CPU components and memory

• Memory Data Register


  • Holds the instruction to be carried out
• Program Counter


  • Hold the memory address of each instruction for each cycle
• Memory Address Register


  • Holds the memory address for each instruction about to be used by the CPU
• Accumulator


  • Holds the result for logical and arithmetic calculations (found in the ALU)
• Memory


  • Holds the program instructions and data
• Virtual memory


  • Secondary storage used as extra RAM if it runs out. It may be needed if there are too many applications being run at the same time or if a memory-intensive application is being run. When switching between virtual memory and RAM if the CPU needs to read data, it is slow. It can make a computer slow to respond
• FDE cycle

  Fetch: Copies the memory address from the PC to MAR. Copies the instruction stored in the memory address to the MDR. Increments the PC to point to the address of the next instruction. Decode: Decodes the instruction in MDR- done by the CU. Execute: Instruction is carried out e.g load data, write data
• RAM

  • Random access memory is a type of primary storage used to store all data, files, and programs while they are being run. It is volatile but is much faster than secondary storage. It is the main memory and can be read to and written to
• Factors affecting CPU performance

  Clock speed: Number of FDE cycles per second. Usually about 3.5GHz. The higher the clock speed the greater number of instructions that can be carried out per second. Number of processor cores: Each core processes data separately. If you increase the number of cores, you increase the instructions that can be carried out at the same time. All three factors increase the CPU performance but also make it more expensive. More RAM makes the computer run faster since it means more applications and memory-intensive applications can be run. If the RAM is too little it will rely on virtual memory which runs at a slower rate. GPU: Graphics processing unit processes images and graphics. This reduces the CPU’s workload allowing it to carry out its other functions. Computers have basic GPUs on the motherboard but a dedicated GPU is often used
• ROM


  • Read-only memory is a non-volatile memory, it comes as a chip on the motherboard. ROM contains all the instructions a computer needs to boot up called basic input output system (BIOS). When the computer is powered on the CPU reads the instructions from ROM
• Primary storage

  • RAM, ROM, cache, registers
• Primary storage

  Fastest read/write times
• Secondary storage

  • HDD, CD, DVD, SD cards
• Secondary storage

  Memory where all data is stored that is not in use
• Secondary storage


  Slower read/write speeds
• Primary storage

  Memory that the CPU can access very quickly
• Primary storage

  Mostly volatile
• Secondary storage

  Non-volatile
• HDD

  • Hard disk drives are the traditional in
• Examples of secondary storage

  • HDD
  • CD
  • DVD
  • SD cards
• Types of memory

  • RAM
  • ROM
  • cache
  • registers
• GPUs


  • Reduces the CPU’s workload allowing it to carry out its other functions
  • Dedicated GPU is often used
• SSD

  • Storage devices with no moving parts
  • Use flash memory
  • Significantly faster read/write time than HDDs
• Examples of flash memory

  • USB pen drives
  • Memory cards
• HDD


  • Traditional internal storage in PCs and laptops
  • Made up of magnetised metal disks that spin
  • Data is stored magnetically in small areas of the disk’s circular tracks
  • Moving arm can access and read/write data
  • Used for backing up and transporting large amounts of data
  • Have moving parts making them unreliable and not durable
• Optical discs


  • Cheap and robust secondary storage
  • CDs-700MB
  • DVDs-4.7GB
  • Blue Ray-25GB
  • 3 forms: read only->CD-ROM, write-once -> CD-R, and rewritable ->CD-RW
  • Uses light and laser to burn marks on the disk
• USB pen drives and memory cards


  • Used to expand storage on small devices
  • High capacity relative to their size
• Advantages of optical discs

  • Very cheap
  • Portable
  • Won’t be damaged by water or shocks
  • Last a long time
• Advantages of HDD and SSD

  • HDD: Cheaper
  • Higher capacity
  • Longer read/write life
  • Faster
  • Don't need defragmenting
  • Shockproof
  • Silent
  • SSD: Portable
  • Cheaper than magnetic
  • Read/write fast
  • Less power
  • Durable
  • Can be unreliable
  • Very expensive
  • Lower capacity
• Disadvantages of optical discs

  • Small
  • Slow read speed
  • Easily scratched
  • Small in capacity