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AM79C970 Datasheet(PDF) 160 Page - Advanced Micro Devices |
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AM79C970 Datasheet(HTML) 160 Page - Advanced Micro Devices |
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160 / 168 page  AMD 1-1027 Am79C970 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 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. Some Examples of LAPP Descriptor Interaction Choose an expected frame size of 1060 bytes. Choose buffer sizes of 800, 200 and 200 bytes. 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: OWN STP ENP OWN STP ENP ¦ (after frame arrival) 1 1 1 X 0 1 0 bytes 1–800 2 1 0 X 0 0 0 bytes 801–1000 3 0 0 X 0 0 1 bytes 1001–1060 4 1 1 X 1 1 X controller’s current location 5 1 0 X 1 0 X not yet used 6 0 0 X 0 0 X not yet used etc. 1 1 X 1 1 X not yet used Descriptor Number Before the Frame Arrives After the Frame has Arrived Comments ¦ ENP or ERR 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. OWN STP ENP OWN STP ENP ¦ (after frame arrival) 1 1 1 X 0 1 0 bytes 1–800 2 1 0 X 0 0 1 bytes 801–900 3 0 0 X 0 0 ?* discarded buffer 4 1 1 X 1 1 X controller’s current location 5 1 0 X 1 0 X not yet used 6 0 0 X 0 0 X not yet used etc. 1 1 X 1 1 X not yet used Descriptor Number Before the Frame Arrives After the Frame has Arrived Comments ¦ ENP or ERR |
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