Frame Recognition
MSC8113 Reference Manual, Rev. 0
Freescale Semiconductor
25-27
25.9.3 Hash Table Algorithm
Using the hash table process for individual and group hash filtering, the Ethernet controller maps
any 48-bit destination address into one of 256 bins, represented by the 256 bits in the Group
Address Registers (GADDR[0–7]) or Individual Address Registers (IADDR[0–7]). The eight
high-order bits of a CRC checksum are used to index into the hash table. The high-order three
bits of this 8-bit field are used to select one of the eight registers in either the individual or group
hash table. The low-order five bits select a bit within the 32-bit register. A value of 0 in the
high-order three bits selects IADDR0/GADDR0.
The same process is used if the Ethernet controller receives a frame. If the CRC checksum selects
a bit that is set in the group/individual hash table, the frame is conditionally accepted pending the
pattern match result. If 32 group addresses are stored in the hash table and random group
addresses are received, the hash table prevents roughly 224/256 (87.5 percent) of the group
address frames from reaching memory. Software must further filter those that reach memory to
determine if they contain the correct addresses. In addition, if pattern matching is enabled, the
Ethernet controller can reduce the burden on the software and further accelerate the reception by
performing additional filtering.
Better performance is achieved when the group and individual hash tables are used in
combination. For instance, if 32 group and 32 physical addresses are stored in their respective
hash tables, because 87.5 percent of all group addresses and 87.5 percent of all individual address
are rejected, then 87.5 percent of all frames are prevented from reaching memory.
The effectiveness of the hash table declines as the number of addresses increases. For instance, as
the number of addresses stored in the 256-bin hash table increases, most of the hash table bits are
set, preventing only a small fraction of frames from reaching memory. Pattern matching can
become extremely valuable because it can reduce the extra memory bus usage due to unintended
hash table hits.
Note:
The hash tables cannot be used to reject frames that match a set of selected addresses
because unintended addresses can map to the same bit in the hash table. Pattern
matching can be used to reject frames with unintended address hits in the hash table.
Содержание MSC8113
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