AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 73

• Manufacturer Part Number: AM79C976
• Description: PCnet-PRO 10/100 Mbps PCI Ethernet Controller
• Manufacturer: Advanced Micro Devices

Since any valid Ethernet Type field value will always be
greater than a normal IEEE 802.3 Length field (Š46),
the Am79C976 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
802.3 Length field is not compliant with either standard
and may cause problems if pad stripping is enabled.
Reception and checking of the received FCS is per-
formed automatically by the Am79C976 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 the Receive Descriptor.
Exception conditions for frame reception fall into two
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 exception events are caused by collisions,
which can distor t and truncate received frames.
Frames shorter than 64 bytes will, by default, be dis-
carded. These fragments will be discarded 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 received message.
8/01/00
Increasing Time
1010....1010
Preamble
Start of Frame
at Time = 0
Bits
56
10101011
SFD
Bits
8
Destination
Address
Bytes
6
P R E L I M I N A R Y
Bit
0
Address
Am79C976
Source
Bytes
Significant
6
Most
Byte
There are two control bits that can be used to cause the
MAC to override normal behavior and accept all frames
that pass address match, regardless of the frame
length. Setting the Runt Packet Accept (RPA) bit
(CMD2, bit 19) causes the MAC to accept runt packets
when the device is operating in either half- or full-du-
plex mode. Setting Full-Duplex Runt Packet Accept
(FDRPA, CMD2, bit 20) causes the MAC to accept runt
packets when the device is operating in full-duplex
mode. (When the value of RPA is 1, runt packets are
accepted regardless of the duplex mode or the value of
FDRPA.) In either case, there is a minimum frame size
of 16 bytes. Frames shorter than this may not be ac-
cepted, regardless of the value of RPA or FDRPA.
Abnormal network conditions include:
These error conditions are reported in the correspond-
ing receive descriptors. The RcvFCSErrors, RcvAlign-
m e n t E r r o r s , o r R c v M i s s P k t s c o u n t e r i s a l s o
incremented when one of these events occurs.
Statistics Counters
In order to provide network management information
with minimum host CPU overhead, the Am79C976 de-
vice automatically maintains a set of 32-bit controller
statistics counters. These counters are mapped di-
Bit
Length
7
Bytes
2
Bit
0
Significant
1 – 1500
Bytes
Least
Byte
Data
LLC
46 – 1500
Bytes
Bit
7
45 – 0
Bytes
Pad
Bytes
FCS
4
73