Motorola MPC823e Reference Manual page 788

Communication Processor Module
In group address recognition, the SCCx Ethernet controller determines whether or not the
group address is a broadcast address. If broadcast addresses are enabled, then the frame
is accepted, but if the group address is not a broadcast address, then you can perform
address recognition on multiple group addresses using the GADDR1–4 hash table. In
promiscuous mode, the SCCx Ethernet controller receives all incoming frames, regardless
of their address.
16.9.22.11 HASH TABLE ALGORITHM. For individual and group hash filtering, the SCCx
Ethernet controller maps any 48-bit address into one of 64 bins, which are represented by
64 bits stored in GADDR1–4 or IADDR1–4. When the SET GROUP ADDRESS command
is executed, the SCCx Ethernet controller maps the selected 48-bit address into one of the
64 bits by passing the 48-bit address through the on-chip 32-bit CRC generator and
selecting 6 bits of the CRC-encoded result to generate a number between 1 and 64. Bits 31–
30 of the CRC result select one of the four GADDRs or IADDRs and bits 29–26 of the CRC
result select the bit within the selected register.
When the SCCx Ethernet controller receives a frame, the same process is used. If the CRC
generator selects a bit that is set in the group/individual hash table, the frame is accepted.
Otherwise, it is rejected. The result is that if eight group addresses are stored in the hash
table and random group addresses are received, the hash table prevents roughly 56/64
(87.5%) of the group address frames from reaching memory. Those that reach memory
must be further filtered by the processor to determine if they truly contain one of the eight
preferred addresses.
Better performance is achieved by using the group and individual hash tables
simultaneously. For instance, if eight group and eight physical addresses are stored in their
respective hash tables, 87.5% of all frames are prevented from reaching memory. The
effectiveness of the hash table declines as the number of addresses increases. For
instance, with 128 addresses stored in a 64-bin hash table, the vast majority of the hash
table bits are set, thus preventing a small fraction of the frames from reaching memory.
Note: The hash tables cannot be used to reject frames that match a set of entered
addresses because unintended addresses are matched to the same bit in the
hash table.
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