Digi NS9750 - Manual (Page 350)

E t h e r n e t f r o n t - e n d m o d u l e
3 2 6 N S 9 7 5 0 H a r d w a r e R e f e r e n c e
used is read from system memory and stored in the registers internal to the
RX_RD
logic.
Figure 66: Receive buffer descriptor format
Field Description
W WRAP bit, which, when set, tells the RX_RD logic that this is the last buffer
descriptor in the ring. In this situation, the next buffer descriptor is found using
the appropriate Buffer Descriptor Pointer register.
When the WRAP bit is not set, the next buffer descriptor is found using an offset
of 0x10 from the current buffer descriptor pointer.
I When set, tells the RX_RD logic to set RXBUFC in the Ethernet Interrupt Status
register (see page 385) after the frame has been transferred to system memory.
E ENABLE bit, which, when set, tells the RX_RD logic that this buffer descriptor
is enabled. When a new frame is received, pools that do not have the E bit set in
their next buffer descriptor are skipped when deciding in which pool to put the
frame.
The receive processor can use up to four different-sized receive buffers in
system memory.
Buffer pointer 32-bit pointer to the start of the buffer in system memory. This pointer must be
aligned on a 32-bit boundary.
Status Lower 16 bits of the Ethernet Receive Status register. The status is taken from
the receive status FIFO and added to the buffer descriptor after the last word of
the frame is written to system memory.
F When set, indicates the buffer is full. The RX_RD logic sets this bit after filling
a buffer. The system software clears this bit, as required, to free the buffer for
future use. When a new frame is received, pools that have the F bit set in their
next buffer descriptor are skipped when deciding in which pool to put the frame.
Destination Address (not used)
Buffer Length (11 lower bits used)
Status
Source Address
0
OFFSET + 0
OFFSET + 4
OFFSET + 8
OFFSET + C F I E W Reserved
31 15 16 30 29 28