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STP2002QFP
Sun Microelectronics
Fast Ethernet, Parallel Port, SCSI (FEPS) - STP2002QFP
5.4.4 Receive Data StructuresProgramming Restrictions:
• Free receive data buffers must be 64-byte aligned.
Table 12: Receive Data Structures Descriptor Layout: Status Word
Field Bits Description
TCP checksum 15:0 This field contains the 16-bit TCP checksum
that was calculated on the entire frame. It will
be updated for every frame that was received
from the network. The software has the choice
of either making use of it or ignoring it.
Free_buffer/Packet_data size 29:16 When the descriptor ownership is passed from
the software to the hardware, this field con-
tains the size of the free buffer that was
allocated for the packet. When the descriptor
ownership is passed from the hardware to the
software, this field indicates the actual number
of packet data bytes that were dumped into the
buffer.
Overflow 30 When an Rx_Buffer_Overflow condition
occurs, this bit will be set to 1 for the frame
that could not fit into the allocated buffer.
Ownership semaphore 31 To turn over ownership, the hardware clears
this bit and the software sets it.
End of packet 29 When set to 1, indicates the last descriptor of
a transmit packet.
Start of packet 30 When set to 1, indicates the first descriptor of
a transmit packet.
Ownership semaphore 31 To turn over ownership, the hardware clears
this bit and the software sets it.
Table 13: Receive Data Structures: Descriptor Layout: Free Buffer Pointer
Field Bits Description
Free buffer pointer 31:2 This 29-bit pointer, points to the beginning of the free
buffer. The first byte of the actual packet data inside the
buffer will always reside at a programmable offset from
this location, but within a double-word range.