Overview

Operating systems provide an environment for program executions and services to programs/users

Functions

User Interface

Shell: A computer program that exposes an OS’s services to a human user or other programs. OS shells use either a command-line interface (CLI) or a graphical user interface (GUI)
An interpreter

CLI

Shell:

• Built-in commands: The interpreter contains the code to execute the command.
  • Executed directly, e.g. cd
• System program commands: The command is a program name.
  • Looks up the system file and executes it, e.g. ls

System Calls

Mode

• User
• Kernel
  Transitions: system calls, interrupts, exceptions

API

System calls are mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use.
Properties:

• Easy to use: no need to understand the underlying implementation, just call it
• Portable: the API is compatible across platforms
• Secure: direct calls interact with the kernel

Parameter Passing

• Registers: fast, but few
• Memory table
• Stack

The last two methods do not limit the number or length of parameters being passed, at a performance cost

Types

Type 1: Process control

• Control the current process: end, abort, execute, load…
• Control a different process
• Allocate memory and release memory
• Debugger
• Locks for managing access to shared data between processes

Type 2: File management

Type 3: Device management

Type 4: Information maintenance

Type 5: Communications

• message passing model: relayed through the kernel, discrete
• shared-memory model: shares physical memory

Type 6: Protection

System Services

In computer hierarchy, system services are higher than system calls.

System services use system calls to interact with the OS kernel

Programs

• System Programs: Login program, shell, window manager
• Application Programs: Email, web browsers, gaming software, word processors

Why Applications are Operating System Specific:

Reason: Each operating system provides its own unique system calls

e.g. file format

Services

• File management
• Status information
  • Some systems implement a registry - used to store and retrieve configuration information
• File modification
• Programming-language support
• Program loading and execution
• Communications
• Background Services
  • Known as services, subsystems, daemons

Operating System Structures

• Simple Structure – MS-DOS
• Monolithic Structure – Original UNIX
• Layered Approach
• Microkernel System Structure - Mach
• Hybrid Systems - windows, macOS, Android

Virtual Machines

Summary

Translated from the Chinese original.