Page "Bus mastering" Paragraph 1

Some types of buses allow only one device ( typically the CPU, or its proxy ) to initiate transactions.

Most modern bus architectures, such as PCI, allow multiple devices to bus master because it significantly improves performance for general purpose operating systems.

Some real-time operating systems prohibit peripherals from becoming bus masters, because the scheduler can no longer arbitrate for the bus and hence cannot provide deterministic latency.