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82575EB Gigabit Ethernet Controller — Transmit Descriptor Write Back Format
Intel
®
82575EB Gigabit Ethernet Controller 324632-003
Software Developer’s Manual and EEPROM Guide Revision: 2.1
136 January 2011
Table 36. Transmit Descriptor (TDESC) Layout – Legacy Mode
5.6.5 Transmit Descriptor Write Back Format
5.6.5.1 Length
Length (TDESC.LENGTH) specifies the length in bytes to be fetched from the buffer address provided.
The maximum length associated with any single legacy descriptor is 9018 bytes.
Note: The maximum allowable packet size for transmits changes based on the value written to the
Packet Buffer Allocation register.
Descriptor length(s) can be limited by the size of the transmit FIFO. All buffers comprising a single
packet must be able to be stored simultaneously in the transmit FIFO. For any individual packet, the
sum of the individual descriptors' lengths must be below 9018 bytes.
Note: Descriptors with zero length (null descriptors) transfer no data. Null descriptors might
appear only between packets and must have their EOP bits set.
5.6.5.2 Checksum Offset and Start (CSO and CSS)
A checksum offset (TDESC.CSO) field indicates where, relative to the start of the packet, to insert a TCP
checksum if this mode is enabled. A checksum start (TDESC.CSS) field indicates where to begin
computing the checksum. Both CSO and CSS are in units of bytes. These must both be in the range of
data provided to the device in the descriptor. This means for short packets that are padded by software,
CSS and CSO must be in the range of the unpadded data length, not the eventual padded length (64
bytes).
With an 802.1Q header, the offset values depend on the VLAN insertion enable bit - CTRL.VME and the
VLE bit. If they are not set (VLAN tagging included in the packet buffers), the offset values should
include the VLAN tagging. If these bits are set (VLAN tagging is taken from the packet descriptor), the
offset values should exclude the VLAN tagging.
Hardware does not add the 802.1Q Ether Type 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.
Note: UDP checksum calculation is not supported by the legacy descriptor because the legacy
descriptor does not support the translation of a checksum result of 0000h to FFFFh needed
to differentiate between an UDP packet with a checksum of zero and an UDP packet without
checksum.
63 48 47 40 39 36 35 32 31 24 23 16 15 0
0 Buffer Address [63:0]
8 Special CSS ExtCMD STA CMD CSO Length
63 48 47 40 39 36 35 32 31 24 23 16 15 0
0 Buffer Address [63:0]
8 Special CSS Reserved STA CMD CSO Length