am79c961 Advanced Micro Devices, am79c961 Datasheet - Page 72

• Manufacturer Part Number: am79c961
• Description: Pcnettm-isa Jumperless Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

1-546
All receive frames can be accepted by setting the PROM
bit in CSR15. When PROM is set, the PCnet-ISA
troller will attempt to receive all messages, subject to
minimum frame enforcement. Promiscuous mode over-
rides the effect of the Disable Receive Broadcast bit on
receiving broadcast frames.
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 dur-
ing reset is 10b, which sets the threshold flag at 64 bytes
empty.
Automatic Pad Stripping
During reception of an 802.3 frame the pad field can be
stripped automatically. ASTRP_RCV (bit 10 in CSR4) =
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.
Receive FCS Checking
Reception and checking of the received FCS is per-
formed automatically by the PCnet-ISA
that if the Automatic Pad Stripping feature is enabled,
the received FCS will be verified against the value com-
puted for the incoming bit stream including pad
characters, but it will not be passed to the host. If a FCS
AMD
1010....1010
Preamble
Bits
56
Increasing Time
Start of Packet
at Time= 0
IEEE/ANSI 802.3 Frame and Length Field Transmission Order
10101011
SYNCH
Bits
8
+
controller. Note
ADDR.
Bytes
Dest.
6
P R E L I M I N A R Y
+
con-
Am79C961
Bit
ADDR.
0
Bytes
Srce.
Significant
6
Most
Byte
The number of bytes to be stripped is calculated from
the embedded length field (as defined in the IEEE 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.
Since any valid Ethernet Type field value will always be
greater than a normal 802.3 Length field ( 46), the
PCnet-ISA
Ethernet frames.
Note that for some network protocols the value passed
in the Ethernet Type and/or 802.3 Length field is not
compliant with either standard and may cause
problems.
The diagram below shows the byte/bit ordering of the re-
ceived length field for an 802.3 compatible frame format.
error is detected, this will be reported by the CRC bit in
RMD1.
Receive Exception Conditions
Exception conditions for frame reception fall into two
distinct categories; those which are the result of normal
Bit
Length
Bytes
7
2
Bit
+
0
controller will not attempt to strip valid
Significant
Least
Byte
1–1500
Bytes
DATA
LLC
Bit
7
46–1500
Bytes
18183B-20
16235C-9
Bytes
45–0
Pad
Bytes
FCS
4