PCI Bus Mastering
As discussed in the section on system bus functions and features, bus mastering is the capability of devices on the PCI bus (other than the system chipset, of course) to take control of the bus and perform transfers directly. The PCI bus is the first bus to popularize bus mastering; probably in part because for the first time there are operating systems and software that are really capable of taking advantage of it.
PCI supports full device bus mastering, and provides bus arbitration facilities through the system chipset. PCI's design allows bus mastering of multiple devices on the bus simultaneously, with the arbitration circuitry working to ensure that no device on the bus (including the processor!) locks out any other device. At the same time though, it allows any given device to use the full bus throughput if no other device needs to transfer anything. In a way, the PCI bus acts like a tiny "local area network" inside the computer, in which multiple devices can each talk to each other, sharing a communication channel that is managed by the chipset.
This is taken from this site:
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