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Ethernet front-end (EFE)
152
NS7520 Hardware Reference, Rev. D 03/2006
The MAC forwards good Ethernet packets through address filtering. Packets can be
filtered based on station address, broadcast, and select multicast packets. Those
packets that pass through address filtering are moved into system memory.
Receive buffer descriptor selection
When an Ethernet frame is received, the Ethernet receiver DMA channel searches the
four buffer descriptors — starting with A and continuing with B, C, and D, as
necessary — for an appropriately sized buffer. The search stops when the DMA
channel finds an available buffer that is large enough to hold the entire Ethernet
receive frame. The search also stops when the DMA channel finds a Control register
whose Channel Enable (CE) bit is zero.
Figure 22 shows receiver buffer size selection.
Figure 22: Receive buffer size selection
2048-byte
receive
FIFO
RX DMA channel
DMA
buffer
descriptor
list
"B"
DMA
buffer
descriptor
list
"C"
DMA
buffer
descriptor
list
"D"
largest
DMA
buffer
descriptor
list
"A"
smallest