Intel PCI-X manual Transmit Descriptor Head, TDH 03810h R/W, TDH Register Bit Description

			Register Descriptions
LEN	19:7	0b	Descriptor Length.
			Number of bytes allocated to the transmit descriptor circular
			buffer.
Reserved	31:20	0b	Reserved
			Reads as 0b. Should be written with 0b for future compatibility.

13.4.39Transmit Descriptor Head

TDH (03810h; R/W)

This register contains the head pointer for the transmit descriptor ring. It holds a value that is an offset from the base, and indicates the in–progress descriptor. It points to a 16-byte datum. Hardware controls this pointer. The only time that software should write to this register is after a reset (TCTL.RST or CTRL.RST) and before enabling the transmit function (TCTL.EN). If software were to write to this register while the transmit function was enabled, the on-chip descriptor buffers can be invalidated and indeterminate operation can result. Reading the transmit descriptor head to determine which buffers have been used (and can be returned to the memory pool) is not reliable.

Table 13-82. TDH Register Bit Description

31 16 15 0

Reserved

TDH

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