Suppose that a system uses dma for data transfer from disk controller to main memory. further assume that it takes t1 nsec on average to acquire the bus and t2 nsec to transfer one word over the bus (t1 > > t2). after the cpu has programmed the dma controller, how long will it take to transfer 1000 words from the disk controller to main memory, if (a) word-at-a-time mode is used, (b) burst mode is used? assume that commanding the disk controller requires acquiring the bus to send one word and acknowledging a transfer also requires acquiring the bus to send one word.