Main components of computer systems

A typical computer system is made up of hardware and software. The figure shows an example of a computer system consisting of input devices, output devices, and secondary storage. These will be discussed in more detail in Chapter 2, but examples include:
  • Input devices: keyboard, mouse
  • Output devices: monitor, printer
  • Secondary storage devices: DVD R/W drive, removable hard drive.
However, one part of the computer system has not yet been mentioned. This is shown as the ‘Processor and internal memory devices’ in the diagram – this consists of four key components called the central processing unit (CPU), internal hard disk, random access memory (RAM), and read-only memory (ROM).



The central processing unit (CPU) is the part of the computer which interprets and executes commands from the computer hardware and software. CPUs used to be made up of discrete components and numerous small integrated circuits, which were combined together on one or more circuit board/s. However, due to modern manufacturing techniques, the term microprocessor is now used instead of CPU.

This is a single integrated circuit that is at the heart of most PCs and is also found in many household devices and equipment where some control or monitoring is needed (e.g. the engine management system in a car). 

The internal hard drive is the computer’s main memory; this is where the applications software, disk operating system, and data files are stored. The main advantage of these memories is the fast data transfer/access times and their large capacity to store data.

Random-access memory (RAM) is an internal chip where data is temporarily stored when running applications. This memory can be written to and read from. Since its contents are lost when power to the computer is turned off, it is often referred to as a ‘volatile’ or ‘temporary’ memory.


Read-only memory (ROM) is a memory used to store information that needs to be permanent. It is often used to contain, for example, configuration data for a computer system. These chips cannot be altered and can only be read from (hence their name). One of the main advantages is that the information stored on the ROM chip is not lost even when power is turned off to the computer. They are often referred to as ‘non-volatile’ memories.

It is worth noting that ROM also contains some coding known as the boot file. This code tells the computer what to do when it first starts up; it is often referred to as the BIOS (basic input/output system). When the computer is turned on, the BIOS carries out a hardware check to find out if all the devices are present and whether they are functional. Then it loads the operating system into the RAM. The BIOS stores the date, time, and system configuration in a non-volatile chip called a CMOS (complementary metal-oxide-semiconductor), which is usually battery powered.


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