[Coco] Fwd: IP packets on my coco

Dave Philipsen dave at davebiz.com
Wed Jun 8 02:13:06 EDT 2016

Actually I don't think it works that way.  The lookup table does not 
contain every possible value of the CRC.  It is used to speed up the 
calculation, not to lookup the final result.  I *think* you could do a 
32-bit CRC with a 256 byte table.


On 6/8/2016 12:57 AM, Barry Nelson wrote:
>   The problem with that approach is that an 8 bit CRC requires a 256 byte lookup table or 2^8 bytes. A 32 bit CRC would need a 2^32 byte table or 4294967296 bytes, that is a 4Gb lookup table.
>> Dave Philipsen dave at davebiz.com
>> Wed Jun 8 01:27:51 EDT 2016
>> I remember from the CRC calculation in X/Ymodem that there is an easy
>> and faster way to get the CRC using lookup tables.  That's how I did it
>> with Supercomm.  I believe the CRC for the networking packets is a
>> 32-bit CRC so I don't know if lookup tables are still practical or not
>> with that size CRC.  What is amazing is that I was able to find that
>> information without the internet back then.  I just now did a few
>> cursory searches on calculating CRCs and I'm guessing that there is a
>> fairly easy method that doesn't require so much overhead.
>> Dave

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