A computer consists of a set of components or modules of three basic types (processor, memory, I/O) that communicate with each other. In effect, a computer is a network of basic modules. Thus, there must be paths for connecting the modules. The collection of paths connecting the various modules is called the interconnection structure. The design of this structure will depend on the exchanges that must be made among modules.
Figure below suggests the types of exchanges that are needed by indicating the major forms of input and output for each module type.
• Memory: Typically, a memory module will consist of N words of equal length. Each word is assigned a unique numerical address (0, 1, . . . , N – 1). A word of data can be read from or written into the memory.The nature of the operation is indicated by read and write control signals. The location for the operation is specified by an address.
• I/O module: From an internal (to the computer system) point of view, I/O is functionally similar to memory. There are two operations, read and write. Further, an I/O module may control more than one external device. We can refer to each of the interfaces to an external device as a port and give each a unique address (e.g., 0, 1,…, M – 1). In addition, there are external data paths for the input and output of data with an external device. Finally, an I/O module may be able to send interrupt signals to the processor.
• Processor: The processor reads in instructions and data, writes out data after processing, and uses control signals to control the overall operation of the system. It also receives interrupt signals. The preceding list defines the data to be exchanged. The interconnection structure must support the following types of transfers:
• Memory to processor: The processor reads an instruction or a unit of data
• Processor to memory: The processor writes a unit of data to memory.
• I/O to processor:The processor reads data from an I/O device via an I/O module.
• Processor to I/O: The processor sends data to the I/O device.
• I/O to or from memory: For these two cases, an I/O module is allowed to exchange data directly with memory, without going through the processor, using direct memory access (DMA).
Though a number of interconnection structures have been tried. By far the most common is the bus and various multiple-bus structures.