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Functional Description
17.4.8 Ethernet Address Recognition
The FEC filters the received frames based on destination address (DA) type — individual
(unicast), group (multicast), or broadcast (all-ones group address). The difference between
an individual address and a group address is determined by the I/G bit in the destination
address field. A flowchart for address recognition on received frames is illustrated in the
figures below.
Address recognition is accomplished through the use of the receive block and microcode
running on the microcontroller. The flowchart shown in Figure 17-2 illustrates the address
recognition decisions made by the receive block, while Figure 17-3 illustrates the decisions
made by the microcontroller.
If the DA is a broadcast address and broadcast reject (RCR[BC_REJ]) is deasserted, then
the frame will be accepted unconditionally, as shown in Figure 17-2. Otherwise, if the DA
is not a broadcast address, then the microcontroller runs the address recognition subroutine,
as shown in Figure 17-3.
If the DA is a group (multicast) address and flow control is disabled, then the
microcontroller will perform a group hash table lookup using the 64-entry hash table
programmed in GAUR and GALR. If a hash match occurs, the receiver accepts the frame.
If flow control is enabled, the microcontroller will do an exact address match check
between the DA and the designated PAUSE DA (01:80:C2:00:00:01). If the receive block
determines that the received frame is a valid PAUSE frame, then the frame will be rejected.
Note the receiver will detect a PAUSE frame with the DA field set to either the designated
PAUSE DA or the unicast physical address.
If the DA is the individual (unicast) address, the microcontroller performs an individual
exact match comparison between the DA and 48-bit physical address that the user programs
in the PALR and PAUR registers. If an exact match occurs, the frame is accepted;
otherwise, the microcontroller does an individual hash table lookup using the 64-entry hash
table programmed in registers, IAUR and IALR. In the case of an individual hash match,
the frame is accepted. Again, the receiver will accept or reject the frame based on PAUSE
frame detection, shown in Figure 17-2.
If neither a hash match (group or individual), nor an exact match (group or individual)
occur, then if promiscuous mode is enabled (RCR[PROM] = 1), then the frame will be
accepted and the MISS bit in the receive buffer descriptor is set; otherwise, the frame will
be rejected.
Similarly, if the DA is a broadcast address, broadcast reject (RCR[BC_REJ]) is asserted,
and promiscuous mode is enabled, then the frame will be accepted and the MISS bit in the
receive buffer descriptor is set; otherwise, the frame will be rejected.
In general, when a frame is rejected, it is flushed from the FIFO.
MOTOROLA
Chapter 17. Fast Ethernet Controller (FEC)
17-11
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