Difference Between CPU and System Unit

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When computers started becoming popular, users were always confusing themselves with the use of CPU and system unit. And even now, some still do. So I am going to take you through the difference between CPU and System Unit in this tutorial.

The Central Processing Unit (CPU)

Simply called the processor and is the brain of the computer. It is plugged in the CPU socket on the motherboard in the system unit. It monitors the operations of all the other components in the computer system. It has two main parts: the Control Unit (CU) and the Arithmetic Logic Unit (ALU).

  • The Control Unit is that part of the processor that ensures that components perform according to the requirements of the software currently in use. It directs the flow of data through the CPU to and from other devices.
  • The Arithmetic Logic Unit also carries out any program instructions requiring the arithmetic or logical processing of data. Thus, actual manipulations of data (arithmetic and logic operations) take place in the ALU. The ALU performs two functions and they are Arithmetic operation - This performs addition, subtraction, multiplication and division and Logic operation - This is also concerned with comparing, selecting, matching, sorting and merging of data etc. 

When comparing data factors, the logical ability of the ALU differentiates between positive and negative differences in accordance with a result of the comparison.
Both the CU and ALU contain registers. Registers or high-speed storage areas are small memory areas that store data and instructions while CPU processes them.

The CPU or microprocessor chips in most PC’s today may either be Intel or Intel-type chips. Examples of microprocessors are:
  • Intel: Since 1978, Intel’s processor has evolved from 8086, 8088, 80286, 80386, 80486 through to the Pentium  family – Pentium, Pentium pro, Pentium with mmx, Pentium II, Pentium III, Celeron, Xeon, Pentium IV, Pentium D (desktops), Pentium M (laptops), Core 2 Duo and now the i-series – i3, i5 and i7.
  • AMD (Advanced Micro Devices) e.g., AMD K6 –III processor.
  • Cyrix
  • Motorola (In Apple Macintosh and PowerPC computers) e.g., G3, G4 PowerPC processors.

Check Me Out: Difference Between Data and Information 


System Unit

The Computer System Unit is normally referred to simply as System Unit. It is also called the System Cabinet or System Case. It houses the processing, memory and other hardware necessary for data processing and storage.

Unlike the computer peripherals, its contents are normally seen. Components can be found on the front, back and the inside of the system as discussed below:

Front Items - Items located at the front of the system unit include:

  • DVD/CD-ROM Drive, Floppy Disk Drive, Switches (reset, power on/off), Zip drive, USB ports, Expansion Bays, Plug for Audio devices.

Inside Items - Items located inside of the system unit include:

  • CPU (Central Processing Unit) or Microprocessor chip
  • The motherboard also called the System Board. It is the main circuit board. It connects all the computer devices together, enabling them to communicate. It is a thin plate that holds the CPU, RAM chips, ROM chips, sound card, network card, connectors for hard drives and optical drives etc.
  • Power Supply Unit, System Fan, Hard Disk drive and Hard Disk, Memory Chips, System Battery, CMOS chip
  • Expansion Slot: They allow for the increase of a computer’s capabilities by adding hardware to perform tasks beyond the existing scope of the computer. Slots may contain FM Card, TV Card, Video Card, Sound Card, Network Interface Card, Modem Card etc. Slots and ports also make it easy to add external device or peripherals to the computer system.

Back Items - Items located at the back of the system unit include:

  • Serial ports for connecting devices like mouse, keyboard
  • Parallel ports (in older PCs) for connecting devices like printers
  • USB (Universal Serial Bus). This may be used for mouse, keyboard or printers with USB cables.
  • Power ports for electricity connection.
  • Other dedicated ports such as telephone ports, speakers, microphones etc.

Now we can boldly conclude that the CPU and the System Unit are two different entities. The CPU is housed in the System Unit on the motherboard.


Let’s get interactive by dropping your questions and suggestions in the comments box below. Enjoy!!!


Level: Beginner

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