Fundamentals of Bus, Crossbar, and Mesh Topologies in CPU Design Quiz

Explore essential aspects of Bus, Crossbar, and Mesh topologies used in CPU design, covering structure, advantages, limitations, and key function comparisons to help you understand interconnection strategies in modern computer architecture.

1. Identifying Bus Topology

   Which topology in CPU design features all components connected to a single shared communication line for data transfer?

   1. Ring topology
   2. Bus topology
   3. Tree topology
   4. Mesh topology

   Explanation: Bus topology uses a single shared communication line where multiple components communicate, making it simple and cost-effective. Mesh topology involves a point-to-point connection between nodes, not a shared line. Ring topology forms a circular structure, and tree topology organizes components hierarchically. These alternatives do not use just one shared medium.

2. Scalability Comparison

   Which topology often faces performance bottlenecks as more CPUs or devices are added due to sharing a single communication pathway?

   1. Bus topology
   2. Grid topology
   3. Crossbar topology
   4. Mesh topology

   Explanation: Bus topology suffers as more devices contend for the shared bus, leading to bottlenecks and reduced efficiency. Mesh topology adds new links for each node, so it scales better. Crossbar topology provides dedicated paths per connection, and grid topology also increases links as nodes are added, both avoiding the single pathway issue.

3. Basic Crossbar Topology

   In which topology does each input have a dedicated connection to each output, allowing multiple simultaneous communications?

   1. Bus topology
   2. Star topology
   3. Crossbar topology
   4. Ring topology

   Explanation: Crossbar topology enables all inputs to connect directly to any output, supporting parallel data transfers. Star topology connects all nodes to a central hub, not each other. A bus topology shares one connection, and ring topology only allows sequential communications, not simultaneous dedicated paths.

4. Mesh Topology Structure

   How are nodes typically connected in a mesh topology used within CPUs or processors?

   1. Each node is connected directly to select neighboring nodes
   2. Nodes connect in a circular loop
   3. Each node connects to a central node only
   4. All nodes share a single wire

   Explanation: In a mesh topology, every node connects to its adjacent or select nearby nodes, forming a grid-like or partial mesh. In contrast, a single wire is characteristic of bus topology, the central node defines a star, and the circular connection describes a ring topology.

5. Advantages of Crossbar Switch

   What is a primary advantage of the crossbar topology in high-performance CPU designs?

   1. It always has the lowest power consumption
   2. Multiple devices can communicate simultaneously without interfering
   3. All devices are connected through a central controller
   4. It is the lowest cost to build

   Explanation: Crossbar topologies enable multiple simultaneous communications thanks to dedicated pathways, minimizing interference. It is not the cheapest or least power-consuming; implementing numerous connections increases cost and power use. No central controller is required in crossbar structures.

6. Mesh Topology Speed

   What makes mesh topology suitable for scalable and high-speed CPU interconnections?

   1. Data must travel through every node in order
   2. Direct connections between neighboring nodes shorten data paths
   3. All messages share one communication channel
   4. Crossbar switches are always used

   Explanation: Mesh topology allows nodes to communicate through short, often direct routes, which improves speed and supports scalability. Unlike ring or bus topologies, not all messages use a single channel or traverse every node. Crossbar switches are not a basic feature of mesh topologies.

7. Bus Topology Limitation

   Why can bus topology be a poor choice for very large multiprocessing systems?

   1. Data traffic congestion can greatly slow overall performance
   2. It always requires the most cabling
   3. Every node must have a unique switch
   4. It supports unlimited simultaneous data transfers

   Explanation: A single bus creates congestion as node number grows, reducing performance. Simultaneous transfers are limited on buses, not unlimited. Bus topology actually reduces cabling compared to mesh or crossbar, and it does not need each node to have its own switch.

8. Crossbar Topology Drawback

   What is a significant limitation of crossbar topology as the number of processors increases?

   1. Communication always goes through a single wire
   2. Hardware cost and complexity rise rapidly due to the growing number of switches
   3. It can only support two devices
   4. It uses only ring connections

   Explanation: As more processors are added, each requires its own set of connections, leading to a large and expensive array of switches in crossbar topology. Data does not travel on a single wire, a hallmark of bus topology. It supports more than two devices, and it is not based on ring connections.

9. Mesh vs. Bus Routing

   In mesh topology, what typically determines the route data takes between source and destination CPUs?

   1. A fixed predetermined circular path
   2. A single central connection point
   3. A set of direct links between neighboring devices
   4. A universal shared communication wire

   Explanation: Mesh topology uses direct or nearly direct links between neighbors, enabling flexible and efficient routing options. It does not have a single connection point as in star topology, nor a circular path like in ring topology, and it does not rely on a shared wire, which is exclusive to bus topology.

10. Topology Selection Factor

    Which factor most often influences choosing mesh topology over bus or crossbar in large multi-core CPU systems?

    1. It requires only a single switch to control traffic
    2. It never requires any direct connections
    3. Scalability in large systems with minimal bottlenecks
    4. It is always the cheapest design

    Explanation: Mesh topology is favored in large systems because it reduces bottlenecks and provides scalable performance. Though not always the cheapest (crossbar and mesh can both be more expensive), mesh doesn't depend on a single switch, nor does it avoid direct connections; in fact, direct connections are its hallmark.