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Receive and Transmit Description
40 Software Developer’s Manual
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).
3.3.4 Transmit Descriptor Special Field Format
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.
Table 3-12. Special Field (TDESC.SPECIAL) Layout
TDESC.STATUS Description
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).
15 13 12 11 0
PRI CFI VLAN
TDESC.SPECIAL Description
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.