Computer science Paper 1

• CPU


  Carries out the fetch decode execute cycle
• Fetch decode execute cycle

  1. Fetch the instruction is moved from memory to the CPU

  2. Decode the instruction is understood by the CPU

  3. Execute the instruction is carried out
• Common CPU components


  • Control unit (CU)
  • Arithmetic logic unit (ALU)
  • Register
  • Cache memory
• Von Neumann architecture


  Program and data are both loaded into memory, making it easier to change and update programs
• Components of Von Neumann architecture

  • Control unit
  • Arithmetic logic unit
  • Buses
  • Registers
  • Input/output
• Registers


  • Program counter (PC)
  • Memory address register (MAR)
  • Memory data register (MDR)
  • Accumulator (ACC)
• CPU performance factors


  • Clock speed
  • Cache memory
  • Number of cores
• Embedded system


  Computer system that is part of a larger device and has a single or limited range of functions
• Characteristics of embedded systems

  • Usually on a single chip
  • Small in size
  • Low power
  • Rugged
  • Low cost
• Primary storage (main memory)

  Where programs and data are stored for the CPU to access
• RAM


  Volatile memory used to store programs and data currently in use
• ROM


  Non-volatile memory that stores hardware settings, BIOS, and boot up instructions
• Virtual memory


  Part of secondary storage used to supplement main memory when RAM is full
• Secondary storage


  Non-volatile storage for long-term or permanent storage of programs and data
• Types of secondary storage

  • Solid state storage (e.g. SSD, SD cards)
  • Magnetic storage (e.g. hard disk drives)
  • Optical storage (e.g. CDs, DVDs, Blu-rays)
• Factors to consider when choosing storage

  • Capacity
  • Speed
  • Portability
  • Durability
  • Reliability
  • Cost
• Units of data storage

  • Bit
  • Nibble
  • Byte
  • Kilobyte
  • Megabyte
  • Gigabyte
  • Terabyte
  • Petabyte
• Converting data to binary

  Computers use switches that can be on or off to represent data as 1s and 0s
• Calculating text file size

  Bits per character x Number of characters
• Calculating image file size

  Color depth x Image height x Image width
• Calculating sound file size

  Sample rate x Duration x Bit depth x Number of channels
• Converting denary to binary

  Repeatedly subtract largest possible binary value from the denary number
• Converting binary to denary

  Add up the binary place values with 1s in them
• Adding binary integers


  1. 0 + 0 = 0

  2. 0 + 1 = 1

  3. 1 + 0 = 1

  4. 1 + 1 = 0, carry 1
• Overflow errors can occur when adding binary integers that exceed the maximum representable value
• Adding two binary integers

  1. Start with least significant bits

  2. 0 + 0 = 0

  3. 0 + 1 = 1

  4. 1 + 0 = 1

  5. 1 + 1 = 0, carry 1 to next column

  6. Carry any remaining 1s
• Cannot just do binary-decimal conversion and add normally, must show binary addition process
• Overflow error occurs when binary addition results in a carry that cannot be represented in the fixed number of bits
• Binary shift


  Moving digits in a binary number left or right
• Shifting left one place

  Multiplies number by 2
• Shifting right one place

  Divides number by 2
• Shifting left with most significant bit 1 causes overflow error, shifting right with least significant bit 1 causes loss of precision
• Converting denary to hexadecimal

  1. Divide by 16, keep whole number and remainder

  2. Whole number is first hex digit, remainder is second hex digit
• Converting hexadecimal to denary


  1. Multiply hex digits by 16^n where n is position (right to left)

  2. Add results
• Converting 8-bit binary to hexadecimal

  1. Split into two 4-bit nibbles

  2. Convert each nibble to hex digit
• ASCII uses 7 bits to represent English characters, extended ASCII uses 8 bits for more characters, Unicode uses 16+ bits for even more characters
• Pixel


  Picture element, smallest unit of an image
• Metadata


  Data about data, information about an image/sound file like color depth, resolution, etc.
• Increasing color depth or resolution

  Increases image quality but also file size
• Sample rate


  Number of samples of sound taken per second, higher rate means better quality but larger file