请高手多多指教

Prefetchable 是什么意思，Being prefetchable means there are no side effects on reads, the device returns all bytes on reads regardless of the byte enables, and host bridges can merge processor writes into this range without causing errors.这句话是什么意思，哪位大侠能给解释一下大概意思？
DDS是数字频率合成器，谁能给介绍一下它的原理？或者给个资料看看。

多谢了！！

 :D