How data flows in a computer

This is the block diagram of a computer system from Grade 10 ICT. Pick a task, then follow the data, the instructions and the control signals from one part to the next.

Central Processing Unit (CPU)Input deviceKeyboardMain memoryemptyOutput deviceMonitorSecondary storageHard diskemptyMemory registersALUControl Unit
  • Data and instructions
  • Control signals

Work out a sum and show the answer

Before it starts

You type 12 + 5 on the keyboard. A moment later the answer is on the monitor. This is input, process and output.

Press Next to go one step at a time, or Play to watch the whole task.

All the steps

The parts of the block diagram

The Central Processing Unit (CPU) is made of three parts: the Control Unit, the Arithmetic and Logical Unit and the memory registers. Around it are main memory, secondary storage, and the input and output devices.

PartWhat it does
Input deviceSends data and instructions into the computer. A keyboard, a mouse and a scanner are input devices.
Main memoryHolds the data and instructions that are in use right now. It is also called primary memory. It forgets everything when the power goes off.
Output deviceShows or gives out the result. A monitor, a printer and a speaker are output devices.
Secondary storageKeeps data and instructions for later, even when the power is off. A hard disk and a flash drive are secondary storage.
Memory registersHold what the CPU is working on at this moment. They are very small and very fast.
ALUThe Arithmetic and Logical Unit. It does the arithmetic, such as 12 + 5, and the logic, such as checking whether 3 is greater than 2.
Control UnitControls every other part. It sends control signals so that data goes to the right place at the right time.

Two kinds of signal

The diagram has two kinds of arrow, and an exam question may ask you to tell them apart.

  • Data and instruction signals are the dashed arrows. Data is what the computer works on, such as the numbers 12 and 5 or the words of a letter. An instruction says what to do with it, such as add, save or print.
  • Control signals are the solid arrows. They all start at the Control Unit. A control signal carries no data. It tells a part when to send and when to receive, so that data goes to the right place at the right time.

The path for each task

Here is the path the data and instructions take in each task above. Every task starts with an input, and none of them skips main memory.

TaskPath
Calculate
  1. Input device → Main memory → Memory registers → ALU → Memory registers → Main memory → Output device
Save
  1. Input device → Main memory → Memory registers
  2. Main memory → Secondary storage
Open
  1. Input device → Main memory → Memory registers
  2. Secondary storage → Main memory → Output device
Print
  1. Input device → Main memory → Memory registers
  2. Main memory → Output device
Open, change, save
  1. Input device → Main memory → Memory registers
  2. Secondary storage → Main memory → Output device
  3. Input device → Main memory → Memory registers → ALU → Memory registers → Main memory → Output device
  4. Input device → Main memory → Memory registers
  5. Main memory → Secondary storage

Why everything goes through main memory

The CPU takes its data and instructions from main memory, and from nowhere else. That is why what you type goes to main memory first, and why a file on the hard disk has to be copied into main memory before the computer can use it.

Main memory is fast, but it forgets everything when the power goes off. Secondary storage is slower, but it keeps what it holds. That is why you save your work: saving copies it from main memory to secondary storage.

The memory registers are faster still, and much smaller. They hold only what the CPU is working on at this moment.

Key terms

Central Processing Unit (CPU)
මධ්‍ය සැකසුම් ඒකකය
Control Unit (CU)
පාලන ඒකකය
Arithmetic and Logical Unit (ALU)
ගණිතමය හා තාර්කික ඒකකය
Memory registers
මතක රෙජිස්තර
Main memory
ප්‍රධාන මතකය
Secondary storage
ද්විතීයික ආචයනය
Input device
ආදාන උපාංගය
Output device
ප්‍රතිදාන උපාංගය
Data and instructions
දත්ත හා උපදෙස්

More to try

  • Play Save and Open. Which task never reaches the ALU, and why?
  • Cover the list of steps, pick Calculate and say where the data goes next before you press Next.
  • Fill in the blanks: data and instructions from the input device are sent to ……… first. They are then sent to the ……… for processing.
  • True or false: the Control Unit sends control signals to every other part of the computer system.
  • During a task, where does the CPU get its data and instructions from: main memory or secondary storage?
  • In Open, change, save, the hard disk still holds 70 after the monitor shows 75. What would happen if the power went off at that moment?

This diagram is part of the Grade 10 lesson on the fundamentals of a computer system. Practise with the Grade 10 ICT past papers.

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