Course Title: Operating system

1. Chapter No: 01 Introduction To O.S Course Title:Operating system Course Instructor: ILTAF MEHDI IT Lecturer By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

2. What is an Operating System? • The 1960’s definition of an operating system is “the software that controls the hardware”. • However, today, due to microcode we need a better definition. “We see an operating system as the programs that make the hardware useable”. • In brief, “an operating system is the set of programs that controls a computer”. • Some examples of operating systems are UNIX, Mach, MS-DOS, MS-Windows, Windows/NT, Chicago, OS/2, MacOS, VMS, MVS, and VM. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

3. Levels of OS • Controlling the computer involves software at several levels. • Kernel services • library services • application-level services • all of which are part of the operating system. • Processes run Applications, which are linked together with libraries that perform standard services. • The kernel supports the processes by providing a path to the peripheral devices. • The kernel responds to service calls from the processes and interrupts from the devices. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

4. Kernel of OS • The core of the operating system is the kernel, “a control program that functions in privileged state (an execution context that allows all hardware instructions to be executed), reacting to interrupts from external devices and to service requests and traps from processes”. • Generally, the kernel is a permanent resident of the computer. • It creates and terminates processes and responds to their request for service. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

5. Multi-Tasking • The ability to execute more than one task at the same time, a task being a program. The terms multitasking and multiprocessing are often used interchangeably, although multiprocessing implies that more than one CPU is involved. • In multitasking, only one CPU is involved, but it switches from one program to another so quickly that it gives the appearance of executing all of the programs at the same time. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

6. Types of Multi-Tasking There are two basic types of multitasking: • Preemptive • Cooperative 1. PreemptiveMultitasking: The operating system parcels out CPU time slices to each program. • OS/2, Windows 95, Windows NT, the Amiga operating system and UNIX use preemptive multitasking. 2. CooperativeMultitasking: Each program can control the CPU for as long as it needs it. • If a program is not using the CPU, however, it can allow another program to use it temporarily. • Microsoft Windows 3.x and the MultiFinder (for Macintosh computers) use cooperative multitasking. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

7. Multi-Processing • (1) Refers to a computer system's ability to support more than one process (program) at the same time. Multiprocessing operating systems enable several programs to run concurrently. UNIX is one of the most widely used multiprocessing systems, but there are many others, including OS/2 for high-end PCs. Multiprocessing systems are much more complicated than single-process systems because the operating system must allocate resources to competing processes in a reasonable manner. • (2) Refers to the utilization of multiple CPUs in a single computer system By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

8. Resource Manager • Operating Systems are resource managers. The main resource is computer hardware in the form of processors, storage, input/output devices, communication devices, and data. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

9. Operating System Functions Some of the operating system functions are: • implementing the user interface • sharing hardware among users • allowing users to share data among themselves • preventing users from interfering with one another • scheduling resources among users • facilitating input/output By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

10. Operating System Functions • recovering from errors • accounting for resource usage • facilitating parallel operations • organizing data for secure and rapid access • and handling network communications. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

11. Objectives of Operating Systems Modern Operating systems generally have following three major goals. To hide details of hardware by creating abstractionAn abstraction is software that hides lower level details and provides a set of higher-level functions. • An operating system transforms the physical world of devices, instructions, memory, and time into virtual world that is the result of abstractions built by the operating system By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

12. Objectives of Operating Systems • There are several reasons for abstraction. First, the code needed to control peripheral devices is not standardized. • Operating systems provide subroutines called device drivers that perform operations on behalf of programs for example, input/output operations. • Second, the operating system introduces new functions as it abstracts the hardware. • For instance, operating system introduces the file abstraction so that programs do not have to deal with disks. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul

13. Objectives of Operating Systems • Third, the operating system transforms the computer hardware into multiple virtual computers, each belonging to a different program. Each program that is running is called a process. Each process views the hardware through the lens of abstraction. • Fourth, the operating system can enforce security through abstraction. By ILTAF MEHDI, IT Lecturer, MIHE, Kabul