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Computing (FOLDOC) dictionary
bus master
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architecture 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
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.