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Receive and Transmit Description
24 Software Developer’s Manual
3.2.3.3 Receive Descriptor Special Field
Hardware stores additional information in the receive descriptor for 802.1q packets. If the packet
type is 802.1q, determined when a packet type field matches the VLAN
1
Ethernet Register (VET)
and RCTL.VME = 1b, then the special field records the VLAN information and the four byte
VLAN information is stripped from the packet data storage. The Ethernet controller stores the Tag
Control Information (TCI) of the 802.1q tag in the Special field. Otherwise, the special field
contains 0000h.
Table 3-4. Special Descriptor Field Layout
802.1q Packets
All Other Packets
SEQ (bit 2)
Sequence Error
When set, indicates a received packet with a bad delimiter sequence (in TBI mode/
internal SerDes). In other 802.3 implementations, this would be classified as a
framing error.
A valid delimiter sequence consists of:
idle →start-of-frame (SOF) → data, →pad (optional) → end-of-frame (EOF) → fill
(optional) → idle.
SE (bit 1)
Symbol Error
When set, indicates a packet received with bad symbol. Applicable only in TBI mode/
internal SerDes.
CE (bit 0)
CRC Error or Alignment Error
CRC errors and alignment errors are both indicated via the CE bit. Software may
distinguish between these errors by monitoring the respective statistics registers.
a. Not applicable to the 82540EP/EM, 82541xx, or 82547GI/EI.
Receive
Descriptor Error
bits
Description
1. Not applicable to the 82541ER.
15 13 12 11 0
PRI CFI VLAN
15 8 7 0
00 00
Receive
Descriptor
Special Field
Description
VLAN
VLAN Identifier
12 bits that records the packet VLAN ID number
CFI
Canonical Form Indicator
1 bit that records the packet’s CFI VLAN field
PRI
User Priority
3 bits that records the packet’s user priority field.