FIFO是FPGA上的内存，通过DMA拷贝到CPU RAM.下面这一段是来自Virtex-6 Family Overview
The built-in FIFO controller for single-clock (synchronous) or dual-clock (asynchronous or multirate) operation increments
the internal addresses and provides four handshaking flags: full, empty, almost full, and almost empty. The almost full and
almost empty flags are freely programmable. Similar to the block RAM, the FIFO width and depth are programmable, but the
write and read ports always have identical width. First-word fall-through mode presents the first-written word on the data
output even before the first read operation. After the first word has been read, there is no difference between this mode and
the standard mode.