我自己似乎找到答案了F4手册:“ The DMA fetches the next descriptor, sets the last
descriptor (RDES0[8]) bit, and releases the RDES0 status bits in the previous frame
descriptor. Then the DMA sets the receive interrupt bit (ETH_DMASR register [6]).”
就是说,RDES0[8]置位,此时产生一个接收中断,意味着DMA从MAC内核搬移了一个帧到描述符。。