am79c961 Advanced Micro Devices, am79c961 Datasheet - Page 162

• Manufacturer Part Number: am79c961
• Description: Pcnettm-isa Jumperless Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

Introduction of the
Look Ahead Packet Processing (LAPP) Concept
A driver for the PCnet-ISA
require that the CPU copy receive frame data from the
controller’s buffer space to the application’s buffer
space after the entire frame has been received by the
controller. For applications that use a ping-pong win-
dowing style, the traffic on the network will be halted
until the current frame has been completely processed
by the entire application stack. This means that the time
between last byte of a receive frame arriving at the
client’s Ethernet controller and the client’s transmission
of the first byte of the next outgoing frame will be sepa-
rated by:
1)
2)
3)
4)
5)
6)
The sum of these times can often be about the same as
the time taken to actually transmit the frames on the
wire, thereby yielding a network utilization rate of less
than 50%.
An important thing to note is that the PCnet-ISA
ler’s data transfers to its buffer space are such that the
system bus is needed by the PCnet-ISA
approximately 4% of the time. This leaves 96% of the
sytem bus bandwidth for the CPU to perform some of
the inter–frame operations in advance of the completion
of network receive activity , if possible. The question
then becomes: how much of the tasks that need to be
performed between reception of a frame and
transmission of the next frame can be performed before
1-636
the time that it takes the client’s CPU’s interrupt
procedure to pass software control from the current
task to the driver
plus the time that it takes the client driver to pass
the header data to the application and request an
application buffer
plus the time that it takes the application to gener-
ate the buffer pointer and then return the buffer
pointer to the driver
plus the time that it takes the client driver to transfer
all of the frame data from the controller’s buffer
space into the application’s buffer space and then
call the application again to process the complete
frame
plus the time that it takes the application to process
the frame and generate the next outgoing frame
plus the time that it takes the client driver to set up
the descriptor for the controller and then write a
TDMD bit to CSR0
APPENDIX E
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controller would normally
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controller for
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control-
Am79C961
the reception of the frame actually ends at the network,
and how can the CPU be instructed to perform these
tasks during the network reception time?
The answer depends upon exactly what is happening in
the driver and application code, but the steps that can be
performed at the same time as the receive data
are arriving include as much as the first three steps and
part of the fourth step shown in the sequence
above. By performing these steps before the entire
frame has arrived, the frame throughput can be sub-
stantially increased.
A good increase in performance can be expected when
the first three steps are performed before the end of the
network receive operation. A much more significant per-
formance increase could be realized if the PCnet-ISA
controller could place the frame data directly into the
application’s buffer space; (i.e. eliminate the need for
step four.) In order to make this work, it is necessary
that the application buffer pointer be determined before
the frame has completely arrived, then the buffer pointer
in the next desriptor for the receive frame would need to
be modified in order to direct the PCnet-ISA
to write directly to the application buffer. More details on
this operation will be given later.
An alternative modification to the existing system can
gain a smaller, but still significant improvement in per-
formance. This alternative leaves step four unchanged
in that the CPU is still required to perform the copy
operation, but it allows a large portion of the copy opera-
tion to be done before the frame has been completely
received by the controller, (i.e. the CPU can perform the
copy operation of the receive data from the PCnet-ISA
controller’s buffer space into the application buffer
space before the frame data has completely arrived
from the network.) This allows the copy operation of
step four to be performed concurrently with the arrival of
network data, rather than sequentially, following the end
of network receive activity.
Outline of the LAPP Flow:
This section gives a suggested outline for a driver that
utilizes the LAPP feature of the PCnet-ISA
Note: The labels in the following text are used as refer-
ences in the timeline diagram that follows.
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controller.
controller
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