DP83815 datasheet(63/108 Pages) NSC

63 / 108 page

4.0 Register Set (Continued)

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Accept on Multicast or Unicast Hash

Multicast and Unicast addresses may be further qualified

by use of the receive filter hash functions. An internal 512

bit (64 byte) RAM-based hash table is used to perform

imperfect filtering of multicast or unicast packets. By

enabling either Multicast Hashing or Unicast Hashing in the

RFCR, the receive filter logic will use the 9 least significant

bits of the destination addresses’ CRC as an index into the

Hash Table memory. The upper 4 bits represent the word

address and the lower 5 bits select the bit within the word.

If the corresponding bit is set, then the packet is accepted,

otherwise the packet is rejected. The hash table memory is

accessed through the RFCR and the RFDR. Refer to

Figure 4-2 for a memory map. Below is example code for

setting/clearing a bit in the hash table.

Figure 4-2 Hash Table Memory - 40h bytes addressed on word boundaries

set HASH_TABLE = 200

crc $DA

# compute the CRC of the destination address

set index = ($crc >> 3)

set bit = ($crc & 01f)

# lower 5 bits select which bit in 32 bit word

# write word address into RFCR

iow l $RFCR ($HASH_TABLE + $index)

# select bit to set/clear

if ($bit > f) set bit = ($bit - 010h)

# use 16 bit register interface into 32bit RAM

set hash_bit = (0001 << $bit)

# read indexed word from table

ior l $RFDR

if ($SetBit) then

set hash_word = ($rc | $hash_bit)

iow l $RFDR ($hash_word)

else

set hash_bit = (~$hash_bit)

set hash_word = ($rc & $hash_bit)

iow l $RFDR ($hash_word)‘

endif

iow l $RFCR ($RFEN|$MHEN|$UHEN)# enable multicast and/or unicast

# address hashing

byte63

byte62

23E

byte61

byte60

23C

byte5

byte4

204

byte3

byte2

202

byte1

byte0

200

Bit#

17 16 15

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