as level computer science

• Base


  The number of digits that a number system can use to represent numbers
• Place value


  Digits in certain positions have a specific value
• Number systems


  • Denary (Base 10)
  • Binary (Base 2)
• Binary


  • Possible bits (binary digits): 0 and 1
  • All data and characters are represented in binary
• Denary prefixes


  • kilo- (k) x10^3
  • mega- (M) x10^6
  • giga- (G) x10^9
  • tera- (T) x10^12
• Binary prefixes


  • kibi- (Ki) x2^10
  • mebi- (Mi) x2^20
  • gebi- (Gi) x2^30
  • tebi- (Ti) x2^40
• Binary Coded Decimal (BCD)

  Binary representation where each positive denary digit is represented by a sequence of 4 bits (nibble)
• Two's Complement


  Represent a negative number in binary by making the most significant bit (MSB) a sign bit, which indicates whether the number is positive or negative
• Converting -42 to two's complement binary


  • 11010110
• Hexadecimal


  Base 16 number system with possible digits 0 to 9 and A to F
• Converting 65 to Hexadecimal

  • 41
• Character encoding standards


  • ASCII
  • Extended ASCII
  • Unicode
• ASCII


  • Only English alphabets can be represented
  • Each character encoding takes up 7 bits, hence 128 possible characters
• Extended ASCII


  • Also includes most European languages' alphabets
  • ASCII extended to 8 bits, hence 256 possible characters
• Unicode


  • Superset for ASCII & extended ASCII - recognized by various global languages
  • Greater range of characters, as it uses 2 or 4 bytes per character
  • Smaller storage space
• Bitmap images


  Data for a bitmapped image is encoded by assigning a solid colour to each pixel, i.e., through bit patterns
• Pixel


  Smallest picture element whose colour can be accurately represented by binary
• Image resolution


  Pixel density, which is measured by no. of pixels/cm
• Screen resolution


  Number of pixels which can be viewed horizontally & vertically on the device's screen
• Colour depth


  Number of bits used to represent the colour of a single pixel
• Colour depth increases


  Colour quality increases but file size increases
• Vector graphics


  Made up of drawing objects which are mathematically defined constructs
• Vector graphics


  • Data is encoded using mathematical formulas to generate properties in order to draw lines & curves to create the image
  • Scalable without losing quality unlike bitmaps
• Analogue data


  Continuous electrical signals
• Digital data


  Discrete electrical signals
• Sound


  Vibrations through a medium, hence analogue in nature
• Digitizing sound


  1. Sampling the sound wave's amplitude (height) at set time intervals

  2. Encoding these samples as a binary number sequence
• Sampling rate


  Number of samples taken per unit time
• Sampling resolution


  Number of bits used to encode each sample
• Bit rate


  Number of bits for storing 1 second of sound
• Bit rate


  File size = Bit rate * Length of sound
• Lossless compression


  Allows original data to be perfectly reconstructed from compressed file
• Lossless compression


  • Bitmap (.bmp), vector graphic (.svg) and .png images, text file compression, database records
• Run-length Encoding (RLE)


  Form of lossless compression which reduces file size of a sequence of elements which has adjacent, identical elements
• Lossy compression


  Irreversibly eliminates unnecessary data, resulting in lower file accuracy/quality but lower file size (~10% of lossless)
• Lossy compression


  • Sound files (.mp3), .jpeg images
• Networking devices


  Interconnected devices that allow a fast means of data transmission within the network
• Networking benefits


  • File sharing
  • Resource sharing
  • Higher storage
• Network types


  • LAN (Local Area Network)
  • WAN (Wide Area Network)
• LAN


  Network that connects devices within a small geographical area