am79c970 Advanced Micro Devices, am79c970 Datasheet - Page 161

• Manufacturer Part Number: am79c970
• Description: Pcnettm-pci Single-chip Ethernet Controller For Pci Local Bus
• Manufacturer: Advanced Micro Devices

Note that the PCnet-PCI controller might write a ZERO
to ENP location in the 3rd descriptor. Here are the two
possibilities:
1. If the controller finishes the data transfers into
2. If the controller finishes the data transfers into
Buffer Size Tuning
For maximum performance, buffer sizes should be ad-
justed depending upon the expected frame size and the
values of the interrupt latency and application call la-
tency. The best driver code will minimize the CPU utili-
zation while also minimizing the latency from frame end
on the network to frame sent to application from driver
(frame latency). These objectives are aimed at increas-
ing throughput on the network while decreasing CPU
utilization.
Note that the buffer sizes in the ring may be altered at
any time that the CPU has ownership of the correspond-
ing descriptor. The best choice for buffer sizes will maxi-
mize the time that the driver is swapped out, while
minimizing the time from the last byte written by the
1-1028
Descriptor
ENP or ERR
Number
buffer number 2 after the driver writes the applica-
tions modified buffer pointer into the third descrip-
tor, then the controller will write a ZERO to ENP for
this buffer and will write a ZERO to OWN and STP.
buffer number 2 before the driver writes the appli-
cations modified buffer pointer into the third de-
scriptor, then the controller will complete the frame
in buffer number two and then skip the then un-
owned third buffer. In this case, the PCnet-PCI
controller will not have had the opportunity to RE-
SET the ENP bit in this descriptor, and it is possi-
ble that the software left this bit as ENP=1 from the
last time through the ring. Therefore, the software
must treat the location as a don’t care; The rule is,
after finding ENP=1 (or ERR=1) in descriptor num-
ber 2, the software must ignore ENP bits until it
finds the next STP=1.
AMD
etc.
1
2
3
4
5
6
OWN
Before the Frame Arrives
1
1
0
1
1
0
1
STP
1
0
0
1
0
0
1
ENP
X
X
X
X
X
X
X
OWN
Am79C970
After the Frame has Arrived
0
0
0
1
1
0
1
* Same as note in case 2 above, except that in this case,
it is very unlikely that the driver can respond to the inter-
rupt and get the pointer from the application before the
PCnet-PCI controller has completed its poll of the next
descriptors. This means that for almost all occurrences
of this case, the PCnet-PCI controller will not find the
OWN bit set for this descriptor and therefore, the ENP
bit will almost always contain the old value, since the
PCnet-PCI controller will not have had an opportunity to
modify it.
** Note that even though the PCnet-PCI controller will
write a ZERO to this ENP location, the software should
treat the location as a don’t care, since after finding the
ENP=1 in descriptor number 2, the software should ig-
nore ENP bits until it finds the next STP=1.
PCnet-PCI controller to the time that the data is passed
from the driver to the application. In the diagram, this
corresponds to maximizing S0, while minimizing the
time between C9 and S8. (The timeline happens to
show a minimal time from C9 to S8.)
Note that by increasing the size of buffer number 1, we
increase the value of S0. However, when we increase
the size of buffer number 1, we also increase the value
of S4. If the size of buffer number 1 is too large, then the
driver will not have enough time to perform tasks S2, S3,
S4, S5 and S6. The result is that there will be delay from
the execution of task C9 until the execution of task S8. A
perfectly timed system will have the values for S5 and
S7 at a minimum.
Assume that instead of the expected 1060 byte
frame, a 100 byte frame arrives, because there
was an error in the network, or because this is the
last frame in a file transmission sequence, or per-
haps because it is an acknowledge frame.
STP
1
0
0
1
0
0
1
ENP
0**
?*
X
X
X
X
1
Comments
(after frame arrival)
bytes 1–100
discarded buffer
discarded buffer
controller’s current location
not yet used
not yet used
not yet used