computer organisation and architecture

• Describe the stored program concept

  Data and instructions are stored in the main memory. Instructions are fetched, decoded, and executed from memory by the processor.
• Describe the fetch stage

  PC holds address of next instruction. Contents of PC are transferred to MAR. Contents of MAR are transferred to main memory via address bus. Contents of addressed memory location are transferred to MBR via data bus. PC increments by 1. Contents of MBR are copied to CIR
• Describe the decode and execute stages

  Instruction in CIR is decoded by the control unit. Type of instruction is identified by opcode. Any necessary data is fetched from memory. Relevant part of processor executes instruction. Result is stored in accumulator, a register, or main memory.
• Compare von Neumann and Harvard architecture

  vN : data and instruction are stored in the same memory location, and share a bus. H : data and instructions are stored in separate memory locations and use different buses
• Advantages of Harvard architecture compared to von Neumann

  In Harvard architecture: data and instructions can be accessed simultaneously. use of separate buses reduces fetching delays. data and instruction can have different word lengths. data and instruction can have different memory structures
• Examples of system hardware decisions that affect processor performance

  more cores - allows tasks to be executed in parallel. Allows tasks to be split up, increasing speed of execution. more cache memory - can be accessed faster than main memory, increasing capacity means main memory doesn't need to be accessed as often therefore faster execution. Increasing address bus width - increases amount of addressable locations. increasing clock speed - increases amount of instructions executed per unit time. increasing word length- increases amount of data able to be processed at one time
• Explain what happens when an interrupt occurs

  Current processor state saved to stack, source of interrupt identified, appropriate interrupt service routine called, processor state restored
• Examples of control signals

  bus grant, memory read/write, I/O read/write, interrupt request
• Describe the laser printer process

  Bitmap of image is built in memory from page description. The print drum is negatively charged. A laser is pointed at the drum; this reverses the charge in certain areas, in line with the image. Mirror is used to direct the laser. The toner (powdered ink) is negatively charged. The toner sticks to the positively charged areas on the drum, creating the image. The toner is transferred to paper by passing a piece of paper under the drum. A heat fuser is used to ensure the toner and the paper are fully stuck together.
• Describe process of barcode readers:

  • Made up of a pattern of black and white
  • laser is shined at barcode. A moving mirror moves the light bean across the barcode. Intensities of reflected light is detected by a photodiode and stored as electrical charge
  • Charge is measured, forms the digital signal representing the barcode's info
  • left side - odd number of black bars, right side - even number
  • guard bars are used to indicate start/end and separate left/right
• Describe the digital camera process:

  • Light enters the lens and hits a sensor. Shutter regulates path.
  • Sensor converts light into electrical charge
  • Charge is stored in cells (1 cell per pixel)
• Describe digital camera process:

  • Light enters through lens. Shutter regulates path between lens and sensor
  • Sensor converts light into electrical charge
  • Charge is stored in cells (1 cell per pixel)
  • Charge in cells are measured, converted to a digital value, stored as digital image
• Advs. of RFID vs barcode:

  • can be read from further away
  • can be read faster
• Describe colours in digital camera:

  • Each pixel has multiple cells, which have different filters to let in different light wavelengths (detecting colours)
  • Measuring the intensity of the filters measures how much of each colour is in the pixel
• Describe operation of HDD:


  • Disk is made up of platters and divided into tracks (which are divided into sectors)
  • Disk spins continuously at high speed while in use
  • Head moves in and out
  • Data is read when correct sector passes under the head
  • Magnetic - head detects change in magnetic polarity as 1, no change as 0
• Describe operation of optical disk:


  • Data is stored on one continuous track
  • High powered laser burns pits into track
  • Low powered laser shines at track, detects reflection or scatter of light
  • Scatter occurs at land/pit boundary, reflection occurs at continuation
• Describe operation of solid state drive:

  • Uses NAND flash memory cells
  • these use floating gate transistors to store electrical charge, non-volatile
  • electrons trapped in middle layer = 0, outside = 1
  • data is stored in blocks, and read as pages
  • data cannot be overwritten, blocks must be erased before rewriting data
• Adv of SSD:

  • No moving parts, purely electronic
  • lighter
  • faster access speed
  • lower power consumption
• principles of operation of RFID:

  reader sends radio frequency wave to antenna of RFID tag. This energises the RFID tag, enabling it to send the data signal. The reader receives the signal.