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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
PeripheralArchitecture
Table3.BasicDescriptorDescription
WordOffsetFieldFieldDescription
0NextDescriptorThenextdescriptorpointerisusedtocreateasinglelinkedlistofdescriptors.Eachdescriptor
Pointerdescribesapacketorapacketfragment.Whenadescriptorpointstoasinglebufferpacket
orthefirstfragmentofapacket,thestartofpacket(SOP)flagissetintheflagsfield.Whena
descriptorpointstoasinglebufferpacketorthelastfragmentofapacket,theendofpacket
(EOP)flagisset.Whenapacketisfragmented,eachfragmentmusthaveitsowndescriptor
andappearsequentiallyinthedescriptorlinkedlist.
1BufferPointerThebufferpointerreferstotheactualmemorybufferthatcontainspacketdataduring
transmitoperations,orisanemptybufferreadytoreceivepacketdataduringreceive
operations.
2BufferOffsetThebufferoffsetistheoffsetfromthestartofthepacketbuffertothefirstbyteofvaliddata.
Thisfieldonlyhasmeaningwhenthebufferdescriptorpointstoabufferthatactuallycontains
data.
BufferLengthThebufferlengthistheactualnumberofvalidpacketdatabytesstoredinthebuffer.Ifthe
bufferisemptyandwaitingtoreceivedata,thisfieldrepresentsthesizeoftheemptybuffer.
3FlagsTheflagsfieldcontainsmoreinformationaboutthebuffer,suchas,isitthefirstfragmentina
packet(SOP),thelastfragmentinapacket(EOP),orcontainsanentirecontiguousEthernet
packet(bothSOPandEOP).TheflagsaredescribedinSection2.5.4andSection2.5.5.
PacketLengthThepacketlengthonlyhasmeaningforbuffersthatbothcontaindataandarethestartofa
newpacket(SOP).InthecaseofSOPdescriptors,thepacketlengthfieldcontainsthelength
oftheentireEthernetpacket,regardlessifitiscontainedinasinglebufferorfragmentedover
severalbuffers.
Figure5.TypicalDescriptorLinkedList
SPRU941A–April2007EthernetMediaAccessController(EMAC)/17
ManagementDataInput/Output(MDIO)
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