dp83820 National Semiconductor Corporation, dp83820 Datasheet - Page 61

• Manufacturer Part Number: dp83820
• Description: 10/100/1000 Mb/s Pci Ethernet Network Interface Controller
• Manufacturer: National Semiconductor Corporation

4.0 Register Set
4.2.22.3 Accept on Multicast or Unicast Hash
Multicast and Unicast addresses may be further qualified by use of the receive filter hash functions. An internal 2048 bit
(256 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 11 most significant bits of the
destination addresses’ CRC as an index into the Hash Table memory. The upper 7 bits represent the word address and
the lower 4 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 a bit in the hash table.
Given a CRC of F9E80000:
RFCR = (IO base + 48h)
RFDR = (IO base + 4Ch)
# Bits 31-25 select which 16-bit word
Word = 7C
#Lower 24-21 bits select which bit in 16-bit word
Bit =F
# Select bit to set/clear
hash_bit = (0001<<bit)
#Write word address into RFCR
iowrite I RFCR (100 + word)
# Read indexed word from table
ioread I RFDR
set hash_word =(result | hash_bit) #OR in the hash bit to set
#Write to the hash table
iowrite I RFDR (hash_word)
# Enable multicast and/or unicast hash
iowrite I RFCR (RFEN | MHEN | UHEN)
Figure 4-2 Hash Table Memory - 100h bytes addressed on word boundaries
(Continued)
Bit#
X X
X X
X X
X X
X X
17 16 15
byte 255
byte 253
byte5
byte3
byte1
61
byte 254
byte 252
byte4
byte2
byte0
1FE
1FC
104
102
100
0
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