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Word Offset
Field
0
Next Descriptor
Pointer
1
Buffer Pointer
2
Buffer Offset
Buffer Length
3
Flags
Packet Length
SPRUEQ6 – December 2007
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Table 4. Basic Descriptor Description
Field Description
The next descriptor pointer is used to create a single linked list of descriptors. Each descriptor
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.
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.
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.
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.
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
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
Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO)
Section 2.5.4
and
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
Architecture
Section
2.5.5.
21
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