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Software Developer’s Manual 317
Register Descriptions
Since write back of transmit descriptors is optional (under the control of RS bit in the descriptor),
not all processed descriptors are counted with respect to WTHRESH. Descriptors start accumu-
lating after a descriptor with RS (or RPS for the 82544GC/EI) is set. Furthermore, with transmit
descriptor bursting enabled, some descriptors are written back that did not have RS (or RPS for the
82544GC/EI) set in their respective descriptors.
LWTHRESH (not applicable to the
82544GC/EI) controls the number of pre-fetched transmit
descriptors at which a transmit descriptor-low interrupt (ICR.TXD_LOW) is reported. This can
enable software to operate more efficiently by maintaining a continuous addition of transmit work,
interrupting only when the hardware nears completion of all submitted work. LWTHRESH
specifies a multiple of eight descriptors. An interrupt is asserted when the number of descriptors
available transitions from (threshold level=8*LWTHRESH)+1 to (threshold
level=8*LWTHRESH). Setting this value to 0b causes this interrupt to be generated only when the
transmit descriptor cache becomes completely empty.
13.4.44 Transmit Absolute Interrupt Delay Value
1
TADV (0382Ch; RW)
Reserved 23:22 0b
Reserved
Reads as 0b. Should be written as 0b for future compatibility.
GRAN 24 0b
Granularity
Set the values of PTHRESH, HTHRESH and WTHRESH in units
of cache lines or descriptors (each descriptor is 16 bytes)
1b = Descriptor granularity.
0b = Cache line granularity.
LWTHRESH
1
31:25 0h
Transmit descriptor Low Threshold
Interrupt asserted when the number of descriptors pending
service in the transmit descriptor queue (processing distance
from the TDT) drops below this threshold.
1. Not applicable to the 82544GC/EI.
Field Bit(s)
Initial
Value
Description
1. Not applicable to the 82544GC/EI.
31 16 15 0
Reserved IDV
Field Bit(s)
Initial
Value
Description
IDV 15:0 0b
Interrupt Delay Value
Counts in units of 1.024 µs. (0b = disabled)
Reserved 31:16 0b Reads as 0b. Should be written to 0b for future compatibility.