1. Operating System Concepts

2. Lecture 3 (Chapter 2) Operating System Services
User Operating-System Interface
System Calls
Types of System Calls
Operating System Design and Implementation
Operating System Structure

3. Objectives
To provide an insight about the services provided by OS to users and designers
To give a brief knowledge about the factors that are to be kept in mind for designing the OS
To give a review on different OS structures which have been developed so far

4. Operating System Services
OS services for convenience of the users:-
User Interface (Command line Interface/ Graphical User Interface): You need an interface to give instructions, OS provides an environment for execution of programs
CLI: Command-line interface (Uses text commands and a method for entering them, you write commands and press enter, it’s executed)
Batch Interface: An interface in which your commands are entered into files and then those files are executed
GUI: Graphical User Interface (You have a mouse from where you choose your I/O devices, choose from menus and keyboard to enter text)
Program execution – system capability to load a program into memory, run, and end it normally or abnormally.
Job Scheduling
CPU scheduling
I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.

5. File-system manipulation – program capability to read, write, create, and delete files, (permission management)
Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing approaches.
Error detection – ensures correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.
OS services for ensuring efficient system utilization:- (Ensuring efficient operating of OS)
Resource allocation – allocating resources to multiple users or multiple jobs running at the same time. (resources like CPU time, main memory, file storage etc)
Accounting – keep track of which users use how many and which kinds of computer resources for account billing or for accumulating usage statistics
Protection – ensuring that all access to system resources is controlled, (extends to defending I/O devices/ modems/network adapters etc)

6. Operating System Interface
On systems with multiple Command Interfaces, the interpreters are known as shells
E.g. MS-DOS
Two ways in which commands can be executed
Direct commands
Files
Uses the command to identify a file to be loaded
Addition of new commands to system by creating with name
GUI- can invoke a program by clicking on mouse

8. System Calls System calls are ENTRY POINTS to OS:
Written in assembly language, C, C++
Concept developed during Dual Mode Operation
E.g. Copy of data from one file to another
Application Programming Interface: Application developers design programs according to API’s (Specifies a set of functions that are available to an application developer, including the parameters that are passed to each function and the return values that the programmer can expect
Functions that make up an API invoke the actual system calls on behalf of application programmer
Three general methods are used to pass parameters between a running program and the operating system:
Pass parameters in registers.
Store the parameters in a table in memory, and the table address is passed as a parameter in a register (used in Linux)
Push (store) the parameters onto the stack by the program, and pop off the stack by OS.
Last two options do not limit the number or length of parameters passed

9. Passing of Parameters in Register

10. Application Programming Interface
OS free the application developers from having to deal with the messy details of manipulating computer hardware to manage memory, perform I/O deal with command lines……
OS provides a series of API calls which programmers use to accomplish detailed hardware operations
API provides the system calls by which a user program instructs the OS to do the work

11. Example of System Call
A system call is used to complete a specific task:
Copy of data from one file to another
Reading input file name
Opening a file
Error message if any
Abnormal termination
Deletion of a file
Prompting a message
Reading data for the file
Writing data to the file or console
Closing the file
Normal termination

12. Types of System Calls Process control File management
End, Abort
Load, Execute
Create process, Terminate process
Allocate memory, Free memory
File management
Create file, Delete file
Open, Close
Read, Write, Reposition
Device management
Request device, Release device
Logically attach devices, Logically detach devices

13. Types of System Calls Information maintenance Communications
Get time or date, Set time or date
Communications
Create communication connection, Delete communication connection
Send messages, Receive messages

14. MS-DOS Execution
At System Start-up
Running a Program

15. UNIX Running Multiple Programs

16. Communication Models
Communication may take place using either message passing or shared memory.
Message Passing
Shared Memory

17. System Programs
System programs are user interfaces to system calls and provide a convenient environment for program development and execution. Categories of system programs are :-
File Manipulation – used for creating, deleting, renaming, printing, dumping, listing, and generally manipulating files and directories.
Status Information – ask the system for date, time, available memory/disk, used memory/disk, number of users etc.
File Modification – text editors to create and modify the content of files stored on disk or tape.
Programming Language Support – compilers, assemblers, and interpreters for common programming languages are provided to the user with the OS.
Program Loading and Execution – OS may provide loaders and debuggers for program loading and execution.
Communications – provide mechanism for creating virtual connections among processes, users, and different computer systems which allows message passing, browse web pages, send messages, log-in remotely, transfer files from one machine to another.
Application Programs or System Utilities – many OSs are supplied with system utilities or application programs (i.e. web browsers, word processors, text formatters, spreadsheets, database systems) to perform common operations.

18. System Programs
Most users’ view of the operation system is defined by system programs, not by the actual system calls.
Command-interpreter is the most important system program and can be implemented in two ways:-
Schemes of commands implementation are:
Command interpreter itself contains the code to execute the commands, jumps to a section of its code that sets up the parameters, and makes the appropriate system call.
Advantages: faster execution; easy to use by the programmers.
Disadvantages: bigger size of the command-interpreter; needs changes for new commands.
All commands are implemented through special system programs. Command file is loaded in the memory and executed.
Advantages : small size of command interpreter; not changed for new commands, new commands can be incorporated easily.
Disadvantages : clumsy task of passing parameters from the command-interpreter to the system program; slower in loading and execution

19. Operating System Design
For designing an OS, you must define certain goals
User Goals- (convenient to use, easy to learn and fast)
Designer/system goals- (easy to maintain, flexible, efficient, error-free)

20. Mechanism and Policy Leave the policy to the user
Mechanism- How to do something
Policy-what should be done
E.g. How to distribute CPU time among multiple processes?
Mechanism- To use a timer, set a fixed amount of time for a process
Policy- how much time should be set
If we fix a time for every process, lets say 15 sec, then it’s hard policy
Leave the policy to the user

21. Implementation
In 1960/1970, in most Operating systems, majority of code was written in assembly.
Linux & Windows XP written in C, although small sections were written in assembly
Advantages of writing in high-level language:
Easy to code
Easy to maintain
Portable

22. OS Structure Simple structure (No structure) (Monolithic)
Layered Approach
Microkernels
Hybrid
Modular
Virtual Machines

23. Simple System Structure (MS-DOS)
MS-DOS was written to provide the most functionality in the least space.
It was not divided into modules.
Monolithic (all functions in one unit)
One big box Everything inside it
Every component can TALK to one another
No concept of User/Kernel Mode

24. UNIX System Structure

25. 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.
Taking different pieces of OS and arranging them in layers
Everything nicely separated
Conceptually clean and very modular
Well-defined boundaries, swapping layers

26. Layered Approach
Advantages: modularization makes debugging and verification much easier; design and implementation is simplified
Disadvantages: level definition and implementation is very difficult; order of levels for the design of layered structure is problematical; interaction between layers and passing of parameters / data is very complex; less efficient as a system call takes longer time for execution than a non-layered system, Communication overhead
Examples: Technische Hogeschool Eindhoven (THE) OS and Venus layer structure.

27. Level 5: User programs
Level 4: buffering for input and output devices
Level 3: operator-console device driver
Level 2: memory management
Level 1: CPU scheduling
Level 0: hardware
Figure : THE layer structure
Level 6: User programs
Level 5: device drivers and schedulers
Level 4: virtual memory
Level 3: I/O channel
Level 2: CPU scheduling
Level 1: instruction interpreter
Level 0: hardware
Figure : Venus layer structure

28. Microkernel System Structure
Larger kernels need more space and are difficult to manage.
Contains only those components which are actually needed to run machine
Why want to keep kernel micro?
Motivation: NO SYSTEM CRASH
A bug in audio driver will cause the sound to be stop but will not crash the computer
Microkernel approach moves all non-essential components from the kernel and implements them as system-level programs and user-level programs, moving as much from the kernel into “user” space.
Microkernel provides minimal process, memory management and communication facility.
Benefits:
easier to extend a microkernel
fewer changes required in the modification of microkernel
easier to port the operating system to new architectures
more reliable (less code is running in kernel mode)
more secure – if a routine fails, the rest of the OS remains untouched

29. Microkernel Disadvantages: Frequent switching
We want efficiency of Monolithic with Reliability of microkernel
Hybrid Approach- MAC OS Kernel

30. Modular Approach Less code: Less bugs
Make small modules and load them dynamically
Can be loaded and unloaded at run time
Compile alongside the kernel
Once your essential part of OS is loaded, then it began to load modules

31. Modular Approach Crash prevention E.g. device drivers
Any module can call any other module without invoking message passing facility
Trading off security of kernel with efficiency

32. Virtual Machine Emulation of underlying hardware
The resources of the physical computer are shared to create the virtual machines
Abstract the hardware of single computer (CPU, memory, disk drives, network interface cards etc) into different environments
Allow you to experiment with another operating system without leaving your current operating system

33. Virtual Machine
E.g. A virtual machine also gives you a way to run another operating system’s software. So, if you’re a Linux user, you can install Windows in a virtual machine and run Windows desktop programs in that virtual machine. Mac users can also use virtual machines to run Windows software.
A virtual machine can be a good place to rest out programs you don’t trust and see what they do.

34. Advantages of Virtual Machine
Share the same resources (hardware) and running different environments
Difficult to implement (providing an exact copy of underlying hardware)
No need to take out the current OS, but simply install VM. (then test and develop)
Programs cannot distinguish between execution on real or virtual hardware

35. System Models

36. Chapter 3 (Processes)
Summary of previous concepts:
A computer consists of a collection of hardware resources, such as the processor, main memory, I/O modules, timers, disk drives, and so on.
Computer applications are developed to perform some task. Typically, they accept input from the outside world, perform some processing, and generate output.
Software is needed to manage the sharing of the processor and other resources by multiple applications at the same time.

37. Chapter 3 Today’s Agenda Review of previous concepts
Processes Concept (Introduction)
Discussion on Process features (scheduling, creation, termination and communication)
Communication in client-server systems

38. When multiple applications are active at the same time, it is necessary to protect the data, I/O use, and other resource use of each application from the others.
The OS was developed to provide a convenient, feature-rich, secure, and consistent interface for applications to use. The OS is a layer of software between the applications and the computer hardware that supports applications

40. Process Concept A process is a program in execution
A process is an active entity
An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
OS books uses the terms job and process almost interchangeably.
Concurrent processing increases system performance
A process includes:
program counter (current activity/ address of the next instruction in the program to be executed.
Text section (program code)
Stack (contains temporary data such as function parameters, return addresses, and local variables)
data section (contains global variables)

41. Program VS Process
We emphasize that a program by itself is not a process; a program is a passive entity, such as a file containing a list of instructions stored on disk (often called an executable file), whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources. A program becomes a process when an executable file is loaded into memory.
Two common techniques for loading executable files are double-clicking an icon representing the executable file and entering the name of the executable file on the command line (as in prog. exe or a. out.)

43. Process State As a process executes, it changes state:
New : The process is being created
Ready : (ready to be allocated CPU, residing in main memory)
running: Instructions are being executed (actually using the CPU)
Waiting: The process is waiting for some external event to occur or waiting for an input that is not yet available
terminated: The process has finished execution

44. Process Attributes Identification…. ?
What are process attributes and why is it necessary to associate certain attributes with a process ?
Identification…. ?
If a processor is executing only one program, then NO ALL JOY…
If I have only one CPU in my system, then how I cheat or give an illusion of multiple things ‘going on’ ?
Time-sharing systems and multiprogramming
systems processor is able to service multiple programs

45. Process Control Block
Attributes should include ‘everything a process cares about’ !
Process State- (running, ready etc)
Program Counter- (address of the next instruction in the program to be executed)
CPU registers (State of CPU registers- Data present in registers in processor while the process is executing)
CPU scheduling priority- (priority relative to other processes, information about scheduling queues)

46. Memory Management Information (location of process’s data and instructions in memory, value of base and limit registers)
Accounting Information (includes amount of processor time used, process number)
I/O status information (data that determines the resources this process can access such as list of I/O devices, files etc)