[Coco] Fwd: IP packets on my coco

Dave Philipsen dave at davebiz.com
Wed Jun 8 02:26:49 EDT 2016

After looking at a few more discussions of this it appears you can 
calculate CRC-32 with a table of 256 *words*  of 32-bits each which 
translates to a table of 1024 bytes in length.


On 6/8/2016 1:13 AM, Dave Philipsen wrote:
> 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.
> Dave
> 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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