This section will discuss the processor, memory and I/O device and their interrelationship, and future sections will build on the topics initiated here.
A computer system is composed of components that may be classified as processor, memory, and I/O that interact or communicate with each other. There must be a path, logically depicted in Figure, between these components in order for this communication to take place.
Figure illustrates a simple single bus structure that may be used for the interaction between components. The location of components relative to each other is important only in the sense that signals travel more rapidly between components that are closer to each other. Components that communicate with each other frequently should be organized so as to minimize the distance between them.
The physical connection between the components is often termed the interconnect structure. Timing considerations must be taken into account and often the interconnection will be more complex than just a set of wires and will include digital circuits used to coordinate the timing of data, address, and control signals between the components.
The reason for this is because different components may operate at different speeds. The data lines are used to transfer information, instructions, and data, and are the sole reason for the existence of the communication address and control lines. It must be possible to move information in either direction between processor and memory or between I/O devices and processor or memory. The interrelationship between the components is defined as the interconnect structure as well as the signals that are transmitted between the components over the interconnection.
Figure appears to allow the exchange of all types of signals in either direction. However, in practice, memory is only capable of sending data and the I/O devices do not send address signals; otherwise all other possible exchanges may occur. The following represents a summary of the activities of each of the components:
The processor is the coordinator of the computer system. It processes abstraction 2 programs by fetching their instructions from memory, decoding them, and executing them. The control portion of the processor is like a director of a play and coordinates the activities of all other portions of the processor, and the other computer system components working from a script called the instructions of a program. The control unit may be strictly hardware, or a combination of hardware and software (micro-program). The processor controls all other activities if it is only processor in the computer system. If multiple processors exist, then one must be the master or arbitration occurs between the processors for control of the other components.
The memory, often called the store or storage, is the location where information is stored. Registers, cache, RAM, ROM, disks, and tapes are different types of memory. The early computers had only one register and it was called an accumulator. However, in the context used so far in this chapter the memory component will be either ROM or RAM. The memory component will contain N words, with each word containing a fixed number of bits and located at one of the addresses 0, 1, 2, . . . , N −1. Memory locations could be compared to tract houses, because all of them look the same and can only be distinguished by their addresses. To address N locations, it is necessary to have ln2 N address line between the processor and memory.
I/O modules: The I/O modules are composed of the I/O interface unit and the peripheral(s). From the user’s view, the I/O modules receive all the instructions and data from the computer user. From the processor point of view, the I/O module is analogous to the memory component in that it will be accessed at a specific address and information can be read / written to it. The I/O modules are an intermediary between the processor and I/O devices to off load some of the processor overhead.
However, it is different from memory in that it may send control signals to the processor that will cause a processor exception to occur. Many computer manufacturers use the term exception to include interrupts, and the two terms will be synonymously used throughout this article. Examples of I/O devices are keyboards, printers, disk drives, CD-ROM drives, DVD drives, tape drives, and ZIP drives.