1.normal fetch ----fetch 1 cache line once
2. pre-fetch mode one ---- fetch 3 cache line once
3.pre-fetch mode two ----{way1: fetch 3 cache lines once , this way is the same as mode one
way2:fetch 2 cache lines once , when fshc in idle state and pre-buffer is empty,hardware would
start a new request automatic}
NOTE: 1 cache line = 4-word width data
pre-fetch data 存于16x32bit buffer(2个8x32bit buffers),每一次预取完会填充其中一个8x32bits的buffer,预取处理完后会产生一个pluse。如果两个8x32bit的buffer都没有数据，在更新时就会清除对应最早的没有使用的buffer的地址及数据。