www.ti.com
|
| Architecture |
|
| Table 4. Basic Descriptor Description |
Word Offset | Field | Field Description |
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 |
|
| or the first fragment of a packet, the start of packet (SOP) flag is set in the flags field. When a |
|
| descriptor points to a single buffer packet or the last fragment of a packet, the end of packet |
|
| (EOP) flag is set. When a packet is fragmented, each fragment must have its own descriptor |
|
| and appear sequentially in the descriptor linked list. |
1 | Buffer Pointer | The buffer pointer refers to the actual memory buffer that contains packet data during |
|
| transmit operations, or is an empty buffer ready to receive packet data during receive |
|
| operations. |
2 | Buffer Offset | The buffer offset is the offset from the start of the packet buffer to the first byte of valid data. |
|
| This field only has meaning when the buffer descriptor points to a buffer that actually contains |
|
| data. |
| Buffer Length | The buffer length is the actual number of valid packet data bytes stored in the buffer. If the |
|
| buffer is empty and waiting to receive data, this field represents the size of the empty buffer. |
3 | Flags | The flags field contains more information about the buffer, such as, is it the first fragment in a |
|
| packet (SOP), the last fragment in a packet (EOP), or contains an entire contiguous Ethernet |
|
| packet (both SOP and EOP). The flags are described in Section 2.5.4 and Section 2.5.5. |
| Packet Length | The packet length only has meaning for buffers that both contain data and are the start of a |
|
| new packet (SOP). In the case of SOP descriptors, the packet length field contains the length |
|
| of the entire Ethernet packet, regardless if it is contained in a single buffer or fragmented over |
|
| several buffers. |
Figure 6. Typical Descriptor Linked List
pNext
pBuffer
| 0 |
| 60 |
|
|
|
|
|
|
| SOP EOP |
| 60 |
|
|
|
|
| |
| pNext |
|
| |
| pBuffer |
|
| |
|
|
| ||
|
|
|
|
|
| 0 |
| 512 |
|
|
|
|
|
|
| SOP |
| 1514 |
|
|
|
|
|
|
|
|
|
| |
| pNext |
|
| |
|
|
|
|
|
| pBuffer |
|
| |
|
|
| ||
|
|
|
|
|
| 0 |
| 502 |
|
|
|
|
|
|
| −−− |
| −−− |
|
|
|
|
|
|
|
|
|
|
|
| pNext |
|
| |
|
|
|
|
|
| pBuffer |
|
| |
|
|
| ||
|
|
|
|
|
| 0 |
| 500 |
|
|
|
|
|
|
| EOP |
| −−− |
|
|
|
|
|
|
|
|
| ||
| pNext (NULL) |
| ||
|
| |||
| pBuffer |
|
| |
|
|
| ||
|
|
|
|
|
| 0 |
| 1514 |
|
|
|
|
|
|
| SOP EOP |
| 1514 |
|
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
Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) | 21 |
Submit Documentation Feedback
