synchronization, inter-process communication, process scheduling, access control, etc.

What we want for OS? Convenience and efficient. But often trade-off between them.

Hardware designed support for OS!

  • protected domain: divide user and kernel(OS) mode, i.e. the privilege level
  • interrupt: help virtualization in preventing illegal instruction
  • timer: Generates periodic interrupt, supports scheduling
  • memory management unit: Supports efficient virtualization of physical memory

protected domain​

We design OS with dual-mode operation: user mode and kernel mode(a.k.a supervisor mode, system mode, privileged mode, etc.).

Hardware is designed with mode bits and designate some instructions as privileged.

  • attempt to execture privileged instruction in user mode: trap to OS
  • protects OS from user programs and user programs from each other

When we have such boundary crossings through user to kernal?

user -> kernal:

  • boot time
  • hardware interrupt
  • software exception (trap or fault)
  • explicit system call - request for service by application
  • hardware has table of "interrupt service routines"

kernal -> user:

  • OS sets up rgisters, MMU, mode for application
  • Jumps to next application instruction

interrupt​

Hardware handles interrupts by:

  1. OS fills in Interrupt Table (at boot time), sets IDTR
  2. CPU execution loop: Forever:
    • Fetch instruction at PC
    • Execute instruction
    • Decode instruction
  3. Interrupt occurs (signal from hardware)
  4. CPU changes mode, disables interrupts
  5. Interrupted PC value is saved
  6. IDTR + interrupt number is used to set PC to start of interrupt handler
  7. Execution continues (saves additional state as first step)

Invoke OS via interrupt, interrupts can:

  • signal CPU that a hardware device has an event that needs attention (e.g. Disk I/O completes, network packet arrived, keyboard input arrived, etc.)
  • signal errors (e.g. divide by zero, invalid memory access, etc.)
  • requests for OS intervention(system call)