am79c961a Advanced Micro Devices, am79c961a Datasheet - Page 195

• Manufacturer Part Number: am79c961a
• Description: Pcnet?-isa Ii Jumperless, Full Duplex Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

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 dis-
cards these desciptors by simply changing the owner-
ship 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 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.
*ENP or ERR
2. Assume that instead of the expected 1060 byte
*ENP or ERR
** Note that the PCnet-ISA II controller might write a ZERO to ENP location in the 3rd descriptor. Here are the two possibilities:
1. If the controller finishes the data transfers into buffer number 2 after the driver writes the application’s modified buffer pointer
2. If the controller finishes the data transfers into buffer number 2 before the driver writes the application’s modified buffer pointer
Descriptor
Descriptor
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.
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.
into the third descriptor, then the controller will complete the frame in buffer number two and then skip the then unowned third
buffer. In this case, the PCnet-ISA II controller will not have had the opportunity to RESET the ENP bit in this descriptor, and
it is possible that the software left this bit as ENP=1 from the last time through the ring. Therefore, the software must treat
the location as a don’t care; The rule is, after finding ENP=1 (or ERR=1) in descriptor number 2, the software must ignore
ENP bits until it finds the next STP=1.
Number
Number
etc.
etc.
1
2
3
4
5
6
1
2
3
4
5
6
OWN
OWN
Before the Frame Arrived
Before the Frame Arrived
1
1
0
1
1
0
1
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
Am79C961A
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.
1. Assume that a 1060 byte frame arrives correctly,
and that the timing of the early interrupt and the soft-
ware is smooth. The descriptors will have changed
from:
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
1
Bytes 1–800
Bytes 801–1000
Bytes 1001–1060
Controller’s current location
Not yet used
Not yet used
Not yet used
Bytes 1–800
Bytes 801–900
Discarded buffer
Controller’s current location
Not yet used
Not yet used
Not yet used
(After Frame Arrival)
(After Frame Arrival)
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