dp83901a National Semiconductor Corporation, dp83901a Datasheet - Page 10

• Manufacturer Part Number: dp83901a
• Description: Serial Network Interface Controller
• Manufacturer: National Semiconductor Corporation

7 0 Packet Reception
The Local DMA receive channel uses a Buffer Ring Struc-
ture comprised of a series of contiguous fixed length
256-byte (128 word) buffers for storage of received packets
The location of the Receive Buffer Ring is programmed in
two registers a Page Start and a Page Stop Register Ether-
net packets consist of a distribution of shorter link control
packets and longer data packets the 256-byte buffer length
provides a good compromise between short packets and
longer packets to most efficiently use memory In addition
these buffers provide memory resources for storage of
back-to-back packets in loaded networks The assignment
of buffers for storing packets is controlled by Buffer Man-
agement Logic in the SNIC The Buffer Management Logic
NIC Receive Buffer Ring
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provides three basic functions linking receive buffers for
long packets recovery of buffers when a packet is rejected
and recirculation of buffer pages that have been read by the
host
At initialization a portion of the 64 kbyte (or 32k word) ad-
dress space is reserved for the receive buffer ring Two 8-bit
registers The Page Start Address Register (PSTART) and
the Page Stop Address Register (PSTOP) define the physi-
cal boundaries of where the buffers reside The SNIC treats
the list of buffers as a logical ring whenever the DMA ad-
dress reaches the Page Stop Address the DMA is reset to
the Page Start Address
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