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EMAC Functional Architecture
2.5Programming Interface
2.5.1Packet Buffer Descriptors
The buffer descriptor is a central part of the EMAC module. It determines how the application software describes ethernet packets to be sent and empty buffers to be filled with incoming packet data.
The basic descriptor format is shown in Figure 7 and described in Table 7.
Figure 7. Basic Descriptor Format
Word Offset |
| Bit Fields |
| |
| 31 | 16 | 15 | 0 |
0 |
| Next Descriptor Pointer |
| |
1 |
| Buffer Pointer |
| |
2 | Buffer Offset |
|
| Buffer Length |
3 | Flags |
|
| Packet Length |
|
| Table 7. Basic Descriptors |
Word |
| Field Description |
Offset | Field |
|
0 | Next Descriptor | The next descriptor pointer creates a single linked list of descriptors. Each descriptor describes a |
| Pointer | 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 memory buffer that either 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 contains data. |
2 | Buffer Length | The buffer length is the 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 whether it is 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). Section 2.5.4 and Section 2.5.5 describe the flags. |
3 | Packet Length | The packet length only has meaning for buffers that both contain data and are the start of a new |
|
| packet (SOP). For SOP descriptors, the packet length field contains the length of the entire Ethernet |
|
| packet, even if it is contained in a single buffer or fragmented over several buffers. |
26 | Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) | SPRU975B |
