am79c970 Advanced Micro Devices, am79c970 Datasheet - Page 160

• Manufacturer Part Number: am79c970
• Description: Pcnettm-pci Single-chip Ethernet Controller For Pci Local Bus
• Manufacturer: Advanced Micro Devices

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 software is
smooth. The descriptors will have changed from:
Assume that instead of the expected 1060 byte frame, a 900 byte frame arrives, either because there was an
error in the network, or because this is the last frame in a file transmission sequence.
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 pur-
poses by the controller, and the driver must recog-
nize 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
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
OWN
OWN
Am79C970
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.
STP
STP
1
0
0
1
0
0
1
1
0
0
1
0
0
1
when searching for a place to begin a new frame.
In other words, the controller is allowed to skip en-
tries 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
AMD
1-1027