September 26th, 2018

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What is a computer?

  • Is a general purpose device
  • that can read, process and write numbers
    • (and things that can be represented by numbers)
  • to and from the memory
  • following a program stored also in the memory
    • many simple steps
  • Changing the program changes the purpose of the machine

Parts of a computer

Processor(CPU) Interface(I/O) Memory(RAM) Secondary storage (Hard disk) Network

One byte = One character

For technical reasons modern computers handle memory in bytes

  • memory big enough to hold a single character
    • like W or @
  • Or a small integer number (0 to 255)
Byte = Integer between 0 and 255 = One letter

Floating point

Numbers with decimals can be represented using scientific notation 1.8466 * 1019

In the computer we write 1.8466E19

Can also represent special values

  • Inf: Positive Infinity, 1/0
  • -Inf: Negative Infinity, -1/0
  • NaN: Not a number, 0/0
  • NA: Not Available, missing data

This has some limitations

We have a fixed number of digits

Not all numbers are represented exactly

For example

1/3 = 0.333333333…

cannot not be represented exactly with 10 digits

We can also represent more complex things things

Example: Sound

  • Sound is transformed into electricity by a microphone.
  • The voltage is measured 44100 times each second
  • Each sample is stored as a number in a CD

Two steps: sampling (in time) and discretization (in voltage)

Example: Greyscale Image

Example: Greyscale Image

  • Each “point” has a value between 0 (black) and 255 (white)
  • correct name is pixel picture element
  • they are stored line by line

Computers have two kinds of memory

Homework: Memory size

How much can you store in your computer? Please answer these two questions:

  1. What is the capacity of the memory of your computer?

  2. What is the capacity of the disk?

Primary memory is like a desk

Secondary storage is like a bookshelf

Structure of secondary memory

The disks store a huge amount of data

To organize it we use files

To organize the files we use folders
also called directories


Like the main memory, a file is just a list of bytes

The meaning of the file depends on the context

You can decide to change their meaning

Most of the times, the name of the file suggests a context

For example, an MP3 file is probably audio

File attributes

Besides the data itself, files have metadata

That is, data about the data. For example

  • Files have a name
  • Files have a modification date, maybe other dates too
  • Files have a size
  • Files have permissions

File names

The names of the files are “words”: a series of letters, numbers and some symbols

Technically, a filenames is a String or list of characters

Maximum length of a filename is 250 characters

Avoid /, :, +, |, <, *, >, " and '

Use letters (A-Z, a-z), numbers (0-9), ., -,   and _

File names

In some systems small caps and BIG CAPS are not equivalent. Be systematic and coherent

If the filename includes ., the text after it is called extension

In Microsoft Windows (c) extensions are usually 3 letters

  • These are hints on how to interpret the file

Kinds of file

At low level there is only one type of file

For us, it is useful to separate in two:

Text Files
each byte is a character, we can read it
Binary Files
bytes are grouped in binary numbers, representing images, sounds, etc.

Among binary files we have EXE files, which are programs for Windows

Representing text

The most natural way to represent a text document is to encode each letter with a single byte

There is a basic standard for English, called ASCII

Each number from 0 to 127 is either a symbol or a special signal

  • New Line
  • End of Message
  • Tab
  • Space
  • Backspace

ASCII code

30 40 50 60 70 80 90 100 110 120
0 ( 2 < F P Z d n x
1 ) 3 = G Q [ e o y
2 4 > H R \ f p z
3 ! + 5 ? I S ] g q {
4 " , 6 @ J T ^ h r |
5 # - 7 A K U i s }
6 $ . 8 B L V ` j t ~
7 % / 9 C M W a k u
8 & 0 : D N X b l v
9 ´ 1 ; E O Y c m w

Non-English languages use numbers between 128 and 255 for symbols like “Ç”, “Ö”, “É”, “Ñ”

Text Files are good

  • They are universal
  • They are easy to read and write from a program


  • They do not have any style like bold or italic
  • They are like books without figures

Microsoft Word files (doc or docx) are NOT text files

Example binary file

Example text file

Text files are for humans and computers

Text files are for humans and computers

  • Binary files are hard to read
    • unless you have the correct program
  • Text files can be read by humans
    • Each byte is a letter
  • Text files can be read by computers
    • Data must be recyclable
    • The output of one program is the input of another program

How to really use computers

Are computers helping us?

To many people, computers are not helping. Instead they feel like computers make things harder.

The same happened when electric engines were invented.

First factories had a single steam engines

Energy was transported using belts

One motor, several machines

Later, electric motors replaced the steam engines

But the factories did not improve

The real change happened when each machine has its own motor

Today we have electric motors everywhere

Doing the same thing gives the same results

Just changing the technology does not change the world

The real change happens when we do things in a different way

What about this?

Computers are not Typewriters

If we only replace typewriters by Word Processors, nothing changes

Microsoft Word is a technology for 19th century

We need a new way to use computers