CH4

• Von Neumann Model

  • Central Processing Unit (CPU/Processor)
  • Processor able to access the memory directly
  • Computer memories that could store programs as well as data
  • Stored programs made up of instructions that could be executed in sequential order

  
• Components of CPU

  • Arithmetic & Logic Unit (ALU)
  • Control Unit (CU)
  • System Clock
  • Immediate Access Store (IAS)
  • Registers

  
• Arithmetic & Logic Unit (ALU)

  • Performs mathematical and logical operations required while running a program
  • Has an "Accumulator" - a temporary register used when carrying out ALU calculations
• Control Unit (CU)


  • Reads an instruction from memory and interprets it
  • During this process, signals are generated along the control bus to tell the other components of the computer on what to do
  • Ensures synchronization of data flow and program instructions throughout the computer

  
• System Clock

  • Produce timing signals on the control bus to ensure the synchronization take place
  • Without it, computer would crash
• Immediate Access Store (IAS)

  • Holds all the data and programs that the CPU needs to access
  • CPU takes data and programs held in backing store and puts them in IAS temporarily
  • This is done because read/write using IAS is faster compared to the backing store
• Registers

  • General Purpose Registers
  • Special Purpose Registers
• General Purpose Registers

  • Holds data that is frequently used by the CPU
  • Example: Accumulator
• Special Purpose Registers

  • CIR
  • IX
  • MAR
  • MDR
  • PC
  • SR
• Accumulator (AC)

  Holds processed data temporarily from ALU
• Current Instruction Register (CIR)

  Stores the current instruction being decoded and executed

  
• Index Register (IX/IR)

  Used when carrying out index addressing operations (assembly code)
• Memory Address Register (MAR)


  Stores the address of the memory location currently being read from or written to
• Memory Data Register / Memory Buffer Register (MDR/MBR)

  Stores data which has just been read from memory or data which is about to be written to memory
• Program Counter (PC)

  Stores the address where the next instruction to be read can be found

  
• Status Register (SR)

  Contains bits (flags) which can be set or cleared depending on the operations
• Status Register Flags

  • Carry Flag (C)
  • Negative Flag (N)
  • Overflow Flag (V)
  • Zero Flag (Z)
• Carry Flag (C)

  Set to 1 if there is CARRY in the following an addition operation
• Negative Flag (N)

  Set to 1 if the result of a calculation has a negative value
• Overflow Flag (V)

  Set to 1 if an arithmetic operation results in an overflow
• Zero Flag (Z)

  Set to 1 if the result of an arithmetic operation is ZERO
• System Buses

  • Address Bus
  • Data Bus
  • Control Bus

  
• Address Bus

  • Carries data throughout the computer system (CPU and memory)
  • Unidirectional (travel in one direction only)
  • Prevents addresses being carried back to the CPU
• Data Bus

  • Bidirectional (allows data to be sent in both directions along the bus)
  • Data can be carried from CPU to memory and to/from input/output devices
  • Data can be an address, an instruction, or a numerical value

  
• Control Bus

  • Bidirectional
  • Carries signals from CPU to all other components
  • 8 bit wide and only carry control signals

  
• Factors that affect the performance of control system

  • Width of a bus
  • Larger word length
  • Clock speed
  • Number of cores
  • Cache Memory
• Width of a bus

  The wider it is, the more memory locations which can be directly addressed at any given time
• Larger word length

  Can improve computer's overall performance
• Clock speed

  • Increasing speed by accessing the Basic Input/Output System (BIOS), processing speed also increase
  • Can lead to unsynchronized operations (unable to complete on time)
  • Overheating the CPU = unreliable performance
• Number of cores

  • Dual core (CPU made up of 2 cores) has one channel and needs the CPU to communicate with both the cores, not really affect performance
  • Quad core (CPU made up of 4 cores) has 6 channels and need the CPU to communicate with all 4 cores, reducing a lot of performance
• Cache Memory

  • Using cache can improve performance
  • Cache have faster access time
  • Stores frequent instructions that is used by CPU
• Universal Serial Bus (USB)

  • 4-wire shielded cable
  • 2 wires for power and the earth
  • 2 wires for data transmission
  • Computers automatically detects that devices is present
  • Appropriate device driver is loaded up so that comp and device can communicate effectively
  • If new device is detected, the comp will look for the device driver which matches thee device
• Advantages of USB

  • Automatically detected and device driver are automatically loaded up
  • Connectors can only fit one way, prevents incorrect connections
  • USB is a standard adopted by many manufacturers = all comps will be equipped with USB ports
  • Fast data transfer = useful when transferring large files such a s video files
• Disadvantages of USB

  • Transmission rate is limited to less than 500 megabits per second
  • Cable length is max 5 meters
• High-Definition Multimedia Interface (HDMI)

  • Allow output from a computer to an HDMI-enabled device
  • The current standard for modern tv and monitors
  • Allows for a very fast data transfer rate
  • Improved security (help prevents piracy) since it uses high bandwidth digital copy protection (HDCP)
  • Support modern digital systems
• Disadvantages of HDMI

  • Not a very robust connection (easy to break connection by moving device)
  • Limited cable length to retain good signal
  • Only 5 cable standards
• Video Graphic Array (VGA)

  • Older version of HDMI
  • Simpler technology
  • Only 1 standard available
  • Easy to split the signal and connect a number of devices from one source
  • Connection is very secure
• Disadvantages of VGA

  • Old out-dated analogue tech
  • Easy to bend the pins when making connections
  • Cables must be a very high grade to ensure good undistorted signals
• Fetch Execute Cycle

  1. Fetch instruction from memory

  2. Decode instruction

  3. Execute instruction

  4. Store result
• Interrupts

  • A signal sent from a device or from software to the processor
  • This will cause the processor to temporarily stop what it is doing and service the interrupt