Intel PCI User Manual

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Note: The DD bit reflects status of all descriptors up to and including the one with the RS bit set (or RPS 

for the 82544GC/EI).

The SPECIAL field is used to provide the 802.1q/802.1ac tagging information. 

When CTRL.VME is set to 1b, all packets transmitted from the Ethernet controller that have VLE 

set in the TDESC.CMD are sent with an 802.1Q header added to the packet. The contents of the 

header come from the transmit descriptor special field and from the VLAN type register. The 
special field is ignored if the VLE bit in the transmit descriptor command field is 0b. The special 

field is valid only for descriptors with EOP set to 1b in TDESC.CMD.

TU
RSV (bit 3)

Transmit Underrun
Indicates a transmit underrun event occurred. Transmit Underrun might occur if Early 
Transmits are enabled (based on ETT.Txthreshold value) and the 82544GC/EI was 
not able to complete the early transmission of the packet due to lack of data in the 
packet buffer. This does not necessarily mean the packet failed to be eventually 
transmitted. The packet is successfully re-transmitted if the TCTL.NRTU bit is 
cleared (and excessive collisions do not occur).
This bit is reserved and should be programmed to 0b for all Ethernet controllers 
except the 82544GC/EI.

LC (bit 2)

Late Collision
Indicates that late collision occurred while working in half-duplex mode. It has no 
meaning while working in full-duplex mode. Note that the collision window is speed 
dependent: 64 bytes for 10/100 Mb/s and 512 bytes for 1000 Mb/s operation.

EC (bit 1)

Excess Collisions
Indicates that the packet has experienced more than the maximum excessive 
collisions as defined by TCTL.CT control field and was not transmitted. It has no 
meaning while working in full-duplex mode.

DD (bit 0)

Descriptor Done
Indicates that the descriptor is finished and is written back either after the descriptor 
has been processed (with RS set) or for the 82544GC/EI, after the packet has been 
transmitted on the wire (with RPS set).

PRI

CFI

VLAN

PRI 

User Priority
3 bits that provide the VLAN user priority field to be inserted in the 802.1Q tag.

CFI

Canonical Form Indicator.

VLAN

VLAN Identifier
12 bits that provide the VLAN identifier field to be inserted in the 802.1Q tag.