Lecture 5 continues here

How to efficiently virtualize the CPU with control?

• The OS needs to share the physical CPU by time sharing
  • Performance : How can we implement virtualization without adding excessive overhead to the system?
  • Control : How can we run processes efficiently while retaining control over the CPU?

Direct Execution| Just run the program directly on the CPU

Direction Execution Protocol

Problems with direct execution

*Problem 1:* How can the OS make sure the program doesn’t do anything that we don’t want it to do 

*Problem 2:* How does the OS stop a process from running and switch to another process

Problem 1: Restricted Operation

• A process must be able to perform I/O and some other restricted operations, but without giving the process complete control over the system. How can the OS and hardware work together to do so?

• Solution: Protected control transfer
  • User mode: Applications do not have full access to hardware resources
  • Kernel mode: The OS has access to the full resources of the machine

System Call

• The OS exposes certain functionalities to a user program, e.g.
  • Accessing the file system
  • Creating and destroying processes
  • Communicating with other processes
  • Allocating more memory

Trap and return

• Trap instruction
  • Simultaneously jumps into the kernel and raises the privilege level to kernel mode

• Return-from-trap instruction
  • returns into the calling user program and reduces the privilege level back to user mode

Limited Direction Execution Protocol

Problem 2: Switching Between Processes

• How does OS regain control of the CPU so that it can switch between processes?

• Cooperative Approach: system calls

• Non-cooperative Approach: OS takes control

A cooperative Approach: Wait for system calls

• Processes periodically give up the CPU by a system call, e.g. yield
  • The OS decides to run some other task
  • Application also transfer control to the OS when they do something illegal.
    • Divide by zero
  • Old Macintosh OS, even older Xerox Alto system

What if a process gets stuck in an infinite loop?

Lecture 6 starts here

A Non-Cooperative Approach: OS Takes Control

 How can the OS gain control of the CPU even if processes are not being cooperative? 

 What can the OS do to ensure a rogue process does not take over the machine?

Control, on time

• A timer interrupt
  • A timer interrupts every few milliseconds
  • Interrupt handler calls OS, which can stop the current process, and start a different one if it likes

The addition of a timer interrupt gives the OS the ability to run again on a CPU

Saving and Restoring Context

• Scheduler makes a decision:

  • Whether to continue running the current process , or switch to a different one

   

  • If the decision is made to switch, the OS executes context switch

Context Switch

• A low-level piece of assembly code
  • Save register values for the current process onto its kernel stack
    • General purpose registers, program counter, kernel stack pointer

• Restore register values for the next process from its kernel stack

• Switch to the kernel stack for the next process

Limited Direction Execution Protocol: Timer variant