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| Table 4. Basic Descriptor Description |
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Word Offset | Field | Field Description |
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0 | Next Descriptor | The next descriptor pointer is used to create a single linked list of descriptors. Each descriptor |
| Pointer | describes a packet or a packet fragment. When a descriptor points to a single buffer packet |
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| or the first fragment of a packet, the start of packet (SOP) flag is set in the flags field. When a |
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| descriptor points to a single buffer packet or the last fragment of a packet, the end of packet |
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| (EOP) flag is set. When a packet is fragmented, each fragment must have its own descriptor |
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| and appear sequentially in the descriptor linked list. |
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1 | Buffer Pointer | The buffer pointer refers to the actual memory buffer that contains packet data during |
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| transmit operations, or is an empty buffer ready to receive packet data during receive |
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| operations. |
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2 | Buffer Offset | The buffer offset is the offset from the start of the packet buffer to the first byte of valid data. |
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| This field only has meaning when the buffer descriptor points to a buffer that actually contains |
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| data. |
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| Buffer Length | The buffer length is the actual number of valid packet data bytes stored in the buffer. If the |
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| buffer is empty and waiting to receive data, this field represents the size of the empty buffer. |
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3 | Flags | The flags field contains more information about the buffer, such as, is it the first fragment in a |
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| packet (SOP), the last fragment in a packet (EOP), or contains an entire contiguous Ethernet |
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| packet (both SOP and EOP). The flags are described in Section 2.5.4 and Section 2.5.5. |
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| Packet Length | The packet length only has meaning for buffers that both contain data and are the start of a |
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| new packet (SOP). In the case of SOP descriptors, the packet length field contains the length |
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| of the entire Ethernet packet, regardless if it is contained in a single buffer or fragmented over |
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| several buffers. |
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Figure 6. Typical Descriptor Linked List
pNext
pBuffer
| 0 |
| 60 |
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| SOP EOP |
| 60 |
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| pNext |
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| pBuffer |
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| 0 |
| 512 |
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| SOP |
| 1514 |
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| pNext |
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| pBuffer |
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| 0 |
| 502 |
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| −−− |
| −−− |
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| pNext |
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| pBuffer |
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| 0 |
| 500 |
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| EOP |
| −−− |
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| pNext (NULL) |
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| pBuffer |
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| 0 |
| 1514 |
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| SOP EOP |
| 1514 |
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Packet A 60 bytes
Packet B
Fragment 1
512 bytes
Packet B
Fragment 2
502 bytes
Packet B
Fragment 3
500 bytes
Packet C
1514 bytes
SPRUFL5B | EMAC/MDIO Module | 19 |
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© 2011, Texas Instruments Incorporated
