Bus Arbitration or Mastering
  
  Bus is a passive device.

  When being used, single master (CPU, DMA controller, etc. ) initiates action.
    Master may interact with one or more client device.

  In early designs, CPU was the only master.

  Newer designs allow multiple masters to optimize system usage.
    CPU - one or more.

    DMA - direct memory access (between memory and hard drive)

    Floating point Unit. (customized Co-processor CPU)

    Video graphics chips/cards