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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 ST-NIC. The Buffer Management Log-
ic 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 ST-NIC
treats the list of buffers as a logical ring; whenever the DMA
address reaches the Page Stop Address, the DMA is reset
to the Page Start Address.
ST-NIC Receive Buffer Ring
TL/F/11157 – 9
14
Obsolete