For each of the memory reference streams given in the following, compare the cost of executing it on a bus-based machine that supports (a) the MESI protocol and (b) the Dragon protocol. Explain the observed performance differences in terms of the characteristics of the streams and the coherence protocols.

a. Stream 1: r1 w1 r1 w1 r2 w2 r2 w2 r3 w3 r3 w3 b. Stream 2: r1 r2 r3 w2 w2 w3 r1 r2 r3 w3 w1 c. Stream 3: r1 r2 r3 r3 w1 w1 w1 w1 w2 w3 All of the references in the streams are to the same location: r/w indicates read or write, and the digit refers to the processor issuing the reference. Assume that all caches are initially empty, and use the following cost model: read/write cache hit 1 cycle; misses requiring simple transaction on bus (BusUpgr, BusUpd) 60 cycles; and misses requiring whole cache block transfer 90 cycles. Assume all caches are write allocated.