AM79C978VCSLASHW datasheet(71/261 Pages) AMD

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Am79C978

The point at which the BMU will start to transfer data

from the receive FIFO to buffer memory is controlled by

the RCVFW bits in CSR80. The default established

during H_RESET is 01b, which sets the watermark flag

at 64 bytes filled.

For test purposes, theAm79C978 controller can be pro-

grammed to accept runt packets by setting RPA in

CSR124.

Address Matching

TheAm79C978 controller supports three types of ad-

dress matching: unicast, multicast, and broadcast. The

normal address matching procedure can be modified

by programming three bits in CSR15, the mode register

(PROM, DRCVPA, and DRCVBC).

If the first bit received after the SFD (the least signifi-

cant bit of the first byte of the destination address field)

is 0, the frame is unicast, which indicates that the frame

is meant to be received by a single node. If the first bit

received is 1, the frame is multicast, which indicates

that the frame is meant to be received by a group of

nodes. If the destination address field contains all 1s,

the frame is broadcast, which is a special type of multi-

cast. Frames with the broadcast address in the desti-

nation address field are meant to be received by all

nodes on the local area network.

When a unicast frame arrives at theAm79C978 control-

ler, the Am79C978 controller will accept the frame if the

destination address field of the incoming frame exactly

matches the 6-byte station address stored in the Phys-

ical Address registers (PADR, CSR12 to CSR14). The

byte ordering is such that the first byte received from

the network (after the SFD) must match the least signif-

icant byte of CSR12 (PADR[7:0]), and the sixth byte re-

ceived must match the most significant byte of CSR14

(PADR[47:40]).

When DRCVPA (CSR15, bit 13) is set to 1,the

Am79C978 controller will not accept unicast frames.

If the incoming frame is multicast, the Am79C978 con-

troller performs a calculation on the contents of the

destination address field to determine whether or not to

accept the frame. This calculation is explained in the

Logical Address Filter (LADRF) bits description.

When all bits of the LADRF registers are 0, no multicast

frames are accepted, except for broadcast frames.

Although broadcast frames are classified as special

multicast frames, they are treated differently by the

Am79C978 controller hardware. Broadcast frames are

always accepted, except when DRCVBC (CSR15, bit

14) is set and there is no Logical Address match.

None of the address filtering described above applies

when the Am79C978 controller is operating in the pro-

miscuous mode. In the promiscuous mode, all properly

formed packets are received, regardless of the con-

tents of their destination address fields. The promiscu-

ous mode overrides the Disable Receive Broadcast bit

(DRCVBC bit l4 in the MODE register) and the Disable

Receive Physical Address bit (DRCVPA, CSR15, bit

13).

TheAm79C978 controller operates in promiscuous

mode when PROM (CSR15, bit 15) is set.

The receive descriptor entry RMD1 contains three bits

that indicate which method of address matching

caused the Am79C978 controller to accept the frame.

Note that these indicator bits are only available when

the Am79C978 controller is programmed to use 32-bit

structures for the descriptor entries (BCR20, bit 7-0,

SWSTYLE is set to 2 or 3).

Physical Address Match (PAM) (RMD1, bit 22) is set by

the Am79C978 controller when it accepts the received

frame due to a match of the frame’s destination ad-

dress with the content of the physical address register.

Logical Address Filter Match (LAFM) (RMD1, bit 21) is

set by the Am79C978 controller when it accepts the re-

ceived frame based on the value in the logical address

filter register.

Broadcast Address Match (BAM) (RMD1, bit 20) is set

by the Am79C978 controller when it accepts the re-

ceived frame because the frame’s destination address

is of the type 'Broadcast.’

If DRCVBC (CSR15, bit 14) is cleared to 0, only BAM,

but not LAFM will be set when a Broadcast frame is re-

ceived, even if the Logical Address Filter is pro-

grammed in such a way that a Broadcast frame would

pass the hash filter. If DRCVBC is set to 1 and the Log-

ical Address Filter is programmed in such a way that a

Broadcast frame would pass the hash filter, LAFM will

be set on the reception of a Broadcast frame.

When the Am79C978 controller operates in promiscu-

ous mode and none of the three match bits is set, it is

an indication that the Am79C978 controller has only

accepted the frame because it was in promiscuous

mode.

When the Am79C978 controller is not programmed to

be in promiscuous mode, then when none of the three

match bits is set, it is an indication that the Am79C978

controller only accepted the frame because it was not

rejected. See Table 10 for receive address matches.

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