In a first-word fall-through (FWFT) FIFO, the data is placed on the DOUT bus before RDEN is asserted. When writing the first word to an EMPTY FIFO, the first word is directly transferred to the output, and appears on the DOUT bus at the same time EMPTY is deasserted.
To prevent data loss in the system, this first output value does not disappear from the DOUT bus until RDEN is asserted. If no more data is available, EMPTY is asserted and the next word written to the FIFO again directly transfers to the output. Because the data appears on DOUT before the read operation, it can take one more last read operation for the FIFO to assert EMPTY (go empty). In either case, the same number of read operations as write operations are required to return the FIFO to an empty state. See the following figures.