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10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual 157
82550 and 82551QM Specific Information
Note: Using software parsing is only allowed with legal TCP/IP or UDP/IP packets. When software
parsing is used, IP and TCP offsets in the IPCB must point to the appropriate headers and the total
TCP/UDP payload should be specified. For all other datagrams, hardware parsing must be used.
Note: VLAN tagging (either in memory or by hardware) and SNAP headers are allowed when software
parsing is used as long as the above guideline is met. IP and TCP header offsets are calculated
according to memory data structures. For example, if VLAN tag is inserted by hardware, offsets do
not change.
The IPCB byte count specifies the number of bytes of transmit data in the immediate data area
(from offset 20h and on).
B.1.1 Maximum TCP Payload / TBD Number and Transmit
Threshold
When the Large Send mode is selected, this field is interpreted as the Maximum TCP Payload.
Otherwise, this 16-bit field is interpreted as the TBD number and Transmit Threshold. The TBD
number specifies the total number of TBDs associated with this IPCB. The Transmit Threshold is
defined in Table 72 above.
Note: A mode bit puts the 82550 and 82551 into a specific mode. When the mode bit is clear, all
parameters that relate to that mode are ignored by the device hardware. Mode bits should obey
consistency rules since not all combinations are allowed. The result of a non-supported
combination is unpredictable.
Note: It is driver responsibility to guarantee that the total length of a frame is not larger than the allowed
MTU size for that connection (which is dependent upon the IP address pair). This is relevant in all
Scheduling
12h:
10h
19:0 Parameter
If the Scheduled Send bit is set, the transmission of the frame
associated with the IPCB is delayed until the internal
scheduling counter of the controller reaches the scheduling
value.
Bits 16:0 are the time stamp and bits 19:17 may be used to
specify time window.
Maximum TCP
Payload
Fh:Eh 15:0 Parameter
This field specifies the maximum number of bytes to be
transmitted in the TCP payload of an Ethernet frame that is
part of the Large Send. All frames use this size except the last
one. This field is ignored when the Large Send bit is clear.
Transmit
Threshold
Eh 7:0 Parameter
In units of 8 bytes, this field defines the number of bytes that
should be present in the Transmit FIFO before transmission
starts. It is relevant only if the checksum requires the whole
frame to be in Transmit FIFO. The value of this parameter
should be in the range of 1h to E0h.
Table 72. IPCB Fields