Introduction to Programming Using Java - Department of ...

Chapter 1Overview: The Mental LandscapeWhen you begin a journey, it’s a good idea to have a mental map of the terrain you’llbe passing through. The same is true for an intellectual journey, such as learning to writecomputer programs. In this case, you’ll need to know the basics of what computers are andhow they work. You’ll want to have some idea of what a computer program is and how one iscreated. Since you will be writing programs in the Java programming language, you’ll want toknow something about that language in particular and about the modern, networked computingenvironment for which Java is designed.As you read this chapter, don’t worry if you can’t understand everything in detail. (In fact,it would be impossible for you to learn all the details from the brief expositions in this chapter.)Concentrate on learning enough about the big ideas to orient yourself, in preparation for therest of the book. Most of what is covered in this chapter will be covered in much greater detaillater in the book.1.1 The Fetch and Execute Cycle: Machine LanguageA computer is a complex system consisting of many different components. But at theheart—or the brain, if you want—of the computer is a single component that does the actualcomputing. This is the Central Processing Unit, or CPU. In a modern desktop computer,the CPU is a single “chip” on the order of one square inch in size. The job of the CPU is toexecute programs.A program is simply a list of unambiguous instructions meant to be followed mechanicallyby a computer. A computer is built to carry out instructions that are written in a very simpletype of language called machine language. Each type of computer has its own machinelanguage, and the computer can directly execute a program only if the program is expressed inthat language. (It can execute programs written in other languages if they are first translatedinto machine language.)When the CPU executes a program, that program is stored in the computer’s main memory(also called the RAM or random access memory). In addition to the program, memorycan also hold data that is being used or processed by the program. Main memory consists of asequence of locations. These locations are numbered, and the sequence number of a locationis called its address. An address provides a way of picking out one particular piece of informationfrom among the millions stored in memory. When the CPU needs to access the programinstruction or data in a particular location, it sends the address of that information as a signalto the memory; the memory responds by sending back the data contained in the specified(online)1