☰ Contents · Computing

How a computer represents information

Lessons 9–10 · 2 lessons · B. Boltayev, M. Mahkamov, A. Azamatov, S. Rahmonqulova. Informatics and Information Technologies, Grade 7. Revised and expanded third edition. “O‘zbekiston milliy ensiklopediyasi” State Scientific Publishing House, Tashkent, 2017
10

Practical lesson

Textbook: pp. 43–44
GoalCalculates the volume of text, pictures and files in bits, bytes, KB and MB, finds the needed code length and determines the transfer time.
New words
colour depth: the number of bits used for the colour of one pixel · rang chuqurligiinformation volume: how many bits or bytes a message takes · axborot hajmitransfer speed: volume sent per second · uzatish tezligiRGB: the red–green–blue colour model · RGB
Explanation

In this practical lesson volume tasks are solved. Text volume: number of characters · 1 byte. Number of characters = pages · lines per page · characters per line. Divide by 1024 to go from bytes to KB and by 8 to go from bits to bytes. Picture volume: number of pixels · colour depth (bits). The colour depth comes from N = 2^r: 2 colours – 1 bit, 16 colours – 4 bits, 256 colours – 8 bits. Transfer time = volume : speed, and the units must match (KB and KB/s). Before writing the answer check the unit and judge whether the size is sensible: a one-page text takes a few KB, while a colour photo can take a few MB.

Worked examples
128 pages, 32 lines per page, 64 characters per line: 128 · 32 · 64 = 262144 characters = 262144 bytes; 262144 : 1024 = 256 KB.
A 600 KB file is sent at 40 KB/s: time = 600 : 40 = 15 seconds.
Class activity

“Volume workshop”: each team writes one day’s lesson timetable as a short message (spaces and punctuation count as characters) and calculates its volume in bits and bytes; then teams swap and check each other’s results.

Practice
1
A book has 96 pages, 40 lines per page and 64 characters per line. How many KB is it?
2
How many bits does the word “SAMARKAND” carry (1 character = 8 bits)?
3
What is the minimum number of bits needed to code 20 different signals?
4
Why is 1 MB = 1024 KB and not 1000? (by the textbook rule)