Intel PCI-X manual Transmit Descriptor Description Legacy, Cmd, Sta, Css

Receive and Transmit Description

Transmit Descriptor	Description
Legacy
CMD	Command field
	See Section 3.3.3.1 for a detailed field description.
STA	Status field
	See Section 3.3.3.2 for a detailed field description.
RSV	Reserved
	Should be written with 0b for future compatibility.
	Checksum Start Field
	The Checksum start field (TDESC.CSS) indicates where to begin computing
	the checksum. The software must compute this offset to back out the bytes
	that should not be included in the TCP checksum. CSS is provided in units
CSS	of bytes and must be in the range of data provided to the Ethernet controller
	in the descriptor (CSS < length). For short packets that ar padded by the
	software, CSS must be in the range of the unpadded data length. A value of
	0b corresponds to the first byte in the packet.
	CSS must be set in the first descriptor of the packet.
Special	Special Field
	See the notes that follow this table for a detailed field description.

Notes:

1.Even though CSO and CSS are in units of bytes, the checksum calculation typically works on 16-bit words. Hardware does not enforce even byte alignment.

2.Hardware does not add the 802.1Q EtherType or the VLAN field following the 802.1Q Ether- Type to the checksum. So for VLAN packets, software can compute the values to back out only on the encapsulated packet rather than on the added fields.

3.Although the Ethernet controller can be programmed to calculate and insert TCP checksum using the legacy descriptor format as described above, it is recommended that software use the newer TCP/IP Context Transmit Descriptor Format. This newer descriptor format allows the hardware to calculate both the IP and TCP checksums for outgoing packets. See Section 3.3.5 for more information about how the new descriptor format can be used to accomplish this task.

Software Developer’s Manual 37