AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 211

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

Note that the PCnet-PCI II 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
increasing 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
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 II
controller will not have had the opportunity to
RESET the ENP bit in this descriptor, and it is pos-
sible that the software left this bit as ENP=1 from
the last time through the ring. Therefore, the soft-
ware must treat the location as a don’t care; The
rule is, after finding ENP=1 (or ERR=1) in descrip-
tor number 2, the software must ignore ENP bits
until it finds the next STP=1.
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
Am79C970A
OWN
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 II controller has completed its poll of the next
descriptors. This means that for almost all occurrences
of this case, the PCnet-PCI II 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 II controller will not have had an opportunity
to modify it.
** Note that even though the PCnet-PCI II 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 II 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
AMD
D-7