Intel PCI User Manual

51

When CTRL.VME is set to 1b, all packets transmitted from the Ethernet controller that has VLE 
set in the DCMD field is 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 when EOP is set.

The transmit descriptor ring structure is shown in 

. A pair of hardware registers 

maintains the transmit queue. New descriptors are added to the ring by writing descriptors into the 

circular buffer memory region and moving the ring’s tail pointer. The tail pointer points one entry 

beyond the last hardware owned descriptor (but at a point still within the descriptor ring). 
Transmission continues up to the descriptor where head equals tail at which point the queue is 

empty.

Descriptors passed to hardware should not be manipulated by software until the head pointer has 

advanced past them.

PRI

CFI

VLAN

PRI 

User Priority
Three 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.