1.2 Hardware Components

The design of the modern personal computer is based on the von Neumann architecture[1] which was described by John von Neumann in 1945[2]. The modern computer is comprised of three hardware components: (1) a processor that contains a control unit, an arithmetic logic unit, and registers; (2) a main memory system; and (3) an Input/Output system. A schematic diagram of a modern desktop computer is illustrated in Figure 1.2.1.

Figure 1.2.1. Schematic design of a modern personal computer.

Processor

Traditionally, we refer to the processor as the central processing unit or CPU for short. The term central was introduced early in the history of digital computers when most of the processing functions were centralized on a single component or unit. Modern systems can consist of a single processor with multiple cores or multiple processors with multiple cores. Thus, the term processor is more commonly used today to reflect this change in architecture, but the term CPU is still widely used.

The processor contains the digital circuitry that executes the instructions of a program. The individual instructions perform simple operations, which typically include those for arithmetic, logic, and memory access. It is comprised of several common components as described below.

Arithmetic-Logic Unit

The arithmetic-logic unit (ALU) performs all arithmetic, logic, and bit-wise operations, which comprise the bulk of the instructions contained in a computer program. Originally, processors could only carry out integer operations. Later, a separate component, known as a floating-point unit (FPU), was included for floating-point operations.

Registers

The processor also contains a small number of high-speed storage elements called registers, which are used to store temporary values. The registers are wired directly to the ALU and provide fast access to data as instructions are being executed. Modern processors also contain several special registers used in the execution of instructions. The instruction register (IR) holds the instruction currently being executed and the program counter (PC) register keeps track of the next instruction to be executed.

Control Unit

The control unit coordinates the activities of the other units. It decodes the instructions and sends signals to direct other units in carrying out or executing the instruction. While it is easier to view the control unit as a single well-defined component, in modern computers, the digital circuitry of the controller is distributed throughout the processor and computer.

Memory System

The program instructions and data are stored in the memory unit. A computer's memory is constructed using semiconductor storage cells, each of which is capable of storing a single bit of information. These storage cells are grouped into larger fixed sizes, typically consisting of eight bits, that are identified by a distinct numerical address. Access to the memory unit is managed through the system bus, which is a digital communications channel that connects the various components of a computer. Accessing data in memory is many times slower than the direct access to the registers on a processor, but memory is much cheaper to construct than are the registers. Thus, the amount of memory in a computer is far greater than the number of available registers.

In modern computers, programs are typically stored on a secondary storage device such as a hard disk drive or some type of compact disk. To be executed, however, a program must reside in the computer's memory unit, which is commonly referred to as primary storage or the main memory. As the program is executed, the processor fetches the instructions, one at time from memory and carries out the appropriate action.

Input/Output System

Computer programs typically follow a common process: receive or input data, process it, then output the results. The input/output system manages the various input/output (I/O) devices that are used to provide input to a program and to receive the output or results from the program. Today, there are a wide variety of I/O devices. Some provide input only, such as keyboards, mice, and joysticks; some only accept output from a program, such as video terminals and printers; while others provide input and accept output, such as disk drives and network adapters. In modern computers, there are separate device controllers used to manage the I/O system, which frees up the main controller and processor to perform their main tasks.

Special Topic

A Look Inside

Early computers were comprised of multiple large units that were physically separated due to the use of vacuum tubes and the excessive heat they produced. These computers required large rooms to house all of the various components. With the invention of the integrated circuit or microchip, the size of computers began to shrink and their speed greatly increased. Integrated circuits are constructed using transistors (transfer resistor), which are far smaller than vacuum tubes. Early integrated circuits included dozens of transistors on a single chip. As manufacturing and chip technology advanced, the number of transistors packed onto a single small chip greatly increased. Today, microchips can contain 100,000 or more transistors. It is this advance in microchip technology that allowed for the introduction of personal computers and the more modern mobile devices. Modern computers are still constructed using similar components as earlier computers, but their size has greatly reduced. Today, the components of a computer are integrated onto a motherboard that is contained within a single physical structure. As illustrated in the image below, the system bus and other communication channels are "printed" onto the board and the various components are physically connected to them. The size of the motherboard can vary depending on how the computer will be used and the number and type of peripheral devices supported. Identifying the various components can be difficult, even if you are familar with the components and their functions. Most motherboards that support the Intel architecture, like the one illustrated above, have a similar design and include similar components. The construction of an actual computer is far more complex than the simple schematic design presented earlier.