EP80579 Datasheet, PDF (1369/1916 Pages) Intel Corporation – Intel® EP80579 Integrated Processor Product Line

Intel^® EP80579 Integrated Processor
Figure 37-19.Transmit Status Layout (TDESC.STATUS)
DD: Descriptor Done
The SPECIAL field is used to provide the 802.1q/802.1ac tagging information. The
special field is ignored if the VLE bit is 0 or if the EOP bit is 0.
Figure 37-20.Transmit Special Field Layout (TDESC.SPECIAL)
15
13 12 11
0
PRI
CFI
VLAN
37.5.6.4 TCP/IP Context Transmit Descriptor Format
The TCP/IP context transmit descriptor provides access to the enhanced checksum off
load and TCP Segmentation facilities available in the GbE. These features allow TCP and
UDP packet types to be handled more efficiently by performing additional work in
hardware, thus reducing the software overhead associated with preparing these
packets for transmission.
The TCP/IP context transmit descriptor is called a “context” descriptor for a reason. A
context descriptor differs from a data descriptor as it does not point to packet data.
Instead, this descriptor provides access to two, GbE hardware contexts that support
the transmit checksum off loading and the segmentation features of the EP80579’s
GbE. A “context” refers to a set of registers loaded or unloaded as a group to provide a
particular function. Only one context is active at any given time.
37.5.6.4.1 Overview of GbE hardware “Contexts”
The previous section mentions access to two, separate GbE hardware contexts. There
are actually three such contexts corresponding to legacy mode, “normal” mode, and
segmentation mode. The first context (legacy) is an implied context as it is not
explicitly specified with a context descriptor. This context is constructed by the device
from the first and last descriptors of a legacy transmit and from some internal
constants. This context then mimics the behavior of the legacy (82542) device. This is
completely transparent to the user and is included here for clarification only.
The other two contexts are explicit and directly accessible via the TCP/IP context
transmit descriptor. One context is used to control the checksum off loading feature for
normal packet transmission. The second context is used to control the packet
segmentation capabilities of the device. The TSE bit selects which context will be
updated. A TCP/IP context transmit descriptor with TSE=0 will update the normal
(checksum off loading only) context. Conversely, the segmentation context is updated
when TSE=1. Refer to Figure 37-22 on page 1371 for details on all of the
TDESC.TUCMD fields, including TSE.
The device automatically selects the appropriate context to use based on the current
packet transmission: legacy, normal, or segmentation.
Note:
While the architecture supports arbitrary ordering rules for the various descriptors,
there are restrictions. The context descriptors should not occur in the middle of a
packet (or of a segmentation) and data descriptors of different packet types (legacy,
normal, or segmentation) should not be intermingled except at the packet (or
segmentation) level.
All three contexts control calculation and insertion of up to two checksums. This portion
of the context is referred to as the checksum context. In addition to a checksum
context, the segmentation context includes information specific to the segmentation
capability. This additional information includes the total payload for the message
(TDESC.PAYLEN), the total size of the header (TDESC.HDRLEN), the amount of payload
August 2009
Order Number: 320066-003US
Intel^® EP80579 Integrated Processor Product Line Datasheet
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