15-10-2012, 12:02 PM
Operating Systems
Operating Systems.ppt (Size: 796.5 KB / Downloads: 72)
Functions performed by OS
User Interface – Process user commands
Program (Process) mgt – Intiation and termination of processes, Scheduling and dispatching, synchronization
Memory Mgt – Allocation and deallocation of memory, Virtual memory mgt
I/O mgt – Scheduling and initiation of I/O ops, I/O interrupt processing
File Mgt – C
Communications mgt – Manage comn between processes, networking
Structure of OS
Monolithic Structure
Layered design
Virtual Machine
Kernel based
Microkernel based
Hybrid
Monolithic Structure
Different parts of the software knows each other’s internal details
Know and directly use addresses of each other’s data
User programs and command interpreter invoked OS functionalities
OS forms a software layer between the user and the hardware
Each component has to interface with hardware
Machine dependent
OS devp became time consuming, difficult and expensive
Porting of OS
Components are not segregated –changes made in one affected others also
Testing and debugging difficult
Layered Structure
Complexity and Cost of OS reduced by dividing it into layers
Large semantic gap between the OS and the bare machine – is reduced
Semantic Gap – Semantics reqd by the application and the semantics of the operations supported by the machine
Extended machine layer between the OS and the bare machine in software – two layer structure
OS components deal with a hypothetical extended machine whose machine instrs can perform many operations reqd by the OS
Algorithm of a component is separated from the implementation of its primitive operations
OS could be structured to contain many layers
Each layer uses the interface provided by the layer below it and provides a more intelligent interface to the layer above it
Virtual Machine OS
Concurrent execution of different OS on a computer
Satellite OS – Different OS executing concurrently on a computer
Host OS – provides concurrent operation of the satellite OS. It is called Virtual Machine OS [VM OS]
Each Satellite OS executes on its own virtual machine
Virtual machine has the same architecture as the bare machine – no semantic gap between virtual machine and the bare machine
Similar to scheduling of different processes
VM OS decides which OS should be in operation next
When VM OS hands the control of the operation to a satellite OS, the satellite OS remains in control of the host computer till VM OS decides to switch to another satellite OS
Kernel based OS
OS has two parts – architecture specific and architecture independent (may be application specific)
Separate them – permits OSs for dissimilar architectures to share their design
Eg – Resource mgt
Policies governing the use of resources
Mechanisms to implement the policies
Policies are architecture independent, while mechanisms are architecture dependent
Architecture of Windows 2000/Xp...
Hybrid between layered architecture and microkernel based architecture
Microkernel provides essential services offered by the OS
Executive provides other services
Environment sub systems layer provide an interface with various OS
Provides extensibility, portability and reliability of a microkernel based architecture
Advantages of a layered architecture – less semantic gap between layers
HAL provides an abstract machine
Provides easy portability to a variety of architectures
HAL and microkernel together are equivalent to a conventional microkernel
Microkernel provides facilities for process synchronization and scheduling, interrupt processing, multiprocessor synchronization
I/O manager consists of file system and device drivers – not part of microkernel