Chapter 3 Operating System Organization TOPICS: i. Factors in Operating System design ii. Basics OS function in computer iii. Basic Implementation consideration

Learning Objectives: OS design factors Essential and basic function for most operating system available Implementation consideration for developing OS Learning Outcomes Student know the factors that should be consider for OS design and its necessity.

OS mechanisms To enable process to create and destroy other process. To request and release resources To use resources To coordinate its own operation with the operations of related processes.

Application Integration Factors in OS Design Performance Protection & security Correctness Maintainability Commercial factors Standards and open systems Application Integration Portability Interoperability

Strategies for open system Explanation Portability Application programs could be build so that they can be moved easily from one type of hardware to another Application integrated All application programs could be built so that they present a common interface to users Interoperability Facilities could be provide in a network environment to standardize access to remote facilities

Basic Functions The OS provides mechanism for supporting an abstraction for computation for the process and for managing resources used by the community of processes. 4 categories OS functions: i. Device Management ii. Process and resource management iii. Memory Management iv. File Management

Process and Resource Management Device Management Refer to the way these generic devices are handled. Process and Resource Management Processes basic unit computation ,and resources are the elements of the computing environment needed by a process to execute. Most OS combine process and resource into a single module and define program execution Memory Management Memory manager administers the allocation and use primary memory resource. Every process request and uses memory according to the its program definition. Modern primary memory managers provide virtual memory.

File Management File are an abstract resource of storage devices. Information stored in the main memory will be overwritten as soon as the memory is deallocated from a process. Information must be copied to a storage device.eg. Disk.,magenetic disk

Basic System Operating Organization This section explains how the basic functions are combined to satisfy the requirements for an OS. Figure above shows the general organization for the basic OS modules (the lines between modules indicate interactions). This interaction is the basic rationale for a monolithic kernel OS design.

Basic Implementation Consideration 3 basic implementation mechanisms are used in every contemporary OS design: 1. Processor Modes. - The mode bit it used to distinguish between execution on behalf of an OS and execution on behalf of a user. 2. Kernels -. The kernel is designed as a trusted software module that supports the correct operation of all over other software. Supervisor mode User mode

3. Method of requesting system device This issue is concerned with the way user processes request services from the operating system: by calling a system function or message passing to a system.

i.Processor Mode 2 types of mode in processor: i. Supervisor mode: processor execute every instruction in hardware ii. User Mode: execute only a subset of the instructions. Supervisor mode also called supervisor, priviledged, protected The mode bit is set by the user mode trap instruction, also called supervisor call instruction

Supervisor and User Memory

ii.Kernels The critical part of the system software that executes in supervisor mode. All application program execute in user mode. Kernels=Nucleus. Operated as trusted software, means implement protection mechanisms that could not be changed actions of untrusted software executing in user mode.

iii. Requesting Services from the OS 2 techniques by which a program executing in user mode can request the kernel’s services: i. System call- a program that request a service from kernel and provide the interface between process and OS. ii.Message passing

Procedure Call & Message Passing Operating Systems