5.1软硬件通信的基本方式 直接存储器存取(DMA)方式 A typical DMA operation S1:Device wishing to perform DMA asserts the processors bus request signal. S2:Processor completes the current bus cycle and then asserts the bus grant signal to the device. S3:The device then asserts the bus grant ack signal. S4:The processor senses in the change in the state of bus grant ack signal and starts listening to the data and address bus for DMA activity.直接存储器存取（DMA）方式 A typical DMA operation S1: Device wishing to perform DMA asserts the processors bus request signal. S2: Processor completes the current bus cycle and then asserts the bus grant signal to the device. S3: The device then asserts the bus grant ack signal. S4: The processor senses in the change in the state of bus grant ack signal and starts listening to the data and address bus for DMA activity. 5.1 软硬件通信的基本方式