Source : Free On-Line Dictionary of Computing

bus master
     
         The device in a computer which is driving the
        {address bus} and bus control signals at some point in time.
        In a simple architecture only the (single) {CPU} can be bus
        master but this means that all communications between
        ("slave") I/O devices must involve the CPU.  More
        sophisticated architectures allow other capable devices (or
        multiple CPUs) to take turns at controling the bus.  This
        allows, for example, a {network controller} card to access a
        {disk controller} directly while the CPU performs other tasks
        which do not require the bus, e.g. fetching code from its
        {cache}.
     
        Note that any device can drive data onto the {data bus} when
        the CPU reads from that device, but only the bus master drives
        the {address bus} and control signals.
     
        {Direct Memory Access} is a simple form of bus mastering where
        the I/O device is set up by the CPU to read from or write to
        one or more contiguous blocks of memory and then signal to the
        CPU when it has done so.  Full bus mastering (or "First Party
        DMA", "bus mastering DMA") implies that the I/O device is
        capable of performing more complex sequences of operations
        without CPU intervention (e.g. servicing a complete {NFS}
        request).  This will normally mean that the I/O device
        contains its own processor or {microcontroller}.
     
        See also {distributed kernel}.
     
        (1996-08-26)