Hardware on the Computer

Given the diversity of modern computers, it is dificult to describe a "typical" computer hardware system. However, certain features are common to most desktop machines, as shown in figure 1.1.
Although specific components and their appearance may vary from computer to computer, virtually all modern computers have the same underlying structure. This structure is known as the von Neumann architecture, named after computer science pioneer John von Neumann, who formalized his method of computer organization in the early 1950s. The von Neumann  architecture identifies three essential components that work together to function as a computer. Input/output devices allow the user to communicate with the computer by entering commands and then viewing the results. Memory stores information for the computer to process, as well as programs, or instructions specifying the steps necessary to complete specific tasks. Finally, the Central processing unit (CPU) carries out programmatic steps in order to process data. These three components are connected by buses, collections of wires that carry information in the form to electrical signals.



Computers that use the von Neumann architecture are known as stored-programs computers, which means that they can initiate different programs from memory. For example, to complete such actions as creating a text doument or performing a computation, the CPU accesses the associated program instructions from memory and executes them in order. If the computer user wishes to perform a new task, he simply loads a new program into memory and instructs the CPU to execute that program. In fact, the von Neumann architecture enables multiple programs to reside in memory at the same time, allowing the CPU to juggle multiple tasks, such as switching back and forth between a Web browser and a text editor.


Central Processing Unit (CPU)
The CPU is the brains of the computer, responsible for controlling the internal workings of the machine. For now that the CPU is made of circuitry, or electronic components wired together control the flow of electrical  signals. The CPU by far the most complex part of computer system. The Intel Core 2 Duo processor, for example, contains circuitry comprised of more than 291 milion individual components. All of this circuitry is packaged onto a small silicon chip encased in plastic, with metal pins for connecting the chip to other hardware components. Common CPUs include the Intel Celeron D. Intel Core 2 Duo, AMD Athlon^TM 64 2X and IBM PowerPC^TM  G5.

Figure 1.1 A common desktop computer system

In controlling the workings of the computer, the CPU has two key tasks: (1) fetching program structions from memory, and (2) executing those instructions. As the ultimate behavior of computer programs can be complex, you might expect that the individual instructions executed by the CPU would be complex, but they are not. Even programs with advanced capabilities are broken into sequences of very simple instructions, such as "add two numbers" or "copy a value from one location to another." Of course,it may require thousands or even milions of such low-level instructionz to specify the vehavior of  a Web browser or word processor. Fortunately, CPUs can process instruction sequences extremely quickly, which enables the processors to handle these immense programs. CPU speed is generally measured in gigahertz (GHz), which indicates how many billions of instructions a CPU can execute in a second. For example, a 2.5GHz CPU can execute approximately 2.5 bilion simple instructions in a second, the producing complex behavior at a speed that appears almost instantaneous to the human user.