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Part 2.1What is a computer?

What is a computer?

A computer system an electronic machine that can be used to carry out specific instructions over and over or programmed to carry out a set of instructions.

Computers can perform calculations and logical operations very quickly, thousands of times faster than a human being. However, it's important to note that computer systems are not intelligent and cannot think for themselves - they rely on humans to program them.

A computer program is a series of instructions that are run on a computer system, Charles Babbage and Ada Lovelace are often considered the first people to create a computer program that could be run on a digital machine. This machine was called the Analytical Engine.

Computer basics

Computer systems are built using physical hardware. Hardware is the tangible parts of a computer. This hardware is able to run software. These days software is programmed to carry out millions of instructions and be able to provide many operations. The software that runs on the computer system can be changed very easily, so computers aren't limited to run specific programs.

Computer software and hardware communicate through a special type of software known as firmware.

Modern computer systems follow an architecture or model called the von Neumann architecture developed by mathematician John von Neumann. Simply put, this describes how all computer systems operate.

Von Neumann architecture

A diagram of a computer system shown using the von Neumann architecture.

The von Neumann architecture describes a computer system. It consists of a central processing unit or CPU which can decode and execute instructions. The CPU should be also be able to read instructions directly from some kind of memory. It should also be able to store data in memory as well.

In the von Neumann architecture at least, the term memory refers to both temporary and permanent memory, but generally speaking memory refers only to temporary memory whereas backing storage refers to the storage used to permanently store data.

The von Neumann architecture describes the operation of a computer system as a machine that receives an input performs some process on it and provides an output. Computer systems can therefore be perceived as a mathematical function. For example take function, f which has some domain X and a codomain of Y, this could be shown as:

$$f: X \to Y$$

Given some input, the same output should occur each time. If this was not the case, computers would be utterly useless since the result would be inconsistent.

Semiconductors

The invention of the semiconductor is one of the most fundamental reasons we have digital computers today.

A semiconductor is neither a perfect electrical conductor nor a perfect electrical insulator and it's electrical resistance drops based on certain variables or conditions such as the temperature of the semiconductor.

Semiconductors are generally made from either silicon or germanium. To make them acts in different ways, they are given impurities by a process called doping. Generally, these impurities are added to increase conductivity and is done by adding an element such as arsenic.

Semiconductors have many uses, but from the point of view of computer systems, the most common use is for constructing transistors.

A transistor is a special kind of semiconductor that acts like a switch. It has three terminals: two inputs and one output. Input one is a basic voltage that is passed through an open switch. The second input is used to determine whether to open the switch or not. Several types of transistors open at 0.7 volts but others are lower or higher than this. Finally, the third terminal is the output which is 0 or the input.

Binary

Since computer systems work with transistors, which are essentially switches, combining these switches together allows a computer to represent many different values.

Encoding using transistors is called binary and is the way in which computer systems represent values.

Binary works with two numbers, 0 and 1, therefore it is often refered to as base 2.

There is more in this tutorial in a later article about binary calculations.

Logic

Since transistors can be combined, it is also possible to combine transistors to make logic representable by computers. This allows computers to perform decision making, mathematical operations and operate on values.

The combinations of transistors into groups allows the creation of logic gates.

There are several gates that computer systems use:

  • AND gate
  • OR gate
  • NOT gate
  • NAND gate
  • NOR gate
  • XOR gate
  • XNOR gate

AND

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