1. Chapter 2: Operating-System Structures

2. Chapter 2: Operating-System Structures
Operating System Services
User Operating System Interface
System Calls
Types of System Calls
System Programs
Operating System Design and Implementation
Operating System Structure
Virtual Machines

3. Objectives
Describe the services an operating system provides to users, processes, and other systems
Discuss various ways of structuring an operating system
Explain how operating systems are installed and customized and how they boot

4. Operating System Services
Provide functions that are helpful to users
User interface
Batch, Command-Line (CLI), Graphics User Interface (GUI)
Program execution
Load a program into memory, Run it, End execution normally or abnormally
I/O operations
File-system manipulation
Create, delete, read, write, and list files and directories
Change permissions

5. Operating System Services (Cont.)
Provide functions that are helpful to users
Communications
Exchange information on the same computer or between computers over a network
Shared memory or message passing
Error detection
Hardware: CPU, memory, I/O devices, etc.
User program
For each type of error, take an appropriate action to ensure correct and consistent computing
Debugging

6. Operating System Services (Cont.)
Ensure efficient system operation via resource sharing
Resource allocation
CPU cycles, main memory, file storage, I/O devices
Accounting: To keep track of which users use how much and what kinds of computer resources
Protection and security
Essential in multi-user/networked environment
Protection: Ensure that all access to system resources is controlled, e.g., mode bit, memory management
Security: User authentication, access control, ...

7. User Operating System Interface: CLI (Command Line Interface)
Direct command entry
Implemented in kernel or by systems program
Multiple flavors can be implemented, e.g., several command shells for UNIX
Fetch a command from user and execute it
Built-in (i.e., implemented in kernel)
Invoke user-level programs
Adding new features requires no shell modification

8. User Operating System Interface: GUI
User-friendly desktop metaphor interface
Icons represent files, programs, actions, etc
Mouse buttons over objects cause various actions
Invented at Xerox PARC
Many systems have both CLI and GUI
Microsoft Windows is GUI with CLI
UNIX/LINUX is CLI with optional GUI interfaces
Apple Mac OS X has Aqua GUI interface with UNIX kernel underneath

9. System Calls Programming interface to OS services
Typically written in C or C++
Usually use high-level Application Program Interface (API) rather than direct system calls
Win32 API for Windows
POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X)
Java API for the Java virtual machine (JVM)

10. Example System Calls
System call sequence to copy the contents of one file to another file

11. Example of Standard API
Consider the ReadFile() function in Win32 API

12. System Call Implementation
System call interface invokes intended system call in OS and returns system status and any return values
Caller needs to know nothing about how the system call is implemented
Use API
Hide implementation details

13. API – System Call – OS Relationship

14. Standard C Library Example
C program invoking printf() library call, which calls write() system call

15. System Call Parameter Passing
Often, more information is required than simple identity of desired system call
How to pass parameters to OS
Pass the parameters in registers
Number & length of parameters can be limited
Parameters stored in a block or table in memory, and pass the block’s address as a parameter in a register
Push parameters onto stack

16. Parameter Passing via Table

17. Types of System Calls Process control File management
Device management
Information maintenance
Communications

18. MS-DOS execution
(a) At system startup (b) running a program

19. FreeBSD Running Multiple Programs

20. System Programs
Convenient environment for program development and execution
Program loading and execution
File manipulation
Programming language support
Communications
Application programs
Most users’ view of the OS is defined by system programs, not the actual system calls

21. System Programs
Provide a convenient environment for program development and execution
Some of them are simply user interfaces to system calls; others are considerably more complex
File management
Create, delete, copy, rename, print, dump, list,
Status information
File modification
Text editors to create and modify files
Commands to search file contents, e.g., grep in UNIX/Linux

22. System Programs (cont’d)
Programming language support
Compilers/interpreters, assemblers, debuggers
Program loading and execution
Absolute loaders, relocatable loaders, linkage editors, and overlay loaders
Communications
Creating virtual connections among processes, users, and computer systems
Send messages to somebody’s screen, send , browse web pages, remote log in, file transfer

23. Operating System Design and Implementation
No silver bullet but some approaches have been successful
Internal structure of different OSs can vary widely
Affected by choice of hardware & OS type
First define goals and specifications
User goals and System goals
User goals: OS should be convenient, easy to learn, reliable, safe, and fast
System goals: OS should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

24. Operating System Design and Implementation (Cont.)
Important to separate
Policy: What will be done? Mechanism: How to do it?
Mechanisms determine how to do something, policies decide what will be done
The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later

25. Simple Structure 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

26. 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

27. Layered Operating System

28. UNIX
UNIX
Limited by hardware functionality, the original UNIX had limited structuring consisted of two separable parts
Systems programs
Kernel
Consists of everything below the system-call interface and above the physical hardware
Provides a large number of functions for one level: CPU scheduling, memory management, file system and other OS functions

29. UNIX System Structure

30. Microkernel System Structure
Moves as much from the kernel into “user” space as possible
Message passing between user modules
Benefits
Easier to extend a microkernel
Easier to port the OS to new architectures
More reliable (less code is running in kernel mode)
More secure
Problem
Performance overhead due to communications between user space to kernel space

31. Mac OS X Structure Hybrid approach (microkernel + layered approach)
Mach microkernel: Memory management, RPC & IPC
BSD kernel: BSD command line interface, networking & file system, POSIX API’s such as Pthreads
In addition, kernel environment provides an I/O kit & dynamically loadable modues

32. 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
Each is loadable as needed within the kernel, e.g., device drivers & system calls
Similar to layers but more flexible

33. Solaris Modular Approach

34. Virtual Machines Take the layered approach
Treat hardware and OS kernel as though they were all hardware
Provide an interface identical to the underlying hardware
An OS creates the illusion of multiple processes executing on its own processor with its own (virtual) memory

35. Virtual Machines (Cont.)
(a) Nonvirtual machine (b) virtual machine
Non-virtual Machine
Virtual Machine

36. Virtual Machines (Cont.)
Provide complete protection of system resources
Each virtual machine is isolated from all other virtual machines
No direct sharing of resources
A perfect vehicle for operating systems research and development
Develop an OS on a virtual machine
Do not disrupt normal system operation.
Difficult to implement due to the effort required to provide an exact duplicate to the underlying machine

37. VMware Architecture

38. The Java Virtual Machine

39. End of Chapter 2 Questions?