AM79C978 Advanced Micro Devices, AM79C978 Datasheet - Page 72

• Manufacturer Part Number: AM79C978
• Description: Single-Chip 1/10 Mbps PCI Home Networking Controller
• Manufacturer: Advanced Micro Devices

Automatic Pad Stripping
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 36 shows the byte/bit ordering of the received
length field for an IEEE 802.3-compatible frame format.
Since any valid Ethernet Type field value will always be
greater than a normal IEEE 802.3 Length field ( 46),
the Am79C978 controller will not attempt to strip valid
Ethernet frames. Note that for some network protocols,
the value passed in the Ethernet Type and/or IEEE
72
PAM
0
1
0
0
0
Table 10. Receive Address Match
LAFM
0
0
1
1
0
BAM
0
0
0
0
1
DRCVBC
X
X
0
1
0
Frame accepted
due to PROM = 1
Physical address
match
Logical address
filter match;
frame is not of
type broadcast
Logical address
filter match;
frame can be of
type broadcast
Broadcast frame
Comment
Am79C978
802.3 Length field is not compliant with either standard
and may cause problems if pad stripping is enabled.
Receive FCS Checking
Reception and checking of the received FCS is per-
formed automatically by the Am79C978 controller.
Note that if the Automatic Pad Stripping feature is en-
abled, the FCS for padded frames will be verified
against the value computed for the incoming bit stream
including pad characters, but the FCS value for a pad-
ded frame will not be passed to the host. If an FCS
error is detected in any frame, the error will be reported
in the CRC bit in RMD1.
Receive Exception Conditions
Exception conditions for frame reception fall into two
distinct categories, i.e., those conditions which are the
result of normal network operation, and those which
occur due to abnormal network and/or host related
events.
Normal events which may occur and which are handled
autonomously by the Am79C978 controller are basi-
cally collisions within the slot time and automatic runt
packet rejection. The Am79C978 controller will ensure
that collisions that occur within 512 bit times from the
start of reception (excluding preamble) will be automat-
ically deleted from the receive FIFO with no host inter-
vention.
The receive FIFO will delete any frame that is com-
posed of fewer than 64 bytes provided that the Runt
Packet Accept (RPA bit in CSR124) feature has not
been enabled and the network interface is operating in
half-duplex mode, or the full-duplex Runt Packet Ac-
cept Disable bit (FDRPAD, BCR9, bit 2) is set. This cri-
terion will be met regardless of whether the receive
frame was the first (or only) frame in the FIFO or if the
receive frame was queued behind a previously re-
ceived message.
Abnormal network conditions include:
n FCS errors
n Late collision
Host related receive exception conditions include
MISS, BUFF, and OFLO. These are described in the
Buffer Management Unit section.