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320 Software Developer’s Manual
Register Descriptions
Table 13-86. TXDCTL Register Bit Description
31 25 24 23 22 21 16 15 14 13 8 7 6 5 0
LWTHRESH
RSV
1
1. 82544GC/EI only.
GRAN RSV WTHRESH RSV HTHRESH RSV PTHRESH
Field Bit(s)
Initial
Value
Description
PTHRESH 5:0 0b
Prefetch Threshold
Used to control when a pre-fetch of descriptors is considered.
This threshold refers to the number of valid, unprocessed
transmit descriptors the Ethernet controller has in its on-chip
buffer. If this number drops below PTHRESH, the algorithm
considers prefetching descriptors from host memory. This fetch
does not happen unless there are at least TXDCTL.HTHRESH
valid descriptors in host memory to fetch. Value of PTHRESH
can be in either cache line units, or based on number of
descriptors based on TXDCTL.GRAN.
Reserved 7:6 0b
Reserved
Reads as 0b. Should be written as 0b for future compatibility.
HTHRESH 15:8 0b
Host Threshold
Provides the threshold of the valid descriptors in host memory A
descriptor prefetch is performed each time enough valid
descriptors (TXDCTL.HTHRESH) are available in host memory,
no other DMA activity of greater priority is pending (descriptor
fetches and write backs or packet data transfers) and the number
of transmit descriptors the Ethernet controller has on its on-chip
buffers drops below TXDCTL.PTHRESH. The value of
HTHRESH can be in either cache line units, or based on number
of descriptors based on TXDCTL.GRAN.
Reserved 15:14 0b
Reserved
Reads as 0b. Should be written as 0b for future compatibility.
WTHRESH 21:16 0b
Write Back Threshold
WTHRESH controls the write back of processed transmit
descriptors. This threshold refers to the number of transmit
descriptors in the Ethernet controller’s on-chip buffer which are
ready to be written back to host memory. In the absence of
external events (explicit flushes), the write back occurs only after
more than WTHRESH descriptors are available for write back.
WTHRESH must contain a non-zero value to take advantage of
the write back bursting capabilities of the Ethernet controller. A
value of 0b causes the descriptors to be written back as soon as
they are processed.
The value of WTHRESH can be in either cache line units, or
based on number of descriptors based on RXDCTL.GRAN.