Intel Intel Gigabit Ethernet Controllers, PCI-X manual Legacy Transmit Descriptor Format

Receive and Transmit Description

Table 3-7. Transmit Descriptor (TDESC) Layout

0

8

63	30	29	28	24 23		20 19	0
	Buffer Address [63:0]
NR		DEXT	NR		DTYP		NR

3.3.3Legacy 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 shown in Table 3-8. The address and length must be supplied by software. Bits in the command byte are optional, as are the Checksum Offset (CSO), and Checksum Start (CSS) fields.

Table 3-8. Transmit Descriptor (TDESC) Layout – Legacy Mode

0

8

63	48 47		40 39		36 35		32 31		24 23	16 15	0
				Buffer Address [63:0]
Special		CSS		RSV		STA		CMD		CSO	Length

Table 3-9. Transmit Descriptor Legacy Descriptions

Transmit Descriptor	Description
Legacy
	Buffer Address
Buffer Address	Address of the transmit descriptor in the host memory. Descriptors with a
	null address transfer no data. If they have the RS bit in the command byte
	set (TDESC.CMD), then the DD field in the status word (TDESC.STATUS) is
	written when the hardware processes them.
	Length is per segment.
	The maximum length associated with any single legacy descriptor is 16288
	bytes. Although a buffer as short as one byte is allowed, the total length of
	the packet, before padding and CRC insertion must be at least 48 bytes.
	Length can be up to a default value of 16288 bytes per descriptor, and
Length	16288 bytes total. In other words, the length of the buffer pointed to by one
	descriptor, or the sum of the lengths of the buffers pointed to by the
	descriptors can be as large as the maximum allowed transmit packet.
	Descriptors with zero length transfer no data. If they have the RS bit in the
	command byte set (TDESC.CMD), then the DD field in the status word
	(TDESC.STATUS) is written when the hardware processes them.
	Checksum Offset
	The Checksum offset field indicates where, relative to the start of the packet,
	to insert a TCP checksum if this mode is enabled. (Insert Checksum bit (IC)
CSO	is set in TDESC.CMD). Hardware ignores CSO unless EOP is set in
	TDESC.CMD. CSO is provided in unit of bytes and must be in the range of
	the data provided to the Ethernet controller in the descriptor. (CSO < length -
	1).
	Should be written with 0b for future compatibility.

36 Software Developer’s Manual