1. Lecture 1-Part 2: Operating-System Structures

2. Operating System Services
Operating systems provide an environment for execution of programs and services to programs and users
One set of operating-system services provides functions that are helpful to the user:
User interface - Almost all operating systems have a user interface (UI).
Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error)
I/O operations - A running program may require I/O, which may involve a file or an I/O device

3. Operating System Services (Cont.)
One set of operating-system services provides functions that are helpful to the user (Cont.):
File-system manipulation - The file system is of particular interest. Programs need to read and write files and directories, create and delete them, search them, list file Information, permission management.
Communications – Processes may exchange information, on the same computer or between computers over a network
Error detection – OS needs to be constantly aware of possible errors
May occur in the CPU and memory hardware, in I/O devices, in user program
For each type of error, OS should take the appropriate action to ensure correct and consistent computing

4. Operating System Services (Cont.)
Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing
Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them
Many types of resources - 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 - The owners of information stored in a multiuser or networked computer system may want to control use of that information
Protection involves ensuring that all access to system resources is controlled
Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts

5. A View of Operating System Services

6. User Operating System Interface - CLI
Command Line Interface (CLI) or command interpreter allows direct command entry
Sometimes implemented in kernel, sometimes by systems program
Sometimes multiple flavors implemented – shells
Primarily fetches a command from user and executes it
Sometimes commands built-in, sometimes just names of programs
If the latter, adding new features doesn’t require shell modification

7. Bourne Shell Command Interpreter

8. User Operating System Interface - GUI
User-friendly desktop interface
Usually mouse, keyboard, and monitor
Icons represent files, programs, actions, etc
Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder)
Many systems now include both CLI and GUI interfaces
Microsoft Windows is GUI with CLI “command” shell
Apple Mac OS X is GUI interface
Unix and Linux have CLI with optional GUI interfaces

9. The Mac OS X GUI

10. Touchscreen Interfaces
Touchscreen devices require new interfaces
Mouse not possible or not desired
Virtual keyboard for text entry
Voice commands.

11. User goals and System 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

12. The Classification of Operating systems
Multi-user: Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.
Multiprocessing: Supports running a program on more than one CPU.
Multitasking : Allows more than one program to run concurrently.
Multithreading : Allows different parts of a single program to run concurrently.
Real time: Responds to input instantly.

13. 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

14. Simple Structure -- MS-DOS
DOS (Disk Operating System) gained a reputation by supporting operating systems developed by IBM and Windows.
DOS was prominent in the industry up until support was terminated for Windows ME.
Though it's not as sophisticated as modern systems, DOS offers many advantages to the user.
This system is very stable and rarely endures crashes.
While the system is still used, DOS has become more difficult to manage as support is limited.

15. Non Simple Structure -- UNIX
UNIX is an operating system which was first developed in the 1960s, and has been under constant development ever since.
By operating system, we mean the suite of programs which make the computer work. It is a stable, multi-user, multi-tasking system for servers, desktops and laptops.
UNIX systems also have a graphical user interface (GUI) similar to Microsoft Windows which provides an easy to use environment.
There are many different versions of UNIX, although they share common similarities.
The most popular varieties of UNIX are Sun Solaris, GNU/Linux, and MacOS X.

16. Linux
The Linux operating system is developed on a kernel based on Unix.
It is known as one of the most secure platforms, yet Linux is often viewed as more complex.
Linux is the result of an open-source project, allowing users and developers to access the source code for free.
Because it requires less hardware, Linux is capable of providing exceptional performance even on a smaller hard disk.
Large communities of users exist for this system, constantly contributing to the code and making improvements.
The server-based concept of Linux has resulted in fewer home users .

17. Mac OS X
Mac OS X is Apple's trademark operating system software for their line of Macintosh computers.
The system was based in part on Unix and mimics it's format with the administrative controls.
Mac OS X software requires a low level of maintenance with fewer occurrences of computers worms, viruses and spyware.
The Apple operating system does have some disadvantages, primarily in regard to software and hardware compatibility.

18. Windows
The Microsoft Windows operating system is the most popular choice and currently has a stronghold over the market.
This platform has made significant advancements from version 1.0 all the way to the new Windows 7 system.
The Windows system is highly compatible, feature-rich and has a much larger selection of software applications.
Despite widespread usage, Windows has been heavily associated with the term "insecure" as a number of security vulnerabilities have made it the most targeted system.
Frequently exploited by hackers and malicious code writers, it is recommended that any Windows operating system with internet access be protected by some form of security software.