Lec2: Operating-System Structures

Operating-System Structures

Operation System Services

​ 以下图片展示了操作系统所能提供的服务：

​ 对于用户来说，操作系统可以提供一系列的服务：

• User Interface：基本上所有的操作系统都有用户界面。
• 多种方式：命令行界面（CLI）、批处理界面（Batch Interface）、图形用户界面（GUI）
• Program Execution：系统应能加载程序到内存，并加以运行。程序应能结束执行，包括正常和不正常。
• I/O 操作：为了安全，用户一般不控制 I/O
• File system manipulation：文件操作
• Conmunications：
  • （同一/不同）电脑的进程与进程之间交换信息
  • 实现：
    • Shared memory
    • Message Passing
• Error detection：

​ Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing

• Resource allocation：
• Accounting：
• 需要记录用户所用资源类型以及数量
• Protection and security：

System calls

• 相对于繁琐的系统调用，程序员们更喜欢使用 API。而对于绝大多数程序设计语言来说，运行时支持系统提供了系统调用接口（System-call Interface）。
• 在调用的时候，我们有一个参数，叫做 System-call number，我们使用这个参数确定我们要使用哪个 System-call。
  • 有两种传参的方式，一种是使用 register；另一种针对参数较多的情况，在 System-call 之前在内存中开辟一段空间，将参数放入这个空间中。
    • 我们知道，对于 user mode 和 kenel mode 来说，它们对 memory 的视角是非常不一样的。
      • 

System Programs

​ 让我们回顾一下计算机的逻辑层次：硬件-->操作系统-->系统程序-->应用程序。系统程序（System Program）也被称为系统工具（System Utility），为程序开发和执行提供了一个方便的环境。有些系统程序只是系统调用的简单用户接口，而其他的可能相当复杂。

​ 系统程序可分为以下几类：

OS design and implementation

Two goals

• User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast
• System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

Policy and Mechanism

• Policy: What will be done? 策略（确定具体做什么事）
  • 它是在拥有机制的基础上，
  • 决定如何使用这些工具的具体规则或决策
• Mechanism: How to do it? 机制（定义做事方式）
  • 机制通常是比较固定且通用的。
  • 它不关心具体的业务逻辑，只负责提供“能力”
  • 策略应该是灵活且易变的。
  • 不同的环境下，我们可以制定不同的策略，
  • 而不需要更换底层的机制

OS Structure

Simple Structure（MS-DOS）

• MS-DOS – written to provide the most functionality in the least space
• Not divided into modules
• Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

Layered Approach

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
• With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers

​ 一个经典的案例就是 Unix，其结构如下：

​ Unix 操作系统包含了两个部分：

1. System programs
2. The kernel
   1. Consists of everything below the system-call interface and above the physical hardware
   2. Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level

Microkernel System Structure

​ 上面我们看到的都是巨核（Monolithic）操作系统，意思是它们的 Kernel 层都很大。Microkernel System Structure 做出了一系列更新：

• Moves as much from the kernel into “user” space
• Communication takes place between user modules using message passing
• Benefits:
  • Easier to extend a microkernel
  • Easier to port the operating system to new architectures
  • More reliable (less code is running in kernel mode)
  • More secure
• Detriments:
  • Performance overhead of user space to kernel space communication

​ 以下是一个示例：

​ 一个现实中的例子就是 Mac OS X 的结构：

Modules

• Most modern operating systems implement kernel modules
  • Uses object-oriented approach
  • Each core component is separate
  • Each talks to the others over known interfaces
    • Modules call each other instead of message passing
  • Each is loadable as needed within the kernel
• Overall, similar to layers but more flexible

Other Structure

Exokernel

Virtual Machine

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware

• A virtual machine provides an interface identical to the underlying bare hardware

• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory

• The resources of the physical computer are shared to create the virtual machines

  • CPU scheduling can create the appearance that users have their own processor
  • Spooling and a file system can provide virtual card readers and virtual line printers
  • A normal user time-sharing terminal serves as the virtual machine operator’s console

• The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.
  • 每台虚拟机与其他虚拟机都是相互隔离独立的
• A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.
  • 虚拟机对开发操作系统很友好
• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine. (for example, virtual user mode and kernel mode) *

​ VMM/Hypervisor 有两种：

• Type 1 Hypervisor：Bare-Metal Hypervisor

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• Type 2 Hypervisor：

  • 

• Paravirtualization：

  • 

Operating System Generation

​ 在刚开机的时候，我们需要从磁盘中调用操作系统，生成操作系统。

• Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site
• SYSGEN program obtains information concerning the specific configuration of the hardware system
• Booting – starting a computer by loading the kernel
• Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution

​ 例子有 lab1 中的 OpenSBI。

System Boot

• Operating system must be made available to hardware so hardware can start it
  • Small piece of code – bootstrap program (a.k.a. bootstrap loader), locates the kernel, loads it into memory, and starts it
  • Sometimes two-step process where boot block at fixed location loads bootstrap loader
  • When power initialized on system, execution starts at a fixed memory location
• Firmware used to hold initial boot code，and test basic hardware components
• Boot loader：找到 Kernel 并加载到内存中，以此来启动操作系统

例如：OpenSBI 放在 0x80000000，用于启动 kernel。