AM79C961AVIW AMD (ADVANCED MICRO DEVICES), AM79C961AVIW Datasheet - Page 197

• Manufacturer Part Number: AM79C961AVIW
• Manufacturer: AMD (ADVANCED MICRO DEVICES)

Specifications of AM79C961AVIW

• Operating Supply Voltage (typ): 5V
• Operating Supply Voltage (min): 4.75V
• Operating Supply Voltage (max): 5.25V
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 144
• Lead Free Status / Rohs Status: Not Compliant

mechanism that examines the amount of time spent in
tasks S5 and S7 as such: While the driver is polling for
each descriptor, it could count the number of poll oper-
ations performed and then adjust the number 1 buffer
size to a larger value, by adding “t” bytes to the buffer
count, if the number of poll operations was greater than
“x”. If fewer than “x” poll operations were needed for
each of S5 and S7, then the software should adjust the
buffer size to a smaller value by, subtracting “y” bytes
from the buffer count. Experiments with such a tuning
mechanism must be performed to determine the best
values for “X” and “y.”
Note whenever the size of buffer number 1 is adjusted,
buffer sizes for buffer number 2 and buffer 3 should
also be adjusted.
In some systems the typical mix of receive frames on a
network for a client application consists mostly of large
data frames, with very few small frames. In this case,
for maximum efficiency of buffer sizing, when a frame
arrives under a certain size limit, the driver should not
adjust the buffer sizes in response to the short frame.
An Alternative LAPP Flow - the TWO Interrupt
Method
An alternative to the above suggested flow is to use
two interrupts, one at the start of the Receive frame
and the other at the end of the receive frame, instead
of just looking for the SRP interrupt as was described
above. This alternative attempts to reduce the
amount of time that the software “wastes” while poll-
ing for descriptor own bits. This time would then be
available for other CPU tasks. It also minimizes the
amount of time the CPU needs for data copying. This
savings can be applied to other CPU tasks.
Am79C961A
The time from the end of frame arrival on the wire to
delivery of the frame to the application is labeled as
frame latency. For the one-interrupt method, frame
latency is minimized, while CPU utilization increases.
For the two-interrupt method, frame latency becomes
greater, while CPU utilization decreases.
Note that some of the CPU time that can be applied to
non-Ethernet tasks is used for task switching in the
CPU. One task switch is required to swap a non-Ether-
net task into the CPU (after S7A) and a second task
switch is needed to swap the Ethernet driver back in
again (at S8A). If the time needed to perform these
task switches exceeds the time saved by not polling
descriptors, then there is a net loss in performance with
this method. Therefore, the NEW WORD method im-
plemented should be carefully chosen.
Figure 3 shows the event flow for the two-interrupt
method.
Figure 4 shows the buffer sizing for the two-interrupt
method. Note that the second buffer size will be about
the same for each method.
There is another alternative which is a marriage of the
two previous methods. This third possibility would use
the buffer sizes set by the two-interrupt method, but
would use the polling method of determining frame
end. This will give good frame latency but at the price
of very high CPU utilization.
And still, there are even more compromise positions
that use various fixed buffer sizes and effectively, the
flow of the one-interrupt method. All of these compro-
mises will reduce the complexity of the one-interrupt
method by removing the heuristic buffer sizing code,
but they all become less efficient than heuristic code
would allow.
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