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Transmit Descriptor Write Back Format — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 137
Because the CSO field is eight bits wide, it puts a limit the location of the checksum to 255 bytes from
the beginning of the packet.
EOP, when set, indicates the last descriptor making up the packet. One or many descriptors can be used
to form a packet. Hardware inserts a checksum at the offset indicated by the CSO field if the Insert
Checksum bit (IC) is set. Checksum calculations are for the entire packet starting at the byte indicated
by the CSS field. A value of 0 corresponds to the first byte in the packet. CSS must be set in the first
descriptor for a packet. In addition, IC is ignored if CSO or CSS are out of range. This occurs if (CSS
length) or (CSO
length - 1).
Note: CSO must be larger than CSS and CSS must be equal or bigger than 14 bytes, and CSO
must be smaller than the packet length minus 4 bytes.
5.6.5.3 Command Byte (CMD)
The CMD byte stores the applicable command and has fields shown in Table 37.
Software must compute an offsetting entry to back out the bytes of the header that are not part of the
IP pseudo header and should not be included in the TCP checksum and store it in the position where the
hardware computed checksum is inserted.