AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 291

• Manufacturer Part Number: AM79C976
• Description: PCnet-PRO 10/100 Mbps PCI Ethernet Controller
• Manufacturer: Advanced Micro Devices

When LAPPEN = 1, then hardware will use a modified
form of descriptor parsing as follows:
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
Note: The Am79C976 controller might write a ZERO
to ENP location in the third descriptor. Here are the two
possibilities:
8/01/00
Descriptor
Descriptor
descriptor, unless the previous STP descriptor in the
ring is also OWNED by the software.
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.
The controller will always obey the OWN bit for de-
termining whether or not it may use the next buffer
for a chain.
The controller will always mark the end of a frame
with either ENP = 1 or ERR = 1.
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-
Number
Number
D & b. ENP or ERR
D
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
ENP
ENP
X
X
X
X
X
X
X
X
X
X
X
X
X
X
a
a
Am79C976
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.
1. If the controller finishes the data transfers into buffer
0
0
0
1
1
0
1
0
0
0
1
1
0
1
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:
cause this is the last frame in a file transmission se-
quence
number 2 after the driver writes the application
modified buffer pointer into the third descriptor, then
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 E
ENP E
?
X
X
X
X
X
X
X
X
0
0
1
0
0
*
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
A-6