Components of a Computer

The five classic components of a computer are briefly described below. Each component is discussed in more detail in its own section. The operation of the processor is best understood in terms of these components.


The datapath manipulates the data coming through the processor. It also provides a small amount of temporary data storage.

The datapath consists of the following components.

  • programmable registers - small units of data storage that are directly visible to assembly language programmers. They can be used like simple variables in a high-level program.
  • the program counter (PC) - holds the address for fetching instructions.
  • multiplexers have control inputs coming from control. They are used for routing data through the datapath.
  • processing elements - compute new data values from old data values. In simple processors the major processing elements are grouped into an Arithmetic-Logic Unit (ALU).
  • special-purpose registers - hold data that is needed for processor operation but is not directly visible to assembly language programmers.


Control generates control signals that direct the operation of memory and the datapath. The control signals do the following.

  • Tell memory to send or receive data.
  • Tell the ALU what operation to perform.
  • Route data between different parts of the datapath.

Memory holds instructions and most of the data for currently executing programs.

The rest of the data is held in programmable registers, which can only hold a limited amount of data.

Input is data coming into the processor from external input devices such as keyboards, mice, disks, and networks.

In modern processors, this data is placed in memory before entering the processor. Input handling is largely under the control of operating system software.

Output is data going from the processor to external output devices such as displays, printers, disks, and networks.

In modern processors, this data is placed in memory before leaving the processor. Output handling is largely under the control of operating system software.

The processor executes a sequence of instructions that are located in memory. Execution of each instruction involves at least the first three of the following activities. The last four activities are required for some, but not all, instructions. The activities are approximately in time order. However, some of the activities can be overlapped in time.

  • Instruction fetch
  • Program counter (PC) update
  • Instruction decode
  • Source operand fetch
  • Arithmetic-logic unit (ALU) operation
  • Memory access
  • Register write

In these activities

  • The program counter (PC) hold the address of the next instruction.
  • For a simple processor, the arithmetic-logic unit (ALU) performs all arithmetic and logical operations.

The organization of the data path can be determined from these activities. Where an activity requires selecting among different options depending on the instruction, there will be a multiplexer that selects the appropriate option as directed by a control signal.

System Interconnection