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On the Fairisle Port Controller VCI 0xff,0xff is needed to determine how deep the switch fabric is. Other (point to point) devices, must not therefore transmit on this VCI.

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eg. the Fairisle Port Controllers cannot distinguish on arrival whether the cell came from the ``loop-back'' or ``receive'' fifos. Incorrect variants would therefore cause considerable chaos.

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ie. Set the portc loop-back bit

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It is of course considered appropriate to generate a diagnostic.

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What this is really doing is: take each word in the cell, convert it from network to host endian, xor it with the checksum and rotate the checksum left one bit. The checksum is finally converted back to network endian before it is stored.

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See the Wanda file kernel/netmsnl/rjbarp.c for an example implementation