82598EB Datasheet, PDF (246/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Transmit Functionality
The transmit data descriptor points to a block of packet data to be transmitted. The TCP/IP transmit
context descriptor does not point to packet data. It contains control/context information that is loaded
into on-chip registers that affect the processing of packets for transmission. The following sections
describe the descriptor formats.
3.5.3.3.1 Description
3.5.3.3.1.1 Legacy Transmit Descriptor Format
To select legacy mode operation, bit 29 (TDESC.DEXT) should be set to 0b. In this case, the descriptor
format is defined as listed in Table 3-62. Address and length must be supplied by software. Bits in the
command byte are optional, as are the CSO, and CSS fields.
Table 3-62. Transmit Descriptor (TDESC) Layout – Legacy Mode
63 48
47 40
39
36
35 32
31 24
23 16
15
0
0
Buffer Address [63:0]
8
VLAN
CSS
Rsvd
STA
CMD
CSO
Length
Table 3-63. Transmit Descriptor Write Back Format
63 48
47 40
39
36
35 32
31 24
23 16
15
0
0
Reserved
Reserved
8
VLAN
CSS
Rsvd
STA
CMD
CSO
Length
Length (16)
Length (TDESC.LENGTH) specifies the length in bytes to be fetched from the buffer address provided.
The maximum length associated with any single legacy descriptor is the supported jumbo frame size 16
kB.
Note: Descriptors with zero length (null descriptors) transfer no data. Null descriptors can appear
only between packets and must have their EOP bits set.
Checksum Offset and Start – CSO (8) and CSS (8)
A checksum offset (TDESC.CSO) field indicates where, relative to the start of the packet, to insert a TCP
checksum if this mode is enabled. A Checksum Start (TDESC.CSS) field indicates where to begin
computing the checksum. Both CSO and CSS are in units of bytes¹. These must both be in 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).
1. Even though these are in units of bytes, the checksum calculations of interest typically work on 16-
bit words. Hardware does not enforce even-byte alignment.
246