Lesson 1 – The CPU

Learning Objectives

  • To understand that a CPU is made up of millions of tightly packed switches called transistors
  • To understand that transistors can generate heat which must be removed
  • To understand that the clock speed of a processor describes how many instructions can be processed each second

Learning Outcomes

All must watch the Royal Christmas Lectures on processors and be able to describe the concepts covered.  With help fill in the clock speed worksheet and be able to describe what clock speed is in simple terms. (Grade D/E)

Most should watch the Royal Christmas Lectures on processors and be able to explain the concepts covered.  Fill in the clock speed worksheet and be able to explain the relationship between clock speed and how many instructions the CPU can carry out. (Grade B/C)

Some could watch the Royal Christmas Lectures on processors and be able to justify the concepts covered.  Fill in the clock speed worksheet and be able to justify the relationship between clock speed and how many instructions the CPU can carry out. (Grade A*/A)

Keywords

Words to learn: processor, clock speed, transistor, heat sink

Starter

The CPU (or central processing unit) is found at the heart of any computer. It carries out literally billions of instructions, or calculations per second.

The processor is responsible for executing a sequence of stored instructions called a program.
This program will take inputs from an input device, process the inputs in some way and output the results to an output device.

Remember IPO.  Input -> Process -> Output

  • Draw a diagram explaining IPO in your book.
  • Imagine you are taking and uploading an image to a social networking site such as Instagram or Facebook.  Break down the process into steps and identify each step into input, process or output.

Main

A CPU has legs on the bottom that connect it to the motherboard.  On top a heatsink and fan is fitted to remove heat from the processor.

Why do CPUs get hot?  Well they are made from millions of electronic components called “transistors” – these are essentially tiny electronic switches that can switch off or on.  If you could look at a CPU’s surface under a microscope you would see the transistors:

processor_microscope

In the picture above each transistor is spaced just 20 nanometres apart.  That’s really small, so how do processor manufacturers make them?  Watch this video to find out.

As you watch, make notes in your book.  Make sure you answer these questions:

  • What is a semiconductor?
  • What is silicon made of?
  • Why is cleanliness so important manufacturing a microprocessor?
  • Just how clean does the factory have to be?
  • How many chips fit on a single wafer?

How quickly can you perform a calculation?

Watch this part of the Royal Institution Christmas Lectures 2009 which explains just how fast processors were then (teachers – 6m40s to 10m23s – click here).  So how do we measure the speed of a CPU? Remember it performs many tasks per second, repetitively, so we measure frequency.

  • Think of your science studies.  How is frequency measured? (if you are stuck look up “cycles per second…”)
  • But we said it performs many many of these calculations per second.  How would we express “millions of cycles per second…”, or “billions of cycles per second…”?
  • Now look at any PC you can find on the  PC world website.  Copy the description of the CPU…  in your own words, what does this actually mean?

Watch this part of the video which shows how processors use switching to perform logic calculations (teachers – 12m1s to 13m29s – click here).

Watch this part of the video which explains why processors performing fast switching has resulted in fast processors (teachers – 27m12s to 28m28s (then do practical) – click here).

Watch this part of the video which goes through the problems of heat and why this is a barrier to processors getting even faster, along with the solutions that manufacturers have come up with (teachers – 31m13s to 35m04s – click here).

Plenary

Fill in this worksheet describing different processor speeds.