Introduction 1

Overview

• “Defining“ operating systems
• What is multi-programming
• Kernel-user mode

Defining Operating Systems

• In the early days, programmers had to deal directly with hardware

  • Real computer hardware is urgly
  • Hardware is extremely difficult to program

• An operating system is a layer[层] of indirection[间接] on top of the hardware:

  • It provide abstractions for application programs
  • it provide a cleaner and easier interface to the hardware

Multi-programming

• Morden OS use multi-programming to improve user experience and maximise the use of resource
  • Disk is slow. CPU is faster than disk.
  • Without multi-programming, CPU time is waste while wating for I/O requests.
  • Multi-programming has important consequences[结果] for operating system design
• The operating system must allocate[分配]/share resources (CPU, memory, I/O devices) fairly and safely between competing processes:
  • In time, e.g. CPUs and printers
  • In space, e.g., memory and disks
• The execution of multiple programs (processes) needs to be interleaved[交错] with one another. This requires:
  • This requires context switches and process scheduling ⇒ mutual exclusion[相互排斥], deadlock avoidance, protection, . . .

Kernel-user mode

• Modern operating systems have multiple modes:
  • The operating system runs in kernel mode and has access to all instructions
  • Applications run in user mode and have access to a subset of instructions
• Transitions from user mode to kernel mode happen in a controlled manner (interrupts, exceptions, system calls) and are mirrored in hardware

Summary

• Some properties:
  • Sits directly on top of the hardware
  • Has access to the full capabilities of the hardware
  • Provides abstractions for the user/programmer
  • Makes sure that everything is organised and runs in order
  • Improve the hardware interface

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