W89C840F
The following figures describe the ring structures of receive descriptor.
first descritpor of Rx descriptor list
status register
data buffer 1
data buffer 2
for storing the first RX packet data
structure constructing register
data buffer 1 pointer
for storing the first RX packet data
data buffer 2 pointer
second descritpor of Rx descriptor list
status register
data buffer 1
data buffer 2
for storing the 2nd RX packet data
for storing the 2nd RX packet data
structure constructing register
data buffer 1 pointer
data buffer 2 pointer
third descritpor of Rx descriptor list
status register
data buffer 1
data buffer 2
for storing the 3rd RX packet data
for storing the 3rd RX packet data
structure constructing register
data buffer 1 pointer
data buffer 2 pointer
last descritpor of Rx descriptor list
status register
data buffer 1
data buffer 2
for storing the nth RX packet data
for storing the nth RX packet data
structure constructing register
data buffer 1 pointer
data buffer 2 pointer
The software driver can request mor e t han one descr i pt or and dat a buf f er at a time. As
described in the above diagram, the total descriptors are constructed as a ring. A packet can be stored in
more than one data buffer. In that case, the data buffer 1 is stored first and then data buffer 2. If a packet
contains more data than the two data buffers can accommodate, it fetches the next descriptor and two new
data buffers to save the extra more data. That is a packet can be stor ed i n mor e t han one descr i pt or .
In the contrary, a descriptor is not allowed to hold more than one packet. If the data buffer 1 can completely
store the received packet, the data buffer 2 will be left empty and the next packet will be firstly stored at the
data buffer 1 in the next descriptor. The diagram shown above is just one case of the buffer application.
When the last descriptor is used by a received packet, the next descriptor should be the first descriptor of
the ring. Once the descriptors are processed by the driver, it can be released to the ring for later use. In the
ring structure, the start address of the next descriptor is specified by the skip length, det er mi ned by bit2
to bit6 of C00/CBCR register, and the start address of the first descriptor is specified by the C0C/CRDLA
register.
. For the descriptors with the chain structure, host is allowed to allocate scatterly a block of memory
with the size of 4 long words, linked by the pointer which located at the next descriptor pointer field.
Each descriptor has only one link to a data buffer to store the received packet data. The descr i pt or s
l ocat e r andoml y linked by the second pointer in each descriptor, which points to the start address of the
next descriptor.
Publication Release Date:April 1997
- 11 -
Revision A1
ETC [ ETC ]
