请教异步FIFO的问题？

yifan225

最近看异步FIFO的资料，有几个问题想请教大家，问题描述如下：
Simulation and Synthesis Techniques for Asynchronous FIFO Design文章2.2Asynchronous FIFO pointers有一段对读指针的描述，看了后，不是很懂！！问题如下：
1、下面粗体字描述的部分，是不是互相矛盾呢？前面的说读指针指向被读的当前FIFO字（the read pointer always points to the current FIFO word to be read）。后面的说指向要读的下一个FIFO字（The fact that the read pointer is always pointing to the next FIFO word to be read ）。是不是我理解错误了呢？
Similarly, the read pointer always points to the current FIFO word to be read. Again on reset, both pointers are set to zero, the FIFO is empty and the read pointer is pointing to invalid data (because the FIFO is empty and the empty flag is asserted). As soon as the first data word is written to the FIFO, the write pointer increments, the empty flag is cleared, and the read pointer that is still addressing the contents of the first FIFO memory word, immediately drives that first valid word onto the FIFO data output port, to be read by the receiver logic. The fact that the read pointer is always pointing to the next FIFO word to be read means that the receiver logic does not have to use two clock periods to read the data word. If the receiver first had to increment the read pointer before reading a FIFO data word,the receiver would clock once to output the data word from the FIFO, and clock a second time to capture the data word into the receiver. That would be needlessly inefficient.
2、就是在如下的图中，问题如我标注的一样？是不是我理解图形错误了呢？我理解的是读写指针都应该是指向下一个要读或要写的地址单元。

老扁

所谓下一个也就是当前的。
对于fifo来说数据线上永远是呆会要读的数据。这样就可以保证高速传输。

yifan225

下面这两句话我理解不是很清楚：
1 the read pointer always points to the current FIFO word to be read和The fact that the read pointer is always pointing to the next FIFO word to be read   我理解的是指向当前的应该是当前时钟正在读的，而指向下一个是指向下一个时钟才读的数据。这样考虑的话，我认为就是矛盾的。我的理解按照你的来说，肯定不清楚，恳请详细解答一下可以么？
2 我认为不管是读指针，还是写指针，都应该是指向下一个时钟周期要读或要写的数据亚？不知道理解是否正确。

onioni

应该这样理解， 在使能信号到来之前，指针指向下一个将要被操作的数据，而当使能信号来了的时候，指针所指的数据就是当前被操作的数据，且在下一拍指针将指向下一个数据。