AM79C978VCSLASHW datasheet(246/261 Pages) AMD

246 / 261 page

B-6

Am79C978

descriptor, unless the previous STP descriptor in the

ring is also OWNED by the software.

When LAPPEN = 1, then hardware will use a modified

form of descriptor parsing as follows:

n The controller will examine OWN and STP to deter-

mine where to begin placing an RCV frame. A new

RCV frame will only begin in a buffer that has

OWN = 1 and STP =1.

n The controller will always obey the OWN bit for de-

termining whether or not it may use the next buffer

for a chain.

n The controller will always mark the end of a frame

with either ENP = 1 or ERR = 1.

The controller will discard all descriptors with OWN = 1

and STP = 0 and move to the next descriptor when

searching for a place to begin a new frame. It discards

these descriptors by simply changing the ownership bit

from OWN = 1 to OWN = 0. Such a descriptor is un-

used for receive purposes by the controller, and the

driver must recognize this. (The driver will recognize

this if it follows the software rules.)

The controller will ignore all descriptors with OWN = 0

and STP = 0 and move to the next descriptor when

searching for a place to begin a new frame. In other

words, the controller is allowed to skip entries in the

ring that it does not own, but only when it is looking for

a place to begin a new frame.

Some Examples of LAPP Descriptor

Interaction

Choose an expected frame size of 1060 bytes. Choose

buffer sizes of 800, 200, and 200 bytes.

n Example 1: Assume that a 1060 byte frame arrives

correctly, and that the timing of the early interrupt

and the software is smooth. The descriptors will

have changed from:

n Example 2: Assume that instead of the expected

1060 byte frame, a 900 byte frame arrives, either

because there was an error in the network, or be-

cause this is the last frame in a file transmission se-

quence.

Note: The controller might write a ZERO to ENP loca-

tion in the third descriptor. Here are the two possibili-

ties:

1. If the controller finishes the data transfers into buffer

number 2 after the driver writes the application

modified buffer pointer into the third descriptor, then

the controller will write a ZERO to ENP for this

buffer and will write a ZERO to OWN and STP.

2. If the controller finishes the data transfers into buffer

number 2 before the driver writes the applications

Descriptor

Number

Before the Frame Arrives

After the Frame Arrives

Comments (After

Frame Arrival)

OWN

STP

ENPa

OWN

STP

ENPb

Bytes 1-800

Bytes 801-1000

Bytes 1001-1060

Controller’s current

location

Not yet used

etc.

Net yet used

a. & b. ENP or ERR.