AM79C973BKC AMD (ADVANCED MICRO DEVICES), AM79C973BKC Datasheet - Page 289

• Manufacturer Part Number: AM79C973BKC
• Manufacturer: AMD (ADVANCED MICRO DEVICES)

Specifications of AM79C973BKC

• Lead Free Status / Rohs Status: Not Compliant

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-
n The controller will always obey the OWN bit for de-
n The controller will always mark the end of a frame
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 unused
n Example 2: Assume that instead of the expected
Note: The Am79C973 controller might write a ZERO
to ENP location in the third descriptor. Here are the two
possibilities:
1. If the controller finishes the data transfers into buffer
Descriptor
Descriptor
descriptor, unless the previous STP descriptor in the
ring is also OWNED by the software.
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.
termining whether or not it may use the next buffer
for a chain.
with either ENP = 1 or ERR = 1.
1060 byte frame, a 900 byte frame arrives, either
because there was an error in the network, or be-
number 2 after the driver writes the application
Number
Number
a. & b. ENP or ERR.
a.
etc.
etc.
& b. ENP or ERR.
1
2
3
4
5
6
1
2
3
4
5
6
OWN
OWN
1
1
0
1
1
0
1
1
1
0
1
1
0
1
Before the Frame Arrives
Before the Frame Arrives
STP
STP
1
0
0
1
0
0
1
1
0
0
1
0
0
1
P R E L I M I N A R Y
Am79C973/Am79C975
ENP
ENP
X
X
X
X
X
X
X
X
X
X
X
X
x
x
a
a
OWN
OWN
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
2. If the controller finishes the data transfers into buffer
0
0
0
1
1
0
1
0
0
0
1
1
0
1
correctly, and that the timing of the early interrupt
and the software is smooth. The descriptors will
have changed from:
cause this is the last frame in a file transmission se-
quence.
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.
number 2 before the driver writes the applications
After the Frame Arrives
After the Frame Arrives
STP
STP
1
0
0
1
0
0
1
1
0
0
1
0
0
1
ENP
ENP
?
X
X
X
X
X
X
X
X
0
0
1
0
0
*
b
b
Controller’s current
Controller’s current
Comments (After
Comments (After
Bytes 1001-1060
Discarded buffer
Bytes 801-1000
Bytes 801-1000
Frame Arrival)
Frame Arrival)
Not yet used
Not yet used
Net yet used
Not yet used
Not yet used
Net yet used
Bytes 1-800
Bytes 1-800
location
location
289