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Receive and Transmit Description
Software Developer’s 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.
Table 3-21. Special Field (TDESC.SPECIAL) Layout
3.4 Transmit Descriptor Ring Structure
The transmit descriptor ring structure is shown in Figure 3-4. 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.
15 13 12 11 0
PRI CFI VLAN
TDESC.SPECIAL Description
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.