56.6.3.8 Receive Packet Buffer

The receive packet buffer stores frames from the MAC receiver along with their status and statistics. Frames with errors are flushed from the packet buffer memory, while good frames are pushed onto the DMA interface.

The receiver packet buffer monitors the FIFO write interface from the MAC receiver and translates the FIFO pushes into packet buffer writes. At the end of the received frame the status and statistics are buffered so that the information can be used when the frame is read out. When programmed in full store and forward mode, if the frame has an error the frame data is immediately flushed from the packet buffer memory allowing subsequent frames to utilise the freed up space. The status and statistics for bad frames are still used to update the GMAC registers.

To accommodate the status and statistics associated with each frame, three words per packet are reserved at the end of the packet data. If the packet is bad and requires to be dropped, the status and statistics are the only information held on that packet.

The receiver packet buffer will also detect a full condition so that an overflow condition can be detected. If this occurs, subsequent packets are dropped and a receive overflow interrupt is raised.

For Full Store and Forward, the DMA only begins packet fetches once the status and statistics for a frame are available. If the frame has a bad status due to a frame error, the status and statistics are passed on to the GMAC registers. If the frame has a good status, the information is used to read the frame from the packet buffer memory and burst onto the system bus using the DMA buffer management protocol. Once the last frame data has been transferred to the packet buffer, the status and statistics are updated to the GMAC registers.

If Partial Store and Forward mode is active, the DMA begins fetching the packet data before the status is available. As soon as the status becomes available, the DMA fetches this information as soon as possible before continuing to fetch the remainder of the frame. Once the last frame data has been transferred to the packet buffer, the status and statistics are updated to the GMAC registers.