A network topology refers to the physical layout of the network in which all the devices are connected. This includes all the hardware that makes up the network. The points of connection to the network by the stations are called nodes or workstations.A number of possible network topologies are available. The choice of network topology for installing a computer network depends upon a combination of following factors:

·         Desired performeace of the system

·         Desired reliability of the entire system

·         Size of the system

·         Expendability of the system

·         cost of the components and services required to implement the network

·         Availability of communication lines.

1. linear bus

  A linear bus topology consists of a main run of cable with a terminator at each end. All nodes are connected to the linear cable

2.Star

A star topology is designed with each node connected directly to a central network hub or conceentrator. Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all function of the network . It also acts as a repeater for the data flow.

3. star wired ring

A star wired ring topology my appear to be the same as a star topology. Internally, the MAU( multistation acces unit) of a star wired ring contains wiring that allows information to pass from one device to another in a circle or ring.

4. Mesh topology

In a mesh topology, each computer on network has redundant data paths. The mesh topology  provides fault tolerance if a wire, hub , switch, or other component fails, data can travel along an alternate path. A digram of a mesh network looks like a fishing net.

The memory management subsystem is one of the most important parts of the operating system. The memory management subsystem includes:

Protection: Each process in th system has its own virtual address space. These virtual address space are compeletly separate from each other and so a process running one application cannot affect another.

large address space:  The operating system makes the system appear as  if it has a larger amount of memory than it actually has. The virtual memory can be many times larger than the physical memory the physical memory in the system.

Memory mapping: Memory mapping is used to map image and data files into a processes address space. In memory mapping, the contents of a file are linked directly into the virtual address space of a process.

Fair Physical memory allocation:  The memory management subsystem allows each running process in the system a fair share of the physical memory of the system.

Shared virtual memory:  There could be several processes in the system running concurrently and simultaneously depending upon the number of processors residing in the system but might be using the common file.