The filter block determines which frames are written to the DMA interface. Filtering is performed on received frames based on the state of the external matching pins, the contents of the specific address, type and hash registers, and the frame’s destination address, and the field type.
GEM is configured to have four specific address filters. Each filter is configured to contain a MAC address, which is specified to be compared against the Source Address (SA) or Destination Address (DA) of each received frame. There is also a mask field to allow certain bytes of the address that are not to be included in the comparison. If the filtering matches for a specific frame, then it is passed on to the DMA memory. Otherwise, the frame is dropped.
Frames may also be filtered using the Type ID field for matching. There are four types of ID registers in the internal register space, and these may be enabled individually. GEM supports the recognition of specific source or destination addresses. The number of a specific source or destination address filters are configurable and can range from 0 (zero) to 4.