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162 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
82550 and 82551QM Specific Information
Note: To use Large Send, the 82550 should be configured to use dynamic Transmit Buffer Descriptors
(TBDs). The driver should ensure transmit buffers associated with the Large Send IPCB contain
enough bytes (headers size and total TCP payload). An end of list (EL) bit should be placed in the
last TBD.
B.3.3 IPCB Large Send
The mode bits in the IP Activation field, control the Large Send operation of the transmit
command. When the Large Send bit is set to 1, the 82550 is forced into Large Send mode for the
processing of this IPCB. The data bytes attached are subject to segmentation by the 82550 and
multiple frames of MTU size (except for the last frame less than or equal to the MTU size) will be
transmitted.
B.3.3.1 Parameters
•
Maximum TCP Payload. This parameter specifies the maximum number of bytes that can be
transmitted in the TCP payload portion of an Ethernet frame that is part of this Large Send. All
frames in this Large Send, except the last one, will have exactly the maximum TCP payload
bytes of TCP payload. The last frame TCP payload size will be the remainder (in other words,
it will be less than or equal to the maximum TCP payload bytes).
Note: The driver must guarantee that the maximum TCP payload value will not cause the
complete frame (including VLAN) to be larger than the allowed MTU for that
connection based on the IP address pair used.
Note: The IEEE 802.3ac committee plans to extend the 10/100 Mbps Ethernet frame size
to account for VLAN fields. This should not impact the TCP/IP MTU.
• Total TCP Payload. This parameter defines the total number of TCP payload bytes (header not
included) requested for transmission as part of this Large Send. This parameter should not be
used as the sole indication for Large Send activation. For example, 1500 bytes can be a Large
Send on an MTU of 576 bytes.
Table 74. IPCB Structure Large Send
Odd Word (D31:D16) Even Word (D15:D0) Offset
E
L
S I CID (5 bits) 000
N
C
CMD = 1001 C X
O
K
U XXXX XXXX XXXX (12 bits)
0h
Link Address (32 bits)
4h
TBD Array Address (32 bits)
8h
Maximum TCP Payload
E
O
F
0 IPCB Byte Count (14 bits)
Ch
IP Activation (12 bits) Scheduling (20 bits)
10h
TCP Header Offset (8 bits) IP Header Offset (8 bits) VLAN (16 bits)
14h
Transmit Buffer #0 Address (32 bits)
18h
Reserved
Total TCP Payload (16 bits)
E
L
0 Transmit Buffer #0 Size (14 bits)
1Ch