Altera Triple Speed Ethernet MegaCore Function - Manual (Page 44)

Unicast Address Checking
When promiscuous mode is disabled, the MAC function only accepts unicast frames if the destination
address matches any of the following addresses:
• The primary address, configured in the registers
mac_0
and
mac_1
• The supplementary addresses, configured in the following registers: smac_0_0/smac_0_1 ,
smac_1_0/smac_1_1
,
smac_2_0/smac_2_1
and
smac_3_0/smac_3_1
Otherwise, the MAC function discards the frame.
Multicast Address Resolution
You can use either a software program running on the host processor or a hardware multicast address
resolution engine to resolve multicast addresses. Address resolution using a software program can affect the
system performance, especially in gigabit mode.
The MAC function uses a 64-entry hash table in the register space, multicast hash table, to implement the
hardware multicast address resolution engine as shown in figure below. The host processor must build the
hash table according to the specified algorithm. A 6-bit code is generated from each multicast address by
XOR ing the address bits as shown in table below. This code represents the address of an entry in the hash
table. Write one to the most significant bit in the table entry. All multicast addresses that hash to the address
of this entry are valid and accepted.
You can choose to generate the 6-bit code from all 48 bits of the destination address by setting the MHASH_SEL
bit in the
command_config
register to 0, or from the lower 24 bits by setting the
MHASH_SEL
bit to 1. The
latter option is provided if you want to omit the manufacturer's code, which typically resides in the upper
24 bits of the destination address, when generating the 6-bit code.
Figure 4-4: Hardware Multicast Address Resolution Engine
Hash
Generate
multicast_match
frame destination
address (47:0)
write_port
din
read_addr(5:0)
dout
hash_addr(5:0) hash_w
ren
hash_d
ata
w
clk
Look-Up Table
(64x1 DPRAM)
Table 4-1: Hash Code Generation—Full Destination Address
Algorithm for generating the 6-bit code from the entire destination address.
Value Hash Code Bit
xor
multicast MAC address bits 7:0 0
xor multicast MAC address bits 15:8 1
xor
multicast MAC address bits 23:16 2
xor
multicast MAC address bits 31:24 3
xor multicast MAC address bits 39:32 4
Functional Description Altera Corporation
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Unicast Address Checking 4-8
2014.06.30