AM79C940VCW datasheet(116/122 Pages) AMD

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A-1

Am79C940

APPENDIX A

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 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 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.

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.

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