Efficiency / Convenience Tradeoff: Efficient systems are often not convenient (e.g. smart phone optimize convenience vs. embedded system optimize efficiency)

OS Definition

Virtualization: Physical device → general form, share resources

• Limited Direct Execution: Physical CPU directly execute user instructions with limited priv.
• Virtual CPU + Virtual Memory: Illusion that each program owns the whole machine
• Abstraction: Process (a running program)
• Protected Domains: OS is allowed specific instructions that user processes cannot use
  • Mode-bit (CPL): Either user mode or system (supervisor / privileged) mode Illegal access cause protection fault
• Exceptions: Software signal or error
• Interrupts: Hardware signal for CPU to jump to a predefined interrupt handler instruction
  • Hardware interrupts: Device events
  • Timer: Periodic interrupt
  • Useful for OS to regain control when the user process fail

Concurrency: Coordinate multiple activities

Persistence: Survive crashes

Implementing Interrupts

OS fill in Interrupt Table & IDTR (pointer to IT) at boot

When interrupt occur:

• CPU change to privileged mode, disable interrupts
• Save interrupted PC address
• Set PC = address at (IDTR + interrupt number) to run handler
• Continue execution

Bootstrapping

BIOS: Basic Input Output System

1. BIOS load the OS boot-loader from disk
2. Boot-loader load and run OS

OS Initialization

1. Initialize machine-dependent operations
2. Create init process (id=1)
3. Switch to user mode, start running first process
4. OS wait for interrupt or exception to happen