Intel PCI-X manual Transmit Descriptor Special Field Format, Special Field TDESC.SPECIAL Layout

Receive and Transmit Description

TDESC.STATUS	Description
	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
TU	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
RSV (bit 3)
	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.
	Late Collision
LC (bit 2)	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.
	Excess Collisions
EC (bit 1)	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.
	Descriptor Done
DD (bit 0)	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).

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).

3.3.4Transmit 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

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.

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