September 27th, 2017

## What is a computer?

### Summary of last week

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

## The smallest information piece

The most simple answer to a question is yes or no

When we bet on a tossed coin, what do we know?

This elementary information unit is called bit (binary digit)

It can be represented by on/off, true/false, 0/1, etc.

## Binary representation

For technical reasons modern computers handle only packs of 8 bits

That is called a byte and can represent a number in the range 0 to 255

## 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

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

## Homework: Memory size

How much can we store in the computer?

What is the size of the memory of your computer?

What is the size of the disk?

The memory (RAM) is like a desk. The disk is like a bookshelf.

## 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, such as

• 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

Numbers between 128 and 255 are not used in ASCII

Non English languages use these values for symbols like “Ç”, “Ö”, “É”, “Ñ”

## Text Files

• are universal
• are easy to read and write from a program
• do not have any style like bold or italic
• are like books without figures

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

You shall not use Word for this course

## 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