Ethernet front-end module
326
NS9750 Hardware Reference
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
WWRAP 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 page385) after the frame has been transferred to system memory.
EENABLE 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 FIEWReserved
31 151630 29 28