AM79C976 datasheet(72/309 Pages) AMD

72 / 309 page

Am79C976

8/01/00

P R E L I M INARY

Although broadcast frames are classified as special

multicast frames, they are treated differently by the

Am79C976 controller hardware. Broadcast frames are

always accepted, except when DRCVBC (CSR15, bit

14) is set. DRCVBC overrides a logical address match.

If DRCVBC is set to 1, broadcast frames are not ac-

cepted even if the Logical Address Filter is pro-

grammed in such a way that a Broadcast frame would

pass the hash filter.

None of the address filtering described above applies

when the Am79C976 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 Am79C976 in the MODE register) and

the Disable Receive Physical Address bit (DRCVPA,

CSR15, bit 13).

The Am79C976 controller operates in promiscuous

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

In addition, the Am79C976 controller provides the Ex-

ternal Address Detection Interface (EADI) to allow ex-

ternal address filtering. See the External Address

Detection Interface section for further details.

The receive descriptor entry RMD1 contains three bits

that indicate which method of address matching

caused the Am79C976 controller to accept the frame.

Note that these indicator bits are not available when the

Am79C976 controller is programmed to use 16-bit

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

SWSTYLE is set to 0).

PAM (RMD1, bit 22) is set by the Am79C976 controller

when it accepts the received frame due to a match of

the frame’s destination address with the content of the

physical address register.

LAFM (RMD1, bit 21) is set by the Am79C976 control-

ler when it accepts the received frame based on the

value in the logical address filter register.

BAM (RMD1, bit 20) is set by the Am79C976 controller

when it accepts the received frame because the

frame’s destination address is of the type ’Broadcast’.

Only BAM, but not LAFM, will be set when a Broadcast

frame is received, even if the Logical Address Filter is

programmed in such a way that a Broadcast frame

would pass the hash filter.

When the Am79C976 controller operates in promiscu-

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

an indication that the Am79C976 controller only ac-

cepted the frame because it was in promiscuous mode.

When the Am79C976 controller is not programmed to

be in promiscuous mode, but the EADI interface is used

and when none of the three match bits is set, it is an in-

dication that the Am79C976 controller only accepted

the frame because it was not rejected by driving the

EAR pin LOW during the receive protect time. The

length of receive protect period can be programmed in

the Receive Protect Register.

See Table 6 for receive address matches.

Table 6. Receive Address Match

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During reception of an IEEE 802.3 frame, the pad field

can be stripped automatically. Setting ASTRP_RCV

(CSR4, bit 0) to 1 enables the automatic pad stripping

feature. The pad field will be stripped before the frame

is passed to the FIFO, thus preserving FIFO space for

additional frames. The FCS field will also be stripped,

since it is computed at the transmitting station based on

the data and pad field characters, and will be invalid for

a receive frame that has had the pad characters

stripped.

The number of bytes to be stripped is calculated from

the embedded length field (as defined in the ISO 8802-

3 (IEEE/ANSI 802.3) definition) contained in the frame.

The length indicates the actual number of LLC data

bytes contained in the message. Any received frame

which contains a length field less than 46 bytes will have

the pad field stripped (if ASTRP_RCV is set). Receive

frames which have a length field of 46 bytes or greater

will be passed to the host unmodified.

Figure 31 shows the byte/bit ordering of the received

length field for an IEEE 802.3-compatible frame format.

PAM

LAFM

BAM

Comment

Frame accepted due to

PROM = 1 or no EADI

reject

Physical address match

Logical address filter

match;

frame is not of type

broadcast

Broadcast frame

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