Time Shared Bus – Interconnection structure in Multiprocessor System

Interconnection structures :
The processors must be able to share a set of main memory modules & I/O devices in a multiprocessor system. This sharing capability can be provided through interconnection structures. The interconnection structure that are commonly used can be given as follows –

1. Time-shared / Common Bus
2. Cross bar Switch
3. Multiport Memory
4. Multistage Switching Network (Covered in 2nd part)
5. Hypercube System

In this article, we will cover Time shared / Common Bus in detail.

1. Time-shared / Common Bus (Interconnection structure in Multiprocessor System) :
In a multiprocessor system, the time shared bus interconnection provides a common communication path connecting all the functional units like processor, I/O processor, memory unit etc. The figure below shows the multiple processors with common communication path (single bus).

Single-Bus Multiprocessor Organization

To communicate with any functional unit, processor needs the bus to transfer the data. To do so, the processor first need to see that whether the bus is available / not by checking the status of the bus. If the bus is used by some other functional unit, the status is busy, else free.

A processor can use bus only when the bus is free. The sender processor puts the address of the destination on the bus & the destination unit identifies it. In order to communicate with any functional unit, a command is issued to tell that unit, what work is to be done. The other processors at that time will be either busy in internal operations or will sit free, waiting to get bus.
We can use a bus controller to resolve conflicts, if any. (Bus controller can set priority of different functional units)
This Single-Bus Multiprocessor Organization is easiest to reconfigure & is simple. This interconnection structure contains only passive elements. The bus interfaces of sender & receiver units controls the transfer operation here.
To decide the access to common bus without conflicts, methods such as static & fixed priorities, First-In-Out (FIFO) queues & daisy chains can be used.

Advantages –

• Inexpensive as no extra hardware is required such as switch.
• Simple & easy to configure as the functional units are directly connected to the bus .

Disadvantages –

• Major fight with this kind of configuration is that if malfunctioning occurs in any of the bus interface circuits, complete system will fail.
• Decreased throughput — 
  At a time, only one processor can communicate with any other functional unit.
• Increased arbitration logic —
  As the number of processors & memory unit increases, the bus contention problem increases.

To solve the above disadvantages, we can use two uni-directional buses as :

Multiprocessor System with unidirectional buses

Both the buses are required in a single transfer operation. Here, the system complexity is increased & the reliability is decreased, The solution is to use multiple bi-directional buses.

Multiple bi-directional buses :
The multiple bi-directional buses means that  in the system there are multiple buses that are bi-directional. It permits simultaneous transfers as many as buses are available. But here also the complexity of the system is increased.

Multiple Bi-Directional Multiprocessor System

Apart from the organization, there are many factors affecting the performance of bus. They are –

• Number of active devices on the bus.
• Data width
• Error Detection method
• Synchronization of data transfer etc.

Advantages of Multiple bi-directional buses –

• Lowest cost for hardware as no extra device is needed such as switch.
• Modifying the hardware system configuration is easy.
• Less complex when compared to other interconnection schemes as there are only 2 buses & all the components are connected via that buses.

Disadvantages of Multiple bi-directional buses –

• System Expansion will degrade the performance because as the number of functional unit increases, more communication is required but at a time only 1 transfer can happen via 1 bus.
• Overall system capacity limits the transfer rate & If bus fails, whole system will fail.
• Suitable for small systems only.

2. Crossbar Switch :
A point is reached at which there is a separate path available for each memory module, if the number of buses in common bus system is increased. Crossbar Switch (for multiprocessors) provides separate path for each module.

3.Multiport Memory :
In Multiport Memory system, the control, switching & priority arbitration logic are distributed throughout the crossbar switch matrix which is distributed at the interfaces to the memory modules.

4.Hypercube Interconnection :
This is a binary n-cube architecture. Here we can connect 2n  processors and each of the processor here forms a node of the cube.  A node can be memory module, I/O interface also, not necessarily processor. The processor at a node has communication path that is direct goes to n other nodes (total 2n nodes). There are total 2n distinct n-bit binary addresses.

Conclusion :
Interconnection structure can decide overall system’s performance in a multi processor environment. Although using common bus system is much easy & simple, but the availability of only 1 path is its major drawback & if the bus fails, whole system fails. To overcome this & improve overall performance, crossbar, multi port, hypercube & then multistage switch network evolved.