Computer Science

Computer Science

1. Data

   • Binary
     • Why binary is used
       • A computer processor contains lots of transistors. Each transistor only has two states: on and off. In binary, 1 represents on and 0 represents off. These are called bits. 4 bits = 1 nibble. 8 bits = 1 byte.
     • Representing data and instructions
       • ASCII
         • The ASCII character encoding system uses 8 bit binary to assign numerical, binary values to textual data- letters, symbols, punctuation marks, etc. Standard ASCII (7 bit) ranges from 0 to 127 whilst extended ASCII (8 bit) ranges from 0 to 255.
         • Extended ASCII table
         • American Standard Code for Information Interchange
     • Number Systems
       • Two common types of number formats
         • Unsigned number
           • A number with a non-negative value of 0 or more
           • A 1-byte unsigned integer number has a range of 0-255
         • Signed numbers
           • A number that can be either positive or negative, indicated by it's sign
           • A 1-byte signed integer has a range of -128 to +127
     • Binary systems
       • Place value increase by powers of two
       • The bit in the leftmost position is the most significant bit (MSB)
       • The rightmost bit of a binary number is the least significant bit (LSB)
       • Base 2
     • Binary arithmetic
       • Overflow error
         • Caused when the result of a calculation is greater than what the computer can score
         • Causes programs to crash or produce incorrect results
     • Negative numbers
       • Two's complement
         • The MSB is designed as the sign bit
           • If the MSB is a 1, the number is positive
           • If the MSB is a 0, the number is negative
           • The MSB of a two's complement is negative. In 8 bit, the 128 becomes a -128
         • 1-byte of two's complement can represent from -127 to 128
     • Binary shifts
       • Done to achieve multiplication and division
       • An operation in which all the bits of a binary pattern are shifted a number of places to either the left or the right
       • There are two types: arithmetic and logical
         • Logical
           • Logical left shifting
             • MSB is shifted left out of pattern. Each other bit is shifted left.
               • Has the effect of multiplying by 2 each 1 space it is shifted left
           • Logical right shifting
             • LSB is shifted right out of pattern with each other bit
               • Has the effect of dividing by 2 each 1 space it is shifted right
             • Causes a loss of precision
               • 23/2 should be 11.5 but with logical right shifting the 0.5 is lost as only integers can be represented
         • Arithmetic
           • Left arithmetic shifting
             • This is the same as logical left shifting
           • Right arithmetic shifting
             • Same as logical right shifting, except the MSB is always maintained
     • Hexadecimal
       • Base 16
       • Has 16 values- 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F
         • The A,B,C,D,E,F represent the numbers 10,11,12,12,13,14,15
       • To convert into decimal
         • Write each character as a binary nibble. Then join the nibbles together and convert from binary into decimal.
           • For example
             • 9A
               • 1001 and 1010
                 • 10011010
                   • 154
       • Only used by humans to shorthand writing out long strings of binary
   • Data Representation
     • Representation of text
       • When you press a key on the keyboard, a stream of 1s and 0s are inputted to the processor
         • Character set
           • The list of characters recognised by the computer
       • ASCII
         • American Standard Code for Information Interchange
         • Allows textual data to be interchanged between computers from different manufacturers
         • Uses 7 bits
           • Holds 128 values
             • Allows it allocate unique binary patterns to all the upper and lower-case letters in the English alphabet and punctuation marks
         • Extended ASCII
           • Uses 8 bits
             • Holds 256 values
               • Extended ASCII table
       • Unicode
         • Used to encode characters
         • Uses more bits than ASCII
           • Permits a larger character set to be represented
     • Representation of bitmap images
       • Bitmap
         • A digital image composed of a matrix of tiny blocks of colour called pixels
       • Each pixel is represented in binary as a single colour
         • The bigger the size of a pixel, the less fine distinctions between shades will be visible
         • Colour depth
           • The number of bits used to encode the colour of each picture
           • The greater the number of bits allocated to represent the colour depth, the more colours that can be used
       • Image size and resolution
         • Image size = height * width
         • Resolution
           • Measured in pixels per inch (ppi)
           • If the resolution is too low, the image will become pixelated
             • Pixelated
               • The individual pixels are visible
       • File size
         • File size = width * height * colour depth
     • Representation of sound
       • Analogue data
         • All sounds are caused by vibrations
           • These vibrations push air molecules, sending waves in all directions
             • When they reach the ears, they cause vibrations in sensory hairs in the inner ear
               • This sends nerve impulses to the brain which interprets them as sound
         • Analogue data is continually changing
         • Analogue data is infinitely long
           • We have to round this so that it can be stored as digital data on the computer
       • Digital data
   • Data Storage and Compression

2. Computers

3. Networks

4. Issues and Impact

5. Computational Thinking

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