Exam practice

• Character code: the number a computer stores for a character (a letter, digit, space or symbol).
• ASCII: an early table of character codes using 7 bits (128 codes) for English letters, digits, punctuation and control codes; for example A = 65.
• Unicode: one table of codes for the characters of every writing system in the world, including Devanagari and emoji; its first 128 codes are ASCII’s.
• Code point: a character’s number in Unicode, such as 2325 for क.
• Font: a file of drawings (shapes) for characters; it decides how the stored codes look on the screen.
• Overflow: when the answer to a calculation needs more bits than the computer keeps for it, so the extra bit is lost and the stored number is wrong.
• RGB: storing a color as three numbers from 0 to 255, for its red, green and blue light.
• Sample: one measurement of a sound wave, kept as a number; sound is stored as many samples a second.

• i.13 → 6 r 1 → 3 r 0 → 1 r 1 → 0 r 1. Read upwards: 1101. Check: 8 + 4 + 1 = 13.
• ii.45 → 22 r 1 → 11 r 0 → 5 r 1 → 2 r 1 → 1 r 0 → 0 r 1. 101101. Check: 32 + 8 + 4 + 1 = 45.
• iii.130 → 65 r 0 → 32 r 1 → 16 r 0 → 8 r 0 → 4 r 0 → 2 r 0 → 1 r 0 → 0 r 1. 10000010. Check: 128 + 2 = 130.
• iv.255 → 127 r 1 → 63 r 1 → 31 r 1 → 15 r 1 → 7 r 1 → 3 r 1 → 1 r 1 → 0 r 1. 11111111. Check: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255.

• i.0011 + 0100 = 0111 (3 + 4 = 7). No carries.
• ii.0111 + 0001 = 1000 (7 + 1 = 8). The carry runs all the way along: 1 + 1 = 10, then 1 + 1 = 10, then 1 + 1 = 10, then 0 + 1 = 1.
• iii.1011 + 0110 = 10001 (11 + 6 = 17). It needs 5 bits: in 4 bits it would overflow.
• iv.00111100 + 00011110 = 01011010 (60 + 30 = 90).

• i.11110000 is 240 and 00100000 is 32. The true answer is 240 + 32 = 272 = 100010000 (9 bits).
• ii.The 9th bit is lost, so the byte holds 00010000 = 16 (272 − 256 = 16).
• iii.Overflow. A program can avoid it by keeping the number in more bits (for example 16 bits, which go up to 65,535), or by checking whether an answer is too big before using it.

75 = K, 97 = a, 116 = t, 104 = h, 109 = m, 97 = a, 110 = n, 100 = d, 117 = u. The message is Kathmandu.

1. ASCII has only 128 codes, enough for English but with no room for the letters of other scripts such as Devanagari, Chinese or Arabic.
2. Before Unicode, different countries and companies made their own different tables for their own letters, so text made on one computer turned into nonsense on another. Unicode is one table that every device in the world uses, so a message shows the same everywhere.

It is stored in the order it is said: क, ◌ि, त, ◌ा, ब (2325, 2367, 2340, 2366, 2348). The vowel sign ि belongs to क, so it is stored after it. The font draws ि on the left of its consonant, because that is how Devanagari is written: the stored order is the order of the sounds, and the font decides the shapes and where they go.

• i.The notice was typed in the Preeti font, which saves English letters and only draws them as Nepali shapes. Rita’s computer doesn’t have the Preeti font (or shows the text in another font), so she sees the English letters that are really stored.
• ii.A search compares stored codes. The file holds the codes of the English letters ; " r g f, not the Nepali codes of स ू च न ा, so a search for सूचना can’t match it.
• iii.She can copy the text into a Preeti-to-Unicode converter and convert it, then save it with a Unicode font such as Kalimati or Mukta. She must read the result carefully and correct any words that came out wrong, and check that the converter was set to the right old font.

It has a long list of correct words, with rules for the endings each word can take (such as मा, को, हरू). It takes the word underneath each typed word’s ending and looks it up. If it isn’t in the list, the word is marked as wrong, and words in the list that differ by one or two letters are suggested.
It can’t catch a real word used in the wrong place, such as घर typed instead of घाम: both are in the list. (It also can’t check Preeti text, which isn’t stored as Nepali letters.)

• i.black: 0, 0, 0
• ii.red: 255, 0, 0
• iii.yellow: 255, 255, 0
• iv.cyan: 0, 255, 255

A screen adds light: red and green light together reach the eye and work the eye’s red-sensitive and green-sensitive cells at once, which the brain sees as yellow. Paint works the other way round: each paint takes away (absorbs) some colors from white light, so mixing two paints takes away even more and gives a darker color.

• i.800 × 600 = 480,000 pixels.
• ii.480,000 × 3 = 1,440,000 bytes.
• iii.1,440,000 ÷ 1,024 ÷ 1,024 ≈ 1.4 MB.

• i.Raster: a photo is full of detail with no simple shapes.
• ii.Vector: a logo is a few clean shapes, used at many sizes.
• iii.Vector: a map of lines and shapes, which must stay sharp when printed large.

A microphone turns the sound’s pressure waves into an electric signal that rises and falls like the wave. The computer measures this signal many times a second; each measurement is a sample, stored as a number in binary. To play it back, the numbers are sent to a speaker at the same speed.

• The sample rate (samples a second): more samples catch quicker changes, the high sounds, but there are more numbers to store.
• The bits a sample: more bits give more levels, so each sample is rounded less and the sound is smoother, but each sample takes more space.

Better sound in both ways means a bigger file.

8 bits = 1 byte a sample. Bytes a second = 8,000 × 1 = 8,000 bytes.
For 30 seconds: 8,000 × 30 = 240,000 bytes.
In KB: 240,000 ÷ 1,024 ≈ 234 KB.