W89C840F データシートの表示（PDF） - Winbond

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W89C840F

100/10Mbps Ethernet Controller

Winbond 

W89C840F

The following figures describe the ring structures of receive descriptor.

first descritpor of Rx descriptor list

status register

structure constructing register

data buffer 1 pointer

data buffer 2 pointer

second descritpor of Rx descriptor list

status register

structure constructing register

data buffer 1 pointer

data buffer 2 pointer

third descritpor of Rx descriptor list

status register

structure constructing register

data buffer 1 pointer

data buffer 2 pointer

data buffer 1

data buffer 2

data buffer 1

data buffer 2

data buffer 1

data buffer 2

last descritpor of Rx descriptor list

status register

structure constructing register

data buffer 1 pointer

data buffer 2 pointer

data buffer 1

data buffer 2

for storing the first RX packet data

for storing the first RX packet data

for storing the 2nd RX packet data

for storing the 2nd RX packet data

for storing the 3rd RX packet data

for storing the 3rd RX packet data

for storing the nth RX packet data

for storing the nth RX packet data

The software driver can request more than one descriptor and data buffer 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 stored in more than one descriptor.

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, determined 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 descriptors

locate randomly linked by the second pointer in each descriptor, which points to the start address of the

next descriptor.

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Publication Release Date:April 1997

Revision A1

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