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Advanced Transmit Data Descriptor — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 143
5.6.8.4 DCMD
DCMD Layout
Field Description
TSE TCP Segmentation Enable
Indicates a TCP segmentation request. When TSE is set in the first descriptor of a TCP packet,
hardware uses the corresponding context descriptor in order to perform TCP segmentation.
Note: TCTL.PSP must be enabled when TSE is true since the last frame can be shorter than 60 bytes,
resulting in a bad frame if TCTL.PSP is disabled.
Note: If a TSO frame spans multiple descriptors, the TSE bit must be set only in the first data
descriptor.
VLE VLAN Packet Enable
Indicates that the packet is a VLAN packet (for example, hardware adds the VLAN Ether type and an
802.1q VLAN tag to the packet).
DEXT Descriptor Extension (1b for Advanced Mode)
Must be set to 1b to indicate advanced descriptor format (not legacy).
Reserved Reserved
RS Report Status
Signals hardware to report the status information. This is used by software that does in-memory
checks of the transmit descriptors to determine which ones are done. For example, if software queues
up to 10 packets to transmit, it can set the RS bit in the last descriptor of the last packet. If software
maintains a list of descriptors with the RS bit set, it can look at them to determine if all packets up to
(and including) the one with the RS bit set have been buffered in the output FIFO. Looking at the
status byte and checking the Descriptor Done (DD) bit do this. If DD is set, the descriptor has been
processed.
Note: Descriptors with a zero length transfer no data.
Reserved Reserved
DTYP Descriptor Type
Always set to 0010b for this type of descriptor.
IFCS Insert FCS
When set, hardware appends the MAC FCS at the end of the packet. When cleared, software should
calculate the FCS for proper CRC check. There are several cases in which software must set IFCS:
Transmission of short packet while padding is enabled by the TCTL.PSP bit.
Checksum offload is enabled by the either TXSM or IXSM bits in the TDESC.DCMD.
VLAN header insertion enabled by the VLE bit in the TDESC.DCMD.
TCP segmentation offload enabled by the TSE bit in the TDESC.DCMD.
EOP End of Packet
EOP indicates whether this is the last buffer of the packet.