Hardware Components of a Computer

A computer system consists of the physical computer, called the hardware, and the programs, called the software, that instruct the machine. The term architecture refers to the way the computer hardware is organized. There are several variations but basically the computer hardware is organized into five functional components. This organization is much the same as the organization outlined by John von Neumann over 50 years ago and is often called the von Neumann architecture.

The Five Hardware Components of a Computer

• Central Processing Unit: This is the hardware component in charge of processing instructions. It consists of two parts:
  • Control Unit (Controller) The Control Unit is in charge of (oversees) the execution of intructions. For each instruction, it must "fetch" the instruction from main memory, "decode" it (figure out what it says to do), then send signals to the unit of the computer that will do the actual execution of the instruction. There are two small memory locations (called registers) in the Control Unit that are used to keep track of things. These are
    • The Program Counter which contains the address of the next instruction to be executed and
    • the Instruction Register which contains the instruction currently being executed.
    The decoding step is done by a circuit called the Instruction Decoder. The sequence of steps the Control Unit goes through as it "controls" the execution of instructions is called the fetch-execute cycle. Click here for a more detailed explanation and example.
  • Arithmetic Logic Unit (ALU) The ALU is responsible for performing arithmetic operations such as addition and multiplication and logic operations such as the comparison of two numbers.

  In personal computers, the circuits that make up the CPU are all on a single silicon chip called the microprocessor. Check out the techweb.com Web site for more information about microprocessor chips. The link takes you to an encyclopedia. Look up the word microprocessor to see the insides of a microprocessor and the word chip to see more information about chips in general (including their history and an explanation of how they are made).

• Main Memory Main memory is for the storage of instructions and data currently being used by the CPU; that is, it is the working memory of the computer. Most of main memory consists of Random Access Memory (RAM) though a small portion is Read Only Memory (ROM). ROM is a permanent, unchangeable (it can only be "read" from not "written" to) part of main memory that contains among other things the instructions that start the process of booting the machine (loading the operating system). RAM, on the other hand, is much larger and changes constantly as the CPU does its work. RAM is volatile, meaning that all data is lost when the power is off; ROM is not volatile. Main memory consists of a sequence of storage locations (cells) each with its own address.

• Auxiliary Memory Auxiliary memory is for long term storage of instructions and data. Magnetic disks are currently the most common type of auxiliary memory. Programs and data must be "saved" to auxiliary memory and "retrieved" or "opened" before they can be used again.

• Input Devices Input devices are for the user to communicate with the computer by entering data (including programs) or issuing commands. Common input devices are the keyboard and the mouse.

• Output Devices Output devices are for the computer to communicate with the user. Common output devices are the monitor, printer, and speakers.

The CPU and main memory are realized in chips mounted on a circuit board called the motherboard or system board. The different components are connected by a group of wires called a bus. Data, instructions, and control signals travel along the bus as the various components communicate with each other.

The above description gives a general overview of basic computer hardware. There are many variations in modern computers designed to speed the computer up. Many modern computers are multiprocessors; that is, they have more than one CPU. Most modern computers have an additional type of memory called cache. Cache is an intermediate form of memory designed to speed up access to data. There is RAM cache which is used to speed up the CPU's access to data in RAM and disk cache which is used to speed up access to data on disk.