83C795 Datasheet, PDF (99/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

83C795
BUFFER STRUCTURING AND DATA MOVEMENT PROCESSES
When the receive DMA process attempts to open
the buffer that BOUND points to for s torage of a
packet, it aborts the reception and s ets the
I NT S T AT .OVW flag (in the Interrupt S tatus
R egis ter) and the R S T AT.MPA bit (in the R eceiver
S tatus R egis ter). T he protectedbuffer is not written
to.
Normally, BOUND is s et up to point to the oldes t
received packet in the ring. T his pointer can be
managed by the host. To dis card an unwanted
frame, the host may s imply rewriteBOUND topoint
to the next packet. A good practice is to write zeros
into the firs t byte of the dis cardedpacket to prevent
future interpretation as a received packet.
When BOUND and CUR R have the same value,
the ring may be either full or empty. T he 83C795
can dis tinguis h between full and empty rings .
CUR R is updated by the DMA controller at the end
of a frame reception. T he ring is cons idered full if
thes e two regis ters are equal and the DMA
controller updated CUR R more recently than the
host updated BOUND. Convers ely, when data is
being removed from the ring, the hos t updates
BOUND after removal. When it has been advanced
past the end of the last received frame, it s hould
have the same value as CUR R . T he chiptreats the
ring as empty when these two regis ter values are
equal and BOUND has been updated after CUR R .
When initializingthebuffer ring, BOUND andCUR R
may be given the s ame or different values. T hese
regis ters may be initialized to point to any buffer
within the ring. T hey may point to R S T AR T but may
notpoint toR S T OP. If CUR R points outsidethering,
the R DMA will s tore frames outside the ring in an
unpredictablemanner. If BOUND points outs idethe
ring, the received frames will not be protectedfrom
overwrites by later frames.
F igure 8-4 illus trates the buffer ring in two common
s tates . T he top ring, INIT R B UF, illus trates the
relations hipbetween thes e pointers in atypical ring
initialization. T he bottom ring, R BUF, s hows a ring
that has received a few frames - the normal
condition for the ring.
FIGURE 8-4. RECEIVER BUFFER RING 1
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