The Evolution of Operating Systems :

2. The Evolution of Operating Systems : • The single processing machines of 1940’s and 1950’s were not very flexible or efficient. • The processing is divided into 3 parts: • Batch Processing:The execution of jobs by collecting them in a batch - The jobs resides in mass storage waited for execution in a job queue - The job queue is referred to term First-In-First-Out (FIFO) -In early batch processing systems, each job was accompanied by a set of instruction explaining the steps require preparing the machine for that particular job.These instructions are called the Job Control Language(JCL)

3. Jobs,Program, Data, and Directions Results User Domain FIFO Job Queue Job Execution Machine Domain The Evolution of Operating Systems : • Batch Processing:

4. Program, Data, Directions, and Results User Domain Machine Domain Program Execution The Evolution of Operating Systems : • Interactive Processing:These systems allow the execution of programs that carried on a dialogue with the userthrough remote terminals or workstations - These interactive systems gave birth of the concept known as real-time processing , which refers to the requirement that the activities taking place in a machine must be coordinated with the activities in the machine’s environment

5. Time Slice P1 P2 P3 P1 The Evolution of Operating Systems : • Time-Sharing : technique of dividing time into intervals , or time slices, and then restricting the execution of a job(process) to only one time slice . - At the end of each time slice, the current job is set aside and another is allowed to execute during the next time slice - By rapidly shuffling of the jobs in this manner, the illusion of several jobs executing simultaneously is created -Today, time-sharing is used in single-user systems as well as multi user systems, the former is called multitasking

6. Multiprocessor Systems : • The word multiprocessor refers to just one computer contains several processors • Coupled computer systems is called a network • The concept of network refers to many small computers connected via network in which users share resources(such as printing capabilities, software packages, data storage facilities, and information) scattered throughout the system • The Internet is an example of a network

7. Software Application System Utility Operating System Shell Kernel Operating System Architecture : • To understand the architecture of a typical operating system(O S), it is helpful to grasp the complete spectrum of software found within a typical computer system • We group pieces of software according to the following classification scheme:

8. O.S Architecture : • Application Software (AS):Consists of the programs for performing tasks particular to the machine’s utilization - Examples of application software include spreadsheets, database systems, publishing systems, program development software , and games • System Software : performs those tasks that are common to computer systems in general. - Provides the environment in which the A.S resides. - Within the class of system software there two categories: • Operating System (OS) • Utility Software (US): - Consists of Software units that extend the capabilities of OS - The distinction between AS and US and between US and OS is often vague

9. User User O.S User O.S Architecture : • Consists of two parts: • The Shell • The Kernel The Shell : - The part of the OS that defines the interface between the operating system and its users - The job of the shell is to communicate with the user, or users , of the machine. Modern shells perform this task by means of Graphical User Interface (GUI)

10. The Kernel : • The internal part of OS, contains those software components that perform the very basic functions required by the computer installation • The kernel consists of the following components: • File Manager • Device Drivers • Memory Manager • Scheduler • Dispatcher

11. Main Memory Disk Storage Bootstrap Program Bootstrap Program ROM Operating System Operating System Volatile Memory Operating System Boot strapping (Booting) : • The procedure performed by the machine each time it turned on • The booting in special-purpose machines performed through the ROM which contains the main part of OS • In general-purpose machines, the ROM contains a short program called the bootstrap , this program executed automatically when the machine is turned on which directs the CPU through the process of transferring material from a predetermined location in mass storage into the volatile area of main memory

12. Coordinating the Machine’s Activities : The Concept of a Process : • The program is merely a static set of directions, while the process is a dynamic activity whose properties change as time progresses • A process encompasses the current status of the activity called the process state, this state includes: • The current position in the program being executed(the value of the program counter) • The value of other CPU registers • The associated memory cells • At different times during the execution of a program( at different times in a process) different snapshots(different process states) will be observed

13. Process Administration : • The tasks associated with process coordination are handled by the scheduler and dispatcher within the operating system’s kernel The Scheduler : • To keep track of all the processes, the scheduler maintains a block of information in main memory called the process table • Each time a new task is assigned to the machine, the scheduler creates a process for that task by placing a new entry in the process table • The entry contains such information as - The memory area assigned to the process(obtained from the memory manager) , the priority of the process, andwhether the process is ready or waiting • The Scheduler maintains this information as the process progresses. The process will shift back and forth between ready and waiting • The priority of the process changes as time passes • The scheduler must remove the process from the process table when the process is completed

14. Interrupt Interrupt Interrupt Interrupt Process B Process B Process Switch Process Switch Process Switch Process A Process A Advancing Time The Dispatcher : • Ensures that the scheduled processes are actually executed • In time-sharing system this task is accomplished by dividing time into short segments, each called a time slice or quantum(typically about 50 ms) • Switching the CPU’s attention among the processes as each allowed to execute for no longer than one time slice process switch • Dispatcher initiates a timer circuit that will measure the next quantum • At the end of the quantum the timer circuit generates a signal called an interrupt

15. Requests Client Server Service S C C S C C C S C S The Client/Server Model : • The various units within an operating system normally execute as individual processes • To coordinate their activities, these processes must communicate with one another • To simplify this interprocess communication, the components of OS are often designed to conform to the client/server model

16. Handling Competition Among Processes : • A universal task among the components of OS’s kernel is the allocation of the machine’s resources to the processes in the system. • This allocation task may has several problems that can lead to system malfunctions unless they accounted for. • Semaphores • Deadlock

17. Semaphores : • Consider the printer example where two users send a file to the printer • Access allocation requires that the OS keeps track whether the printer has been allocated. One approach is to use flag(Set + Clear) • Clear: means the printer is available • Set: means that the printer is busy • The OS clears the flag to begin, then it checks the flag each time a request for printer access is made. If the flag is set the OS makes the request wait. Two solutions Test-and-Set and Interrupt Enable and Disable

18. Deadlock : • Two processes are blocked from processing because each is waiting for access to resources allocated to another. Like a printer waiting for tape drive while the tape drive is waiting for the printer • Analysis of deadlock has revealed that it cannot occur unless all three of the following conditions are satisfied: • There is a competition for non-sharable resources • The resources are requested on a partial basis; that is, having received some resources, a process will return later to request more • Once a resource has been allocated, it can not be forcibly retrieved

19. Networks : • Networks are classified as : • Local Area Networks(LANs) and Wide Area Networks (WANs) : LAN: Normally consists of collection of computers in a single building or building complex WAN: Links machines that may be on opposite sides of a city or the world, like satellite link • Closed Networks and Open Networks: Closed Network:Network comprised of HW and SW from a single vendor Open Network: Implemented as a conglomerate of products from different manufactures (e.g. Internet) • Topology of the network

20. Networks • Topology of the network: the pattern in which the machines are connected: - Ring topology - Bus topology - Star topology - Irregular topology • Internet • Internet addressing • WWW

21. Computer Computer Computer Messages move in only one direction Computer Computer “Communication over ring topology” Network Protocols : • Controlling Transmission Privileges :

22. Message’s Origin Message’s Destination Application Layer Application Layer Transport Layer Transport Layer Network Layer Network Layer Link Layer Link Layer Internet Software Layers The Layered Approach to Network Software :

23. Application Transport Application Network Transport Network Link Network Network Link Link Link The layered Approach to Network Software : A Path of a data unit through the Internet involving two intermediate machines