Motorola ColdFire MCF5281 - Manual (Page 348)

Fast Ethernet Controller (FEC)
17-38 Freescale Semiconductor
Figure 17-28. Ethernet Address Recognition—Microcode Decisions
17.5.10 Hash Algorithm
The hash table algorithm used in the group and individual hash filtering operates as follows. The 48-bit
destination address is mapped into one of 64 bits, represented by 64 bits stored in GAUR, GALR (group
address hash match), or IAUR, IALR (individual address hash match). This mapping is performed by
passing the 48-bit address through the on-chip 32-bit CRC generator and selecting the six most significant
bits of the CRC-encoded result to generate a number between 0 and 63. The msb of the CRC result selects
GAUR (msb = 1) or GALR (msb = 0). The five least significant bits of the hash result select the bit within
the selected register. If the CRC generator selects a bit set in the hash table, the frame is accepted;
otherwise, it is rejected.
For example, 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 do reach memory must be further filtered by the processor to determine if they truly contain one of the
eight desired addresses.
The effectiveness of the hash table declines as the number of addresses increases.
Receive Address
I/G Address
?
Exact Match
?
Hash Search
Group Table
Match
?
Hash Search
Individual Table
False
Match
?
False False
True
True
True
Individual Group
True False
True
False
?
Pause Address
FCE
?
Recognition
Reject Frame
Flush from FIFO
Reject Frame
Flush from FIFO
Receive Frame
Receive Frame
Receive Frame
Receive Frame
Notes:
FCE - field in RCR register (flow control enable)
I/G - Individual/Group bit in destination address (lsb in first byte received in MAC frame)
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3