Computer structure

The computer interacts in some fashion with its external environment. In general, all of its linkages to the external environment can be classified as peripheral devices or communication lines.We will have something to say about both types of linkages.

But of greater concern in this book ismthe internal structure of the computer itself, which is shown in Figure 1.4.There are four main structural components:

• Central processing unit (CPU): Controls the operation of the computer and performs its data processing functions; often simply referred to as processor.
• Main memory: Stores data.
• I/O: Moves data between the computer and its external environment.
• System interconnection: Some mechanism that provides for communication among CPU, main memory, and I/O. A common example of system

interconnection is by means of a system bus, consisting of a number of conducting
wires to which all the other components attach. There may be one or more of each of the aforementioned components.Traditionally, there has been just a single processor. In recent years, there has been increasing use of multiple processors in a single computer. Some design issues relating to multiple processors crop up and are discussed as the text proceeds; Part Five focuses on such computers.
Each of these components will be examined in some detail in Part Two. However, for our purposes, the most interesting and in some ways the most complex component is the CPU. Its major structural components are as follows:

• Control unit: Controls the operation of the CPU and hence the computer
• Arithmetic and logic unit (ALU): Performs the computer’s data processing functions
• Registers: Provides storage internal to the CPU
• CPU interconnection: Some mechanism that provides for communication among the control unit,ALU, and registers

Each of these components will be examined in some detail in Part Three, where we will see that complexity is added by the use of parallel and pipelined organizational techniques. Finally, there are several approaches to the implementation of the control unit; one common approach is a microprogrammed implementation. In essence, a microprogrammed control unit operates by executing microinstructions that define the functionality of the control unit.With this approach, the structure of the control unit can be depicted, as in Figure 1.4.This structure will be examined in Part Four.