AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 210

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

D-6
Descriptor
Descriptor
ENP or ERR
ENP or ERR
Number
Number
Assume that a 1060 byte frame arrives correctly,
and that the timing of the early interrupt and the
Assume that instead of the expected 1060 byte
frame, a 900 byte frame arrives, either because
AMD
etc.
etc.
The controller will discard all descriptors with
OWN = 1 and STP = 0 and move to the next de-
scriptor 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 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
1
2
3
4
5
6
1
2
3
4
5
6
OWN
OWN
Before the Frame Arrives
1
1
0
1
1
0
1
Before the Frame Arrives
1
1
0
1
1
0
1
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
Am79C970A
OWN
OWN
After the Frame has Arrived
After the Frame has Arrived
0
0
0
1
1
0
1
0
0
0
1
1
0
1
Some Examples of LAPP Descriptor
Interaction
Choose an expected frame size of 1060 bytes. Choose
buffer sizes of 800, 200 and 200 bytes.
software is smooth. The descriptors will have
changed from:
there was an error in the network, or because this
is the last frame in a file transmission sequence.
STP
STP
1
0
0
1
0
0
1
1
0
0
1
0
0
1
descriptor when searching for a place to begin a
new frame. In other words, the controller is al-
lowed 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.
ENP
ENP
?*
X
X
X
X
X
X
X
X
0
0
1
0
1
Comments
(After frame arrival)
Bytes 1–800
Bytes 801–1000
Bytes 1001–1060
Controller’s current location
Not yet used
Not yet used
Not yet used
Comments
(After frame arrival)
Bytes 1–800
Bytes 801–900
Discarded buffer
Controller’s current location
Not yet used
Not yet used
Not yet used