Computers as Components, 2nd Edition

162 CHAPTER 4 Bus-Based Computer Systems

A device that can initiate its own bus transfer is known as a bus master. Devices

that do not have the capability to be bus masters do not need to connect to a bus

request and bus grant. The DMA controller uses these two signals to gain control

of the bus using a classic four-cycle handshake. The bus request is asserted by the

DMA controller when it wants to control the bus, and the bus grant is asserted by

the CPU when the bus is ready.

The CPU will finish all pending bus transactions before granting control of the

bus to the DMA controller. When it does grant control, it stops driving the other

bus signals: