am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 116

• Manufacturer Part Number: am79c940
• Description: Media Access Controller For Ethernet Mace
• Manufacturer: Advanced Micro Devices

Logical Address Filtering
For Ethernet
The purpose of logical (or group or multicast) addresses
is to allow a group of nodes in a network to receive the
same message. Each node can maintain a list of multi-
cast addresses that it will respond to. The logical ad-
dress filter mechanism in AMD Ethernet controllers 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 destination address
should be accepted.
The logical address filter hardware is an implementation
of a hash code searching technique commonly used by
software programmers. If the multicast bit in the destina-
tion address of an incoming packet is set, the hardware
maps this address into one of 64 categories then ac-
cepts or rejects the packet depending on whether or not
the bit in the logical address filter register corresponding
the selected category is set. For example, 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
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 fi-
nal decision whether to accept or discard the packet.
However, the hardware prevents the software from hav-
ing to deal with the vast majority of the unacceptable
packets.
The efficiency of this scheme depends on the number of
multicast groups that are used on a particular network
and the number of groups to which a node belongs. 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 filtering must be
done by software. At the other extreme (which is closer
to a practical network), if multicast addresses are as-
signed by the local administrator, and fewer than 65
groups are set up, the addresses can be assigned so
that each address maps into a different category, and no
software filtering will be needed at all.
APPENDIX A
14
possible multicast ad-
Am79C940
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 par-
ticular 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 receive 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 significant bit appearing
first).
Driver software that manages a list of multicast ad-
dresses can work as follows. First the multicast address
list and the logical address filter must be initialized.
Some sort of management function such as the driver
initialization routine passes to the driver a list of ad-
dresses. 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 soft-
ware copy of the logical address filter register. When the
complete list of addresses has been processed, the reg-
ister 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 ad-
dress. 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.
A-1