dp83820 National Semiconductor Corporation, dp83820 Datasheet - Page 22

• Manufacturer Part Number: dp83820
• Description: 10/100/1000 Mb/s Pci Ethernet Network Interface Controller
• Manufacturer: National Semiconductor Corporation

3.0 Functional Description
3.13.2.1 Single Descriptor Packets
To represent a packet in a single descriptor, the MORE bit in the cmdsts field is set to 0.
3.13.2.2 Multiple Descriptor Packets
A single packet may also cross descriptor boundaries. This
is indicated by setting the MORE bit in all descriptors
except the last one in the packet. Ethernet inter-networking
applications (bridges, switches, routers, etc.) can optimize
3.13.2.3 Descriptor Rings
The
descriptors
initialization, the driver can set up a fixed list of descriptors
complete with links connecting the descriptors in a ring. All
descriptors will initially be owned by the producer of the
data (the driver for transmit, the DP83820 for receive). The
OWN bit is used by both driver and the DP83820 to
simplest
is
in
and
a
recommended
fixed
MAC hdr
1
bufptr
link
ring
14
implementation. At
(Continued)
Figure 3-11 Multiple Descriptor Packets
Figure 3-10 Single Descriptor Packets
organization
multiple descriptor / single fragment
single descriptor / single fragment
netwk hdr
1
bufptr
netwk hdr
link
MAC hdr
0
bufptr
of
data
link
22
20
memory utilization by using a single small buffer per
receive descriptor, and allowing the DP83820 hardware to
use the minimum number of buffers necessary to store an
incoming packet.
indicate data availability and to release descriptors back to
the producer. When using a descriptor ring, the driver
should never need to modify any fields of a descriptor it
does not own. For transmit, the driver should never assign
all descriptors to the device, reserving one descriptor to
terminate the list, preventing the device from wrapping
completely around the ring.
64
0
bufptr
link
data
30
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