The tx_client_fifo is built
around a dual-port inferred RAM, giving a total memory capacity of 4,096 bytes.
When a full frame has been written into the transmit FIFO,
the FIFO presents data to the Ethernet MAC transmitter. The Ethernet MAC uses
tx_axis_mac_tready to throttle the data until it has control of the
medium.
If the FIFO memory fills up, the
tx_axis_fifo_tready signal halts the AXI4-Stream interface
from writing in data until space becomes available in the FIFO. If the FIFO memory fills up
but no full frames are available for transmission, such as when a frame larger than 4,000
bytes is written into the FIFO, the FIFO asserts the tx_overflow signal and
continues to accept the rest of the frame. The overflow frame is dropped by the FIFO ensuring
that the AXI4-Stream FIFO interface does not lock up.