AM79C975VCW datasheet(284/304 Pages) AMD

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APPENDIX E

Am79C973/Am79C975

284

Look-Ahead Packet Processing

(LAPP) Concept

APPENDIX E: LOOK-AHEAD PACKET PROCESSING (LAPP) CONCEPT

Introduction

A driver for the Am79C973 controller would normally

require that the CPU copy receive frame data from the

controllers buffer space to the applications buffer space

after the entire frame has been received by the control-

ler. For applications that use a ping-pong windowing

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 be-

tween 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 separated

by:

1. The time that it takes the client’s CPU interrupt pro-

cedure to pass software control from the current

task to the driver,

2. Plus the time that it takes the client driver to pass

the header data to the application and request an

application buffer,

3. Plus the time that it takes the application to gener-

ate the buffer pointer and then return the buffer

pointer to the driver,

4. 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,

5. Plus the time that it takes the application to process

the frame and generate the next outgoing frame,

and

6. 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.

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 percent.

An important thing to note is that the Am79C973 con-

troller’s data transfers to its buffer space are such that

the system bus is needed by the Am79C973 controller

for approximately 4 percent of the time. This leaves 96

percent of the system bus bandwidth for the CPU to

perform some of the interframe 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 per-

formed before 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 substantially

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

perfo r m ance increa se could be re alized if th e

Am79C973 controller could place the frame data di-

rectly into the application’s buffer space; (i.e., eliminate

the need for step 4.) In order to make this work, it is

necessary that the application buffer pointer be deter-

mined before the frame has completely arrived, then

the buffer pointer in the next descriptor for the receive

frame would need to be modified in order to direct the

Am79C973 controller 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 4 unchanged in

that the CPU is still required to perform the copy oper-

ation, but is allows a large portion of the copy operation

to be done before the frame has been completely re-

ceived by the controller, i.e., the CPU can perform the

copy operation of the receive data from the Am79C973

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 4 to be performed concurrently with the arrival of

network data, rather than sequentially, following the

end of network receive activity.

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