AM79C978 Advanced Micro Devices, AM79C978 Datasheet - Page 246

• Manufacturer Part Number: AM79C978
• Description: Single-Chip 1/10 Mbps PCI Home Networking Controller
• Manufacturer: Advanced Micro Devices

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 un-
n Example 2: Assume that instead of the expected
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
B-6
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
ENP
ENP
X
X
X
X
X
X
X
X
X
X
X
X
x
x
a
a
Am79C978
OWN
OWN
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
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