1.3 Operating system services An operating system provide services to programs and to the users of the program. It provides an environment for the execution of programs. The services provided by one operating system is different from the services provided by the other operating system. Operating system makes programming tasks easier. 1

Common classes of services Program Execution: operating system loads a program into memory and executes the program. I/O operation: a running program may require I/o from a file or an I/o device; for protection and efficiency, users cannot control these device directly. File system manipulation: programs needs to read, write, create and delete files. 2

– The operating system gives permission to the program for operation on files. Communication: between processes on the same computer or between different computers and which is implemented either by shared memory or message passing. Error detection: error may occur in the CPU, I/O and memory, connection failure on a network. – Lack of paper on the printer, division by zero on programs are examples of errors. 3

– For each type of error the operating system must take the appropriate action to ensure correct and consistent computing. Resource allocation and conflict resolution: when there are multiple users or multiple jobs running at the same time, resources must be allocated to each of them. Accounting: to keep track of which users use how much and what kinds of computer resources for accumulating usage statistics. 4

Protection: of information, one process not to interfere with another System calls: modern processors provide instructions that can be used as system calls. – System calls provide as an interface between a process and OS. – A system call instruction is an instruction that generates interrupt which cause the operating system to gain control of the processor. 5

1.4 Operating system structures Modern operating systems are large and complex consists of different components interconnected and melded into a kernel. For designing such system, different types of system structures are used. Let see some of them 1.Simple structure 2.Layered structure 3.Microkernel 6

1. Simple structure( – Small, simple and limited systems – The structure is not well defined – Many commercial systems do not have well-defined structures. – Frequently, such operating systems started as small, simple, and limited systems and then grew beyond their original scope. – MS-DOS is an example of such a system. It was originally designed and implemented by a few people who had no idea that it would become so popular. 2. Layered structure – Functions are organized hierarchal and interaction takes place only between adjacent layers. – Most or all of the layers operate in kernel mode. – The bottom layer is the hardware. 7

3.Micro kernel – Is a small operating system core that provides the foundation for modular extensions’ – Main function is to provide a communication facility between the client program and the various services running on the user space. – Micro kernel assigns only a few essential functions to the kernel including the address space,IPC and basic scheduling. 8