System Bus

• The data carried to and processed by The Processor are transmitted by a system bus
• Three different types of communication
  • Data bus
    • The bidirectional data bus transports data around the system
  • Address bus
    • The address bus holds the area of memory or I/O to which data will be passed or retrieved, it is one way (unidirectional)
  • Control bus
    • The control bus carries commands from the CPU and returns status signals from the devices, for example, if data is being read/written to a device the appropriate line (read or write) will be active
    • Consists of 8 wires
      • Used to transport control signals between the components
      • Typical control lines include:
        • System clock - used to synchronise operations (e.g. timing).
        • Reset - initialises all components (hard reboot).
        • Memory write - causes data on the data bus to be written into the addressed location.
        • Memory read - causes data from the addressed location to be placed on the data bus.
        • I/O Write - causes data on the data bus to be output to the addressed I/O port.
        • I/O Read - causes data from the addressed I/O port to be placed on the data bus.
        • Transfer ACK - indicates that data have been accepted from or placed on the data bus.
        • Bus Request - indicates that component needs to gain control of bus.
        • Bus Grant - indicates that a requesting component has been granted control of the system bus
        • Interrupt request - indicates that an interrupt is pending.
        • Interrupt ACK - acknowledges that the pending interrupt has been recognised.