AM79C940KCSLASHW datasheet(121/144 Pages) AMD

121 / 144 page

APPENDIX A

Am79C940

121

Logical Address Filtering

For Ethernet

The purpose of logical (or group or m ulticast)

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 1014 possible multicast

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

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.

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