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The Central Processing Unit

Summary

Objectives

By the end of this session you should be able to:

  • Introduce logic gates
  • Describe what a transistor is
  • Describe how transistors can be arranged into logic gates
  • Explain what a CPU Architecture is

Key Points

  • Logic Gates are devices that combine binary inputs together using fixed rules.
  • Transistors are small electrical devices that we use as switches in modern computers.
  • Logic Gates can be made out of transistors.
    • This is an example of abstraction.
  • A computer itself is made of several layers of abstraction.

Breakdown

Transistors

Transistors are simple electronic components. They have three pins, and current - electricity - can flow between the first two when electricity is applied to the third. For this reason they are often thought of like electronically-activated switches.

The Transistor became the dominant method for switching in circuits, and we can squeeze lots of transistors into a microchip. Transistors can be arranged into complex circuits like the full adder, which can take binary numbers and add them together! This is the basis of all modern computers.

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Transistors are made with silicon, a product made from sand. This is the "silicon" in "silicon valley" - the name comes from the large quantity of microchips needed by the fast-growing tech companies in the San Francisco Bay Area.

You might hear some people joke that we're "making sand do maths" - which is sort-of true!

Logic gates

As computers got more complex, we needed a way of abstracting away the complexity by representing it in simpler terms. We created logic gates, which are arrangements of transistors that can complete simple logical functions like AND, OR and NOT. Using a series of logic gates we can design instructions like adders, subtractors and multiplers - as well as far more complex circuits.

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Abstraction is an important concept in tech. Trying to understand computers as a series of switching transistors is basically impossible, let alone programming them as such. For this reason, we come up with models which allow us to ignore the underlying complexity.

The network stack is another example of abstraction

Layers of abstraction

There are many other layers of abstraction between a transistor and your first "Hello World" program:

  • Instructions - arrangements of logic gates - are captured in instruction sets, which are then built into a microchip which we call the processor.
  • An Assembly language is written which commands these instructions in a way that a human can read.
  • While some applications are written directly in Assembly, Low-level programming languages like C are written to make programming easier than directly commanding the instructions on the processor.
  • High-level programming languages like Python are then written which seek to make programming even more human-friendly!

A "Hello World" program written in Python is interpreted by the Python interpreter, which is written in C. This then compiles the program to Assembly language, which directly commands the instructions in the CPU's instruction set. These instructions are arrangements of logic gates, which themselves are just arrangements of transistors.

Specialisation

Computers are very complex machines. Not only have we made them human-friendly through layers of abstraction, we have different engineers specialising in different layers of the stack. Nobody understands the full stack in depth - but having an awareness of it, and a curiosity about how problems are solved in different parts of it, will make you a better engineer.