SOP | EOP
60
0
60
pBuffer
pNext
Packet A
60 bytes
0
SOP
Fragment 1
Packet B
512
1514
pBuffer
pNext
512 bytes
EOP
0
0
−−−
Packet B
Fragment 3
500 bytes
502
pBuffer
−−−
500
pNext
−−−
pBuffer
pNext
Packet B
Fragment 2
502 bytes
SOP | EOP
0
1514 bytes
Packet C
1514
pBuffer
pNext (NULL)
1514
Architecture
1813
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
EMAC/MDIO Module
Table 32-6. Basic Descriptor Description
Word Offset
Field
Field Description
0
Next Descriptor
Pointer
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.
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
and
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 32-6. Typical Descriptor Linked List