82583V Datasheet, PDF (143/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

Inline Functions—82583V GbE Controller
7.2.9.1.2
Note:
7.2.9.1.3
Note:
Note:
Note:
Note:
7.2.9.1.4
Table 39.
Length
Length (TDESC.LENGTH) specifies the length in bytes to be fetched from the buffer
address. The maximum length associated with any single legacy descriptor is 16288
bytes.
The maximum allowable packet size for transmits might change based on the value
configured for the transmit FIFO size written to the Packet Buffer Allocation (PBA)
register. For any individual packet, the sum of the individual descriptors' lengths must
be at least 80 bytes less than the allocated size of the transmit FIFO.
Checksum Offset and Checksum Start - CSO and CSS
The checksum start (TDESC.CSS) field indicates where to begin computing the
checksum. CSS must be set in the first descriptor of a packet. The checksum offset
(TDESC.CSO) field indicates where to insert the TCP checksum, relative to the start of
the packet. Both CSO and CSS are in units of bytes while they must be within the range
of data provided to the device in the descriptor. This means, for short packets that are
padded by software, CSS and CSO must be in the range of the unpadded data length,
not the eventual padded length (64 bytes).
CSO must be set in the last descriptor of the packet. Only when EOP is set does the
hardware interpret Insert Checksum (IC), and CSO bits.
In the case of 802.1Q header, the offset values depend on the VLAN insertion enable bit
- CTRL.VME and the VLE bit. When the CTRL.VME and the VLE bit are not set (VLAN
tagging included in the packet buffers), the offset values should include the VLAN
tagging. When these bits are set (VLAN tagging is taken from the packet descriptor),
the offset values should exclude the VLAN tagging.
Although the 82583V can be programmed to calculate and insert TCP checksum using
the legacy descriptor format as previously described, it is recommended that software
use the newer context descriptor format. This newer descriptor format enables
hardware to calculate both the IP and TCP checksums for outgoing packets. See
section 7.2.6 for more information about how the new descriptor format can be used to
accomplish this task.
UDP checksum calculation is not supported by the legacy descriptor.
As the CSO field is eight bits wide, it limits the location of the checksum to 255 bytes
from the beginning of the packet.
Software must compute an offsetting entry and store it in the position where the
hardware computed checksum is to be inserted. This offset is needed to back out the
bytes of the header that should not be included in the TCP checksum.
Command Byte - CMD
The CMD byte stores the applicable command and has the fields shown in Table 39.
Command Byte Fields
7
6
IDE
VLE
5
DEXT
4
RSV
3
2
RS
IC
1
IFCS
0
EOP
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