AM79C940JC/W AMD [Advanced Micro Devices], AM79C940JC/W Datasheet - Page 121

• Manufacturer Part Number: AM79C940JC/W
• Manufacturer: AMD [Advanced Micro Devices]

Logical Address Filtering
For Ethernet
The purpose of logical (or group or multicast)
ad dresses is to allow a group of nodes in a network to
receive the same message. Each node can maintain a
list of multicast addresses that it will respond to. The
logical address filter mechanism in AMD Ethernet con-
trollers is a hardware aide that reduces the average
amount of host computer time required to determine
whether or not an incoming packet with a multicast
des tination address should be accepted.
The logical address filter hardware is an implementa-
tion of a hash code searching technique commonly
used by software programmers. If the multicast bit in
the destination address of an incoming packet is set,
the hardware maps this address into one of 64 catego-
ries then accepts or rejects the packet depending on
whether or not the bit in the logical address filter regis-
ter corresponding the selected category is set. For
ex ample, if the address maps into category 24, and bit
24 of the logical address filter register is set, the packet
is accepted.
Since there are more than 10
ad dresses and only 64 categories, this scheme is far
from unambiguous. This means that the software will
still have to compare the address of a received packet
with its list of acceptable multicast addresses to make
the final decision whether to accept or discard the
packet. However, the hardware prevents the software
from having to deal with the vast majority of the
unac ceptable packets.
The efficiency of this scheme depends on the number
of multicast groups that are used on a particular net-
work and the number of groups to which a node
be longs. At one extreme if a node happens to belong
to 64 groups that map into 64 different categories, the
hardware will accept all multicast addresses, and all fil-
tering must be done by software. At the other extreme
(which is closer to a practical network), if multicast
ad dresses are assigned by the local administrator, and
fewer than 65 groups are set up, the addresses can be
APPENDIX A
14
possible multicast
Am79C940
assigned so that each address maps into a different
category, and no software filtering will be needed at all.
In the latter case described above, a node can be made
a member of several groups by setting the appropriate
bits in the logical address filter register. The administra-
tor can use the table Mapping of Logical Address to
Fil ter Mask to find a multicast address that maps into a
particular address filter bit. For example address 0000
0000 00BB maps into bit 15. Therefore, any node that
has bit 15 set in its logical address filter register will
re ceive all packets addressed to 0000 0000 00BB.
(Addresses in this table are not shown in the standard
Ethernet format. In the table the rightmost byte is the
first byte to appear on the network with the least
signif icant bit appearing first).
Driver software that manages a list of multicast
ad dresses can work as follows. First the multicast
ad dress list and the logical address filter must be
ini tialized. Some sort of management function such as
the driver initialization routine passes to the driver a list
of addresses. For each address in the list the driver
uses a subroutine similar to the one listed in the
Am7990 LANCE data sheet to set the appropriate bit in
a software copy of the logical address filter register.
When the complete list of addresses has been
pro cessed, the register is loaded.
Later, when a packet is received, the driver first looks
at the Individual/Group bit of the destination address of
the packet to find out whether or not this is a multicast
address. If it is, the driver must search the multicast
ad dress list to see if this address is in the list. If it is not
in the list, the packet is discarded.
The broadcast address, which consists of all ones is a
special multicast address. Packets addressed to the
broadcast address must be received by all nodes.
Since broadcast packets are usually more common
than other multicast packets, the broadcast address
should be the first address in the multicast address list.
121